The Average Chemical Composition of Igneous Rocks

4 Igneous Processes and Volcanoes
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The solids are made of crystallized minerals floating in the liquid melt. The presence and amount of these three components affect the physical behavior of the magma and will be discussed more below. Below the surface, the temperature of the Earth rises. This heat is caused by residual heat left from the formation of Earth and ongoing radioactive decay. The rate at which temperature increases with depth is called the geothermal gradient. Pressure-temperature diagram showing temperature in degrees Celsius on the x-axis and depth below the surface in kilometers km on the y-axis.

The red line is the geothermal gradient and the green solidus line represents the temperature and pressure regime at which melting begins. Rocks at pressures and temperatures left of the green line are solid. Source: Woudloper The depth- temperature graph see figure illustrates the relationship between the geothermal gradient geotherm, red line and the start of rock melting solidus, green line.

The geothermal gradient changes with depth which has a direct relationship to pressure through the crust into upper mantle. The area to the left of the green line includes solid components; to the right is where liquid components start to form. The increasing temperature with depth makes the depth of about kilometers 78 miles where the natural geothermal gradient is closest to the solidus. At bottom of the crust , 35 km 22 mi deep, the pressure is about 10, bars. A bar is a measure of pressure, with 1 bar being normal atmospheric pressure at sea level. At these pressures and temperatures, the crust and mantle are solid.

Igneous Rocks

To a depth of km 93 mi , the geothermal gradient line stays to the left of the solidus line. This relationship continues through the mantle to the core The innermost chemical layer of the Earth, made chiefly of iron and nickel. It has both liquid and solid components. The solidus line slopes to the right because the melting temperature of any substance depends on pressure. The higher pressure created at greater depth increases the temperature needed to melt rock. But if the pressure is lowered, as shown on the video below, water boils at a much lower temperature.

There are three principal ways rock behavior crosses to the right of the green solidus line to create molten magma : 1 decompression melting caused by lowering the pressure, 2 flux melting caused by adding volatiles see more below , and 3 heat-induced melting caused by increasing the temperature. Since magma is a mixture of different minerals , the solidus boundary is more of a fuzzy zone rather than a well-defined line; some minerals are melted and some remain solid. This type of rock behavior is called partial melting and represents real-world magmas , which typically contain solid, liquid, and volatile components.

The figure below uses P-T diagrams to show how melting can occur at three different plate tectonic settings. The green line is called the solidus , the melting point temperature of the rock at that pressure. In the other three situations, rock at a lettered location with a temperature at the geothermal gradient is moved to a new P-T situation on the diagram. This shift is indicated by the arrow and its temperature relative to the solidus is shown by the red line. Partial melting occurs where the red line temperature of the rock crosses the green solidus on the diagram.

Setting B is at a mid-ocean ridge decompression melting where reduction of pressure carries the rock at its temperature across the solidus. Setting C is a hotspot where decompression melting plus addition of heat carries the rock across the solidus, and setting D is a subduction zone where a process called flux melting takes place where the solidus melting point is actually shifted to below the temperature of the rock.

Graph A illustrates a normal situation, located in the middle of a stable plate , where no melted rock can be found. The remaining three graphs illustrate rock behavior relative to shifts in the geothermal gradient or solidus lines. Partial melting occurs when the geothermal gradient line crosses the solidus line. Graph B illustrates behavior of rock located at a mid-ocean ridge , labeled X in the graph and side view. Reduced pressure shifts the geotherm to the right of the solidus, causing decompression melting.

Graph C and label Y illustrate a hotspot situation. Decompression melting , plus an addition of heat, shifts the geotherm across the solidus. Graph D and label Z show a subduction zone, where an addition of volatiles lowers the melting point, shifting the solidus to the left of the geothermal gradient.

B, C, and D all show different ways the Earth produces intersections of the geothermal gradient and the solidus, which results in melting each time. Progression from rift to mid-ocean ridge, the divergent boundary types. Note the rising material in the center. Magma is created at mid-ocean ridges via decompression melting. Strong convection currents cause the solid asthenosphere to slowly flow beneath the lithosphere. The upper part of the lithosphere crust is a poor heat conductor, so the temperature remains about the same throughout the underlying mantle material.

Where the convection currents cause mantle material to rise, the pressure decreases, which causes the melting point to drop. In this situation, the rock at the temperature of the geothermal gradient is rising toward the surface, thus hotter rock is now shallower, at a lower pressure, and the rock, still at the temperature of the geothermal gradient at its old location, shifts past the its melting point shown as the red line crossing over the solidus or green line in example B in previous figure and partial melting starts.

As this magma continues to rise, it cools and crystallizes to form new lithospheric crust. Flux melting or fluid-induced melting occurs in island arcs and subduction zones when volatile gases are added to mantle material see figure: graph D, label Z. Flux-melted magma produces many of the volcanoes in the circum-Pacific subduction zones, also known as the Ring of Fire.

The subducting slab Name given to the subducting plate, where volatiles are driven out at depth, causing volcanism. As covered in Chapter 2 , these hydrated forms are created when water ions bond Two or more atoms or ions that are connected chemically. As the slab Name given to the subducting plate, where volatiles are driven out at depth, causing volcanism. The volatiles dissolve into the overlying asthenospheric mantle and decrease its melting point. The previous figure graph D shows the green solidus line shifting to the left of and below the red geothermal gradient line, and melting begins.

This is analogous to adding salt to an icy roadway. The salt lowers the freezing temperature of the solid ice so it turns into liquid water. Heat-induced melting, transforming solid mantle into liquid magma by simply applying heat, is the least common process for generating magma see figure: graph C, label Y.

Heat-induced melting occurs at a mantle plumes or hotspots. The rock surrounding the plume is exposed to higher temperatures, the geothermal gradient crosses to the right of the green solidus line, and the rock begins to melt. The mantle plume includes rising mantle material, meaning some decompression melting is occurring as well.

A small amount of magma is also generated by intense regional metamorphism see Chapter 6. This magma becomes a hybrid metamorphic - igneous rock called migmatite. This explains the wide variety of resulting igneous rocks that are found all over Earth. Because the mantle is composed of many different minerals , it does not melt uniformly.

As minerals with lower melting points turn into liquid magma , those with higher melting points remain as solid crystals. This is known as partial melting. As magma slowly rises and cools into solid rock, it undergoes physical and chemical changes in a process called magmatic differentiation. Since most rocks are made of many different minerals , when they start to melt, some minerals begin melting sooner than others.

This is known as partial melting , and creates magma with a different composition than the original mantle material. The most important example occurs as magma is generated from mantle rocks as discussed in Section 4. The chemistry of mantle rock peridotite is ultramafic , low in silicates and high in iron and magnesium. When peridotite begins to melt, the silica-rich portions melt first due to their lower melting point.

If this continues, the magma becomes increasingly silica-rich, turning ultramafic mantle into mafic magma , and mafic mantle into intermediate magma. The magma rises to the surface because it is more buoyant than the mantle. Geologic provinces with the Shield orange and Platform pink comprising the Craton, the stable interior of continents. Partial melting also occurs as existing crustal rocks melt in the presence of heat from magmas. In this process, existing rocks melt, allowing the magma formed to be more felsic and less mafic than the pre-existing rock.

In the figure, the old granitic cores of the continents, called shields , are shown in orange. Liquid magma is less dense than the surrounding solid rock, so it rises through the mantle and crust. As magma begins to cool and crystallize, a process known as magmatic differentiation changes the chemistry of the resultant rock towards a more felsic composition. This happens via two main methods: assimilation and fractionation. During assimilation , pieces of country rock with a different, often more felsic , composition are added to the magma.

These solid pieces may melt, which changes the composition of the original magma. At times, the solid fragments may remain intact within the cooling magma and only partially melt. The unmelted country rocks within an igneous rock mass are called xenoliths. Xenoliths are also common in the processes of magma mixing and rejuvenation, two other processes that can contribute to magmatic differentiation.

Magma mixing occurs when two different magmas come into contact and mix, though at times, the magmas can remain heterogeneous and create xenoliths , dike A narrow igneous intrusion that cuts through existing rock, not along bedding planes. Magmatic rejuvenation happens when a cooled and crystallized body of rock is remelted and pieces of the original rock may remain as xenoliths.

Much of the continental lithosphere is felsic i. When mafic magma rises through thick continental crust , it does so slowly, more slowly than when magma rises through oceanic plates. This gives the magma lots of time to react with the surrounding country rock. The mafic magma tends to assimilate felsic rock, becoming more silica-rich as it migrates through the lithosphere and changing into intermediate or felsic magma by the time it reaches the surface. This is why felsic magmas are much more common within continents.

Rising magma diapirs in mantle and crust. Fractional crystallization occurs in the diapirs in the crust. Source: Woudloper Fractionation or fractional crystallization is another process that increase magma silica content, making it more felsic. As the temperature drops within a magma diapir rising through the crust , some minerals will crystallize and settle to the bottom of the magma chamber , leaving the remaining melt depleted of those ions.

When ultramafic magma cools, the olivine crystallizes first and settles to the bottom of the magma chamber see figure. This means the remaining melt becomes more silica-rich and felsic. This crystal fractionation can occur in oceanic lithosphere , but the formation of more differentiated, highly evolved felsic magmas is largely confined to continental regions where the longer time to the surface allows more fractionation to occur.

Schematic diagram illustrating fractional crystallization. If magma at composition A is ultramafic, as the magma cools it changes composition as different minerals crystallize from the melt and settle to the bottom of the magma chamber. In section 1, olivine crystallizes; section 2: olivine and pyroxene crystallize; section 3: pyroxene and plagioclase crystallize; and section 4: plagioclase crystallizes. The crystals are separated from the melt and the remaining magma composition B is more silica-rich. Source: Woudloper.

A volcano is a type of land formation created when lava Liquid rock on the surface of the Earth. Volcanoes have been an important part of human society for centuries, though their understanding has greatly increased as our understanding of plate tectonics has made them less mysterious. Most volcanoes are interplate volcanoes.

Igneous rock texture

Waters, A. Traces of that energy are released all over the surface of the planet, but that release of energy is not evenly distributed on the surface. A granite, for example, can be distinguished from a sandstone because rather than being a mixture of weathered, rounded grains compressed and cemented together, granite consists of a small number of minerals in shiny black, white, or pink colors, with excellent crystal forms, grown together into a completely interlocking pattern. Andesitic rocks are associated with oceanic island arc volcanoes associated with subduction zones, such as those found in Japan or the Aleutian Islands volcanic chain. Felsic Rocks Felsic rocks include rocks of grantitic appearance looking like any of a variety of granite-like rocks and felsic mineral composition composed mostly of low-temperature minerals including quartz, micas, and feldspars.

Interplate volcanoes are located at active plate boundaries created by volcanism at mid-ocean ridges , subduction zones, and continental rift Area of extended continental lithosphere, forming a depression. Some volcanoes are intraplate volcanoes. Many intraplate volcanoes are formed by hotspots. Map of mid-ocean ridges throughout the world.

Most volcanism on Earth occurs on the ocean floor along mid-ocean ridges , a type of divergent plate boundary see Chapter 2. These interplate volcanoes are also the least observed and famous, since most of them are located under 3,, m 10,, ft of ocean and the eruptions are slow, gentle, and oozing. One exception is the interplate volcanoes of Iceland. The diverging and thinning oceanic plates allow hot mantle rock to rise, releasing pressure and causing decompression melting. Ultramafic mantle rock, consisting largely of peridotite , partially melts and generates magma that is basaltic.

Because of this, almost all volcanoes on the ocean floor are basaltic. In fact, most oceanic lithosphere is basaltic near the surface, with phaneritic gabbro and ultramafic peridotite underneath. When basaltic lava Liquid rock on the surface of the Earth. These seafloor eruptions enable entire underwater ecosystems to thrive in the deep ocean around mid-ocean ridges.

This ecosystem exists around tall vents emitting black, hot mineral -rich water called deep-sea hydrothermal vents, also known as black smokers. Distribution of hydrothermal vent fields. Without sunlight to support photosynthesis, these organisms instead utilize a process called chemosynthesis. Certain bacteria are able to turn hydrogen sulfide H 2 S , a gas that smells like rotten eggs, into life-supporting nutrients and water. Larger organisms may eat these bacteria or absorb nutrients and water produced by bacteria living symbiotically inside their bodies.

The three videos show some of the ecosystems found around deep-sea hydrothermal vents. The second most commonly found location for volcanism is adjacent to subduction zones, a type of convergent plate boundary see Chapter 2. The process of subduction expels water from hydrated minerals in the descending slab Name given to the subducting plate, where volatiles are driven out at depth, causing volcanism. Because subduction volcanism occurs in a volcanic arc , the thickened crust promotes partial melting and magma differentiation. These evolve the mafic magma from the mantle into more silica-rich magma.

The Ring of Fire surrounding the Pacific Ocean, for example, is dominated by subduction -generated eruptions of mostly silica-rich lava Liquid rock on the surface of the Earth. Some volcanoes are created at continental rift Area of extended continental lithosphere, forming a depression. Volcanism caused by crustal thinning without continental rift Area of extended continental lithosphere, forming a depression.

In this location, volcanic activity is produced by rising magma that stretches the overlying crust see figure. Lower crust or upper mantle material rises through the thinned crust , releases pressure, and undergoes decompression-induced partial melting. This magma is less dense than the surrounding rock and continues to rise through the crust to the surface, erupting as basaltic lava Liquid rock on the surface of the Earth. These eruptions usually result in flood basalts , cinder cones, and basaltic lava Liquid rock on the surface of the Earth.

Relatively young cinder cones of basaltic lava Liquid rock on the surface of the Earth. These Utah cinder cones and lava Liquid rock on the surface of the Earth. Diagram showing a non-moving source of magma mantle plume and a moving overriding plate. Hotspots are the main source of intraplate volcanism. Hotspots occur when lithospheric plates glide over a hot mantle plume , an ascending column of solid heated rock originating from deep within the mantle. The mantle plume generates melts as material rises, with the magma rising even more.

When the ascending magma reaches the lithospheric crust , it spreads out into a mushroom-shaped head that is tens to hundreds of kilometers across. Since most mantle plumes are located beneath the oceanic lithosphere , the early stages of intraplate volcanism typically take place underwater. Over time, basaltic volcanoes may build up from the sea floor into islands, such as the Hawaiian Islands.

Where a hotspot is found under a continental plate , contact with the hot mafic magma may cause the overlying felsic rock to melt and mix with the mafic material below, forming intermediate magma. Or the felsic magma may continue to rise, and cool into a granitic batholith or erupt as a felsic volcano. The Yellowstone caldera is an example of hotspot volcanism that resulted in an explosive eruption.

A zone of actively erupting volcanism connected to a chain of extinct volcanoes indicates intraplate volcanism located over a hotspot. These volcanic chains are created by the overriding oceanic plate slowly moving over a hotspot mantle plume. These chains are seen on the seafloor and continents and include volcanoes that have been inactive for millions of years. The Hawaiian Islands on the Pacific Oceanic plate are the active end of a long volcanic chain that extends from the northwest Pacific Ocean to the Emperor Seamounts , all the way to the to the subduction zone beneath the Kamchatka Peninsula.

The overriding North American continental plate moved across a mantle plume hotspot for several million years, creating a chain of volcanic calderas that extends from Southwestern Idaho to the presently active Yellowstone caldera in Wyoming. Two three -minute videos below illustrates hotspot volcanoes.

There are several different types of volcanoes based on their shape, eruption style, magmatic composition , and other aspects. Kilauea sitting on the flank of Mauna Loa is not considered a parasitic cone because it has its own separate magma chamber. Viscosity is the resistance to flowing by a fluid. Low viscosity magma flows easily more like syrup, the basaltic volcanism that occurs in Hawaii on shield volcanoes.

High viscosity means a thick and sticky magma , typically felsic or intermediate , that flows slowly, similar to toothpaste. The largest volcanoes are shield volcanoes. They are characterized by broad low-angle flanks, small vents at the top, and mafic magma chambers. They are typically associated with hotspots , mid-ocean ridges , or continental rift Area of extended continental lithosphere, forming a depression.

The low-angle flanks are built up slowly from numerous low- viscosity basaltic lava Liquid rock on the surface of the Earth. The basaltic lava Liquid rock on the surface of the Earth. Typically, shield volcano eruptions are not much of a hazard to human life—although non-explosive eruptions of Kilauea Hawaii in produced uncharacteristically large lavas that damaged roads and structures. Shield volcanoes are also found in Iceland, the Galapagos Islands, Northern California, Oregon, and the East African rift Area of extended continental lithosphere, forming a depression.

This possibly extinct shield volcano covers an area the size of the state of Arizona. This may indicate the volcano erupted over a hotspot for millions of years, which means Mars had little, if any, plate tectonic activity. Basaltic lava Liquid rock on the surface of the Earth.

The two main types of basaltic volcanic rock have Hawaiian names— pahoehoe and aa A blocky, stubby, rubble-like lava. Pahoehoe might come from low- viscosity lava Liquid rock on the surface of the Earth. The exact details of what forms the two types of flows are still up for debate. Felsic lavas have lower temperatures and more silica, and thus are higher viscosity.

These also form aa A blocky, stubby, rubble-like lava. Low- viscosity , fast-flowing basaltic lava Liquid rock on the surface of the Earth. Once lava Liquid rock on the surface of the Earth. Fissures are cracks that commonly originate from shield -style eruptions. The Kiluaea eruption included fissures associated with the lava Liquid rock on the surface of the Earth. Some fissures are caused by the volcanic seismic activity rather than lava Liquid rock on the surface of the Earth. Some fissures are influenced by plate tectonics , such as the common fissures located parallel to the divergent boundary in Iceland.

Cooling lava Liquid rock on the surface of the Earth. This feature forms the famous Devils Tower in Wyoming, possibly an ancient volcanic vent Opening of a volcano where lava can erupt. A stratovolcano , also called a composite cone volcano , has steep flanks, a symmetrical cone shape, distinct crater, and rises prominently above the surrounding landscape. The term composite refers to the alternating layers of pyroclastic fragments like ash Volcanic tephra that is less than 2 mm in diameter. Stratovolcanoes usually have felsic to intermediate magma chambers, but can even produce mafic lavas.

Stratovolcanoes have viscous lava Liquid rock on the surface of the Earth. This produces volcanoes with steep flanks. Too viscous to flow easily, the felsic lava Liquid rock on the surface of the Earth. As the dome A rock up-warping of symmetrical anticlines. Mount Saint Helens has a good example of a lava dome inside of a collapsed stratovolcano crater.

Examples of stand-alone lava Liquid rock on the surface of the Earth. Calderas are steep-walled, basin -shaped depressions formed by the collapse of a volcanic edifice into an empty magma chamber. Calderas are generally very large, with diameters of up to 25 km The term caldera specifically refers to a volcanic vent Opening of a volcano where lava can erupt.

The caldera at Crater Lake National Park in Oregon was created about 6, years ago when Mount Mazama, a composite volcano , erupted in a huge explosive blast. The volcano ejected large amounts of volcanic ash Volcanic tephra that is less than 2 mm in diameter. Wizard Island in the middle of the lake is a later resurgent lava dome that formed within the caldera basin. The Yellowstone volcanic system erupted three times in the recent geologic past—2. Each eruption created large rhyolite lava Liquid rock on the surface of the Earth. These extra-large eruptions rapidly emptied the magma chamber , causing the roof to collapse and form a caldera.

The youngest of the three calderas contains most of Yellowstone National Park, as well as two resurgent lava Liquid rock on the surface of the Earth. The calderas are difficult to see today due to the amount of time since their eruptions and subsequent erosion and glaciation. As the plate moved to the southwest over the stationary hotspot , it left behind a track of past volcanic activities.

The plate eventually arrived at its current location in northwestern Wyoming, where hotspot volcanism formed the Yellowstone calderas. The Long Valley Caldera near Mammoth, California, is the result of a large volcanic eruption that occurred , years ago. The explosive eruption dumped enormous amounts of ash Volcanic tephra that is less than 2 mm in diameter. The current caldera basin is 17 km by 32 km 10 mi by 20 mi , large enough to contain the town of Mammoth Lakes, major ski resort, airport, major highway, resurgent dome A rock up-warping of symmetrical anticlines.

Cinder cones are small volcanoes with steep sides, and made of pyroclastic fragments that have been ejected from a pronounced central vent Opening of a volcano where lava can erupt. The small fragments are called cinders and the largest are volcanic bomb Large volcanic tephra greater than 64 mm in diameter. The eruptions are usually short-lived events, typically consisting of mafic lavas with a high content of volatiles.

Hot lava Liquid rock on the surface of the Earth. Cinder cones are found throughout western North America. The cinder cone started explosively shooting cinders out of the vent Opening of a volcano where lava can erupt. The volcanism quickly built up the cone to a height of over 90 m ft within a week, and m 1, ft within the first 8 months. After the initial explosive eruption of gases and cinders , basaltic lava Liquid rock on the surface of the Earth. This is a common order of events for cinder cones: violent eruption, cone and crater formation , low- viscosity lava Liquid rock on the surface of the Earth.

The cinder cone is not strong enough to support a column of lava Liquid rock on the surface of the Earth. During nine years of eruption activity, the ashfall covered about km 2 mi 2 and destroyed the nearby town of San Juan. Flood basalts are some of the largest and lowest viscosity types of eruptions known. They are not known from any eruption in human history, so the exact mechanisms of eruption are still mysterious. Some famous examples include the Columbia River Flood Basalts in Washington, Oregon, and Idaho, the Deccan trap A geologic circumstance such as a fold, fault, change in lithology, etc.

Arguably the most unusual volcanic activity are carbonatite eruptions. Only one actively erupting carbonatite volcano exists on Earth today, Ol Doinyo Lengai, in the East African rift Area of extended continental lithosphere, forming a depression. Carbonatite lavas are very low viscosity and relatively cold for lava Liquid rock on the surface of the Earth. The erupting lava Liquid rock on the surface of the Earth. These rocks are occasionally found in the geologic record and require special study to distinguish them from metamorphic marbles see Chapter 6. They are mostly associated with continental rift Area of extended continental lithosphere, forming a depression.

Igneous rock types and related volcano types. Mid-ocean ridges and shield volcanoes represent more mafic compositions, and strato composite volcanoes generally represent a more intermediate or felsic composition and a convergent plate tectonic boundary. Note that there are exceptions to this generalized layout of volcano types and igneous rock composition. Get Started! While the most obvious volcanic hazard is lava Liquid rock on the surface of the Earth. For example, on May 18, , Mount Saint Helens Washington, United States erupted with an explosion and landslide that removed the upper m 1, ft of the mountain.

The initial explosion was immediately followed by a lateral blast, which produced a pyroclastic flow that covered nearly km 2 mi 2 of forest with hot ash Volcanic tephra that is less than 2 mm in diameter. The pyroclastic flow moved at speeds of kph mph , flattening trees and ejecting clouds of ash Volcanic tephra that is less than 2 mm in diameter. The USGS video provides an account of this explosive eruption that killed 57 people. In 79 AD, Mount Vesuvius, located near Naples, Italy, violently erupted sending a pyroclastic flow over the Roman countryside, including the cities of Herculaneum and Pompeii.

The buried towns were discovered in an archeological expedition in the 18th century. The most dangerous volcanic hazard are pyroclastic flows video. These flows are a mix of lava Liquid rock on the surface of the Earth. The turbulent cloud of ash Volcanic tephra that is less than 2 mm in diameter. Most explosive, silica-rich, high viscosity magma volcanoes such as composite cones usually have pyroclastic flows. There are numerous examples of deadly pyroclastic flows. In , the Mount Ontake pyroclastic flow in Japan killed 47 people. The flow was caused by magma heating groundwater into steam, which then rapidly ejected with ash Volcanic tephra that is less than 2 mm in diameter.

Some were killed by inhalation of toxic gases and hot ash Volcanic tephra that is less than 2 mm in diameter. The volatile components both lower the temperature that mineral can form, but also enhance diffusivity the ability for elements to move around within a fluid to interact with crystal nucleation sites. This diffusivity allows large and rapid crystal growth—allowing minerals to separate into pockets, some with rare and unusual compositions.

These late stage fluids can be enriched in the elements necessary for important gems. Not all pegmatites form the same way. Pegmatites frequently occur in association with aplite dikes and veins Figures to Aplite is a light-colored granitic rock composed of quartz and feldspar with sugary texture.

Pegmatites occur in pod-shaped mass, usually as small pods to long linear zones, in some rare cases hundreds of feet in size and thickness. Some pegmatites may contain open pockets called vugs that are typically surrounded by well-formed crystal masses radiating into the open voids. Some pegmatites are best described as metamorphic in origin, formed as rocks rich in fluids only begin to melt and separate from host rocks into isolated pockets and veins. Economic Value: Gem minerals found in pegmatites include: apatite, beryls including emerald , cassiterite, corundum sapphires , feldspars including aquamarine and perthite , fluorite, garnet, lepidolite quartz varieties crystal, rose, and smoky , spodumene, topaz, and tourmaline.

Pegmatites sometimes containing rare minerals rich in uncommon elements, including important rare-earth elements. Pegmatite ore around the world have been mined for such economically strategic elements, primarily for beryllium and lithium, but also aluminum, bismuth, boron, cesium, molybdenum, niobium, potassium, tantalum, thorium, tin, tungsten, and uranium.

Low- and high-temperature "common" igneous minerals. Bowen's Reaction Series illustrates the order that minerals form in a cooling melt. Common igneous rock-forming minerals formed through Bowen's Reaction Series. Igneous rocks formed through Bowen's Reaction Series. Isostasy and the role of density of oceanic crust and continental crust. Both types of crust float on the asthenosphere, but because continental rocks are less dense, they rise above the ocean crust. As the old, denser, "mafic rich" rock sinks, it heats up.

First low-temperature minerals begin to melt, assisted by fluids trapped in the rock including water, carbon dioxide, etc. This new molten material is enriched in felsic minerals. As the process proceeds, the remaining mafic minerals are concentrated in the material sinking back into the mantle, whereas the lighter felsic material rises toward the surface forming plutons that may or may not erupt on the surface as volcanic eruptions. Over time, the continents grow by the accretion of felsic-rich rocks Figure Refining minerals into the crust through subduction, plutonism and volcanism.

Over time, felsic materials accumulate in continental crust.

A simple classification of volcanic rocks

Felsic Rocks Felsic rocks include rocks of grantitic appearance looking like any of a variety of granite-like rocks and felsic mineral composition composed mostly of low-temperature minerals including quartz, micas, and feldspars. Granitic rocks typically is found in association within continental land masses , particularly in the cores of mountain ranges and ancient shield regions. Two main types include granite and rhyolite.

Intrusive Rock. A volcano forms where magma erupts on the surface. Eruptions produce both tephra deposits and lava flows. Types of volcanoes are based on shape of their cones including fissure eruptions, shield cones, cinder cones, composite cones, and lava domes. Gas and lava venting at Pu'u'o'o Volcano on Hawaii. Hawaii volcanoes release some of the hottest lava on Earth. Explosive volcanic eruptions like this one in the Aleutian volcanic chain are associated with relatively cool and wet magmas.

Halemaumau Crater on top of Kilauea Volcano on Hawaii. The crater is one of the vents on the volcano, sometime filling with a lava lake, rising and sinking with different phases of the erupt. Crater Lake in Oregon is a small caldera about 2 miles in diameter. It formed from the eruption of Mt. Mazama about years ago. It is now hosts the deepest lake in North America. View of a crater on top of Mount Rainier , Washington. Glaciers are actively carving the margins of the crater on top of the volcano. Satellite view of Valles Caldera , New Mexico. The semicircular light-green colored area is the caldera on this massive volcano.

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Lava tube in Lavabeds NM, California. Igneous provinces of North America : plutonic and volcanic rocks and volcanic features are found throughout North America. All recent historic activity has been in the western US. Columbia River Basalts - a region covered by extensive lava flows from great fissure eruptions. California's volcanic areas.

Many volcanic areas are now within state and national parks. Major tephra falls map of the Western United States - large regions were blanketed with tephra by large volcanic eruptions. Volcanic disasters that have impacted the work in historic times Fig. Volcano hazards Fig. Worst Volcanic Disasters Fig. Tambora Volcano Fig. Krakatoa Volcano Fig. Pelee is a very active volcano in the Caribbean island chain. Santorini is what is left of the Thera Volcano , blown apart in BC. Santorini Thera Volcano destroyed the Minoan Empire. Vesuvius buried the cities of Pompeii and Herculaneum in 79 AD.

Dark areas on the Moon are great lava-filled basins that formed early in its history. With surface temperatures around degrees, the surface of Venus is a volcanic planet. Olympus Mons on Mars is the largest known volcano in the Solar System. Jupiter's moon Io is covered with volcanoes, many are currently active. Introduction to Geology. The word igneous is derived from the Latin word ignus , meaning fire or fiery. The term igneous applies to natural process relating to the formation, movement, and cooling of molten materials— magma hot, molten material underground , and lava molten material flowing on or near the surface Figure In geology, the word igneous applies to materials that have solidified from molten rock material, it also applies to the processes associated with the movement of molten material underground or erupting or the surface.

Igneous rocks are classified by their unique properties and characteristics which are related to composition of their host melt and the environmental setting where they form, underground or on the surface. Volcanoes form where molten material erupts on the surface. Regions where volcanoes occur, both modern active or dormant or ancient extinct have a unique variety of landforms.

There are at least active volcanoes around the world, of which about 25 are actively erupting, spewing out lava, rock, ash, and noxious gases. It is estimated that nearly million people around the world live within volcanic hazard zones regions that could be potentially impacted by catastrophic volcanic eruptions. When magma reaches the surface it becomes lava.

Origin of Igneous Rocks The Earth stores vast quantities of energy in the form of kinetic heat. Portions of the Earth's interior are molten, such as the outer liquid core. However, rocky materials in the overlying mantle are thought to be under too much pressure to melt. However, in many places, heat flow near the Earth's surface is high enough and confining pressure is low enough for rocks to melt.

This molten material may find zones of weakness, such as along fault zones, to inject, melt, or under great pressure, find means to migrate to the surface. The heat released by volcanoes and volcanic activity ultimately comes from heat convection from the Earth's core and mantle. Traces of that energy are released all over the surface of the planet, but that release of energy is not evenly distributed on the surface. The amount of energy released as geothermal energy is only a very tiny fraction compared to the solar energy the Earth receives from the Sun.

The Geothermal Gradient In most regions, the average temperature increases an average of 20 to 30 degrees C per kilometer with increasing depth in the upper crust. This temperature gradient called the geothermal gradient varies considerably with depth and location, depending on geologic settings. Regions actively experiencing plutonism and volcanism are locations where hot material may be located closer to the surface.

What causes rocks to melt? Rocks will melt generating magma if heat flow increases in an area to the point that minerals reach their melting points. Other factors that cause melting include the introduction of hot volatile fluids water and gases into rocks under pressure, or if there is a decrease in pressure confining hot rocks such as the release of pressure caused by a great earthquake.

Melting ranges from only partial melting of select low-temperature minerals to complete melting under higher temperature conditions. As heat flow increases, more materials will potentially melt. Heat convection from the mantle is the source of heat driving volcanic activity. This heat is remnant from its period of Earth's formation and the decay of radioactive elements. Intrusive igneous rocks form in naturally insulated settings rock is a poor conductor of heat so that minerals crystallize slowly, forming large, visible crystals.

Rocks formed from cooling lava on or very near the surface are called extrusive igneous rocks. When lava cools rapidly it crystallizes quickly, preventing visible crystals from forming. Extrusive rocks include lava flows and pyroclastic material such as volcanic ash, cinders, etc. Plutonism refers magma moving, cooling, and crystallizing underground. A pluton is a body of igneous rock formed underground.

It is usually medium- to coarse-grained with a granitic phaneritic texture. Volcanism is any of various processes and phenomena associated with the surface discharge of molten rock or hot water, steam, or gases. Volcanic rock is any rock formed by volcanism. A volcanic eruption occurs when molten material under pressure is expelled on the surface. Some may be discharged into the atmosphere or oceans producing a variety of rock fragments including large blocks , volcanic bombs blobs that may have a hard crust and a partially molten interior , cinders, and ash.

Molten material may flow on the surface under the influence of gravity. A volcano is a pile of volcanic rock that forms around a vent. Volcanic activity can also produce hot springs , geysers erupting hot springs and fumaroles gas vents.

Plutonic rocks

Parts of a volcano illustrating select features. Why do volcanoes erupt? Magma migrates to the surface under extreme pressure created by the weight of the rock above it and also from the pressure of gases dissolved within it much like a warm can of a carbonated soda or beer. Magma of granitic composition can have as much at 5 percent water dissolved in it. The release and expansion of gases is the driving force in volcanic explosions and eruptions Figures and Where do volcanic gases come from? Some of it is new from the mantle such as at divergent zone and some of it is recycled from older crust sinking into a subduction zone.

Figure shows the composition of volcanic gases from different geologic settings. Water and carbon dioxide are the dominant gazes released by volcanoes. Because magma contains large amount of dissolved gases! When magma containing dissolved water and gases is released in a volcanic eruption it expands hundreds of times in volume creating ash-filled clouds.

Gases from volcanic eruptions Fig. Volcanic gases from different plate-tectonic settings. The table below illustrates the temperature of lava sampled from a variety of volcanoes around the world. The table compares temperature of selected lava samples with their rock composition, geologic setting, and types of volcanic eruptions they produce. Note that the hottest lava is found on Hawaii's Kilauea volcano. Note that the hot lava volcanoes tend to produce the more gentle, yet extensive eruptions.

In contrast, the cooler the lava the more explosive the eruptions tend to be. Temperature C sample estimate range. Temperature F sample estimate range. General Characteristics of Intrusive and Extrusive Igneous Rocks The factors the help produce the texture and appearance of igneous rocks include: 1 the amount of dissolved gases in magma, 2 the rate at which magma cools , and 3 the amount of silica SiO 2 present. Extrusive igneous rocks that form from rapidly cooling magma or lava near or on the surface, crystallize quickly, preventing visible crystals from forming.

Note: Both rocks shown below have the same mineral and chemical composition, but different texture due to the rate of cooling of the molten material. Figure Phaneritic texture is a term usually used to refer to igneous rock with a larger crystal grain size and texture. It means that the size of matrix grains in the rock are large enough to be distinguished with the unaided eye as opposed to aphanitic which is too small to see with the naked eye.

Rocks with phaneritic texture are usually intrusive igneous rocks. Magma underground cools and crystallizes very slowly allowing large crystals to form. Aphanitic texture applies to dense, homogeneous rock with constituents that are so fine grained that they cannot be seen by the naked eye. Rocks with aphanitic texture are usually extrusive igneous rocks. Magma or lava at or near the surface tends to cool very quickly, preventing large crystals from forming, so the rock typically has a very fine-grained texture. Intrusive igneous rock. This example is granite. E xtrusive igneous rock.

This example is rhyolite. General Classification of Igneous Rocks Figure illustrates a general classification of igneous rocks. Igneous rocks are named based on combinations of their: 1 mineral composition, 2 crystalline sizes, 3 general color , and 4 textural characteristics. However, this method is difficult for fine-grained rocks without a microscope Figure Igneous rocks with larger crystals are easiest to identify by mineral composition.

The terms felsic, intermediate , and mafic are general terms used to describe and classify fine-grained igneous rocks Figure ; also see discussion below.

A textural name often relates to how a rock forms. For example, pumice is a volcanic rock that has so many gas bubbles in it that it can float! Obsidian , a rock that is sometimes used in jewelry, has a glass-like texture with a conchoidal fracture. Classification of igneous rocks based on mineral composition.

This method is useful only if larger crystal sizes allows mineral identification. Color of volcanic rocks. Light colored volcanic rocks are described as felsic; dark colored are mafic, and color shades in the middle are considered intermediate. How do different types of igneous rocks form from one original supply of magma? Igneous rocks form as molten material cools and crystallizes into rock. As the molten material cools, chemical compounds in the melt crystallize into minerals at different temperatures, with high-temperature minerals crystallizing first.

These high-temperature minerals are denser than the molten material and tend to settle out in the bottom of a magma chamber pluton. As the melt continues to cool, the composition of the melt changes as more crystals form and settle out. Finally the melt completely cools with the composition of the rock enriched in low-temperature minerals. This process is called magmatic differentiation.

A simple comparison is what happens when seawater freezes. The ice that forms directly from seawater is nearly pure water in composition. As sea ice forms, the remaining seawater becomes enriched in dissolved salts, lowering the freezing temperature of the remaining seawater concentrated as brine. As seawater freezes and crystalline ice forms, and liquid brine concentrated salt water and air are trapped in tiny pore spaces within a matrix of pure ice crystals. With further cooling, solid salt crystals subsequently precipitate in pockets of brine within the ice.

The net volume of the ice, volume of brine, and chemical composition of the solid salts are temperature-dependent. Magmatic differentiation involves processes by which chemically different igneous rocks, such as basalt and granite , can form from the same initial magma Figure High-temperature minerals can crystallize and settle out, causing the remaining molten material to be concentrated with component that may later form rock enriched in low temperature minerals such as granite.

The last rocks to crystallize in a magmatic intrusion will be enriched in low temperature minerals quartz, mica, and potassium- and sodium- feldspars. Gases and fluids including water, carbon dioxide, nitrogen and other compounds are also dissolved in magma and will be concentrated in the remaining lava before being expelled as the last traces of magma cools into rock. Different rocks from from one magma by magmatic differentiation. High temperature vs. This was demonstrated by the work of a 19th century petrologist, Norman Bowen , who showed that as a silicate-rich melt cools, minerals that form at higher temperatures will crystallize first.

As these minerals crystallize, the chemistry of the remaining melt will change as it cools, allowing different minerals to form as the melt proceeds cooling. High-temperature minerals like olivine and Ca-rich feldspar cool first, minerals like quartz, K-rich feldspar, and biotite crystallize last Figure In addition, fluids, such as gases and water, are concentrated in the remnants of a melt. This pattern of mineral and rock formation is called the Bowen's Reaction Series Figures to For example, in Bowen's Reaction Series, the first mineral that would crystallize from a high-temperature melt would be olivine.

Conversely, quartz would be the last mineral to crystallize, and therefore granite is the last rock type to form as a melt cools and crystallizes if all the silica SiO 2 in a melt is not consumed as other high-temperature minerals form first. What is the significance of the composition of igneous rocks?

Felsic is a term used to describe molten material magma , minerals, and rocks which are enriched in the elements such as silicon, oxygen, aluminum, sodium, and potassium. Felsic minerals produce felsic rocks. Common felsic minerals include quartz, muscovite, and feldspars. Granite and rhyolite are common felsic rocks.

Mafic is an adjective describing molten material magma , minerals, or rocks that are enriched in magnesium and iron;. Common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt and gabbro. Mafic rocks rich in iron and magnesium are generally denser and darker-colored than felsic rocks rich in silica and aluminum. The term ultramafic is applied to rock composed chiefly of mafic minerals rich in iron and magnesium, and less than about 45 percent silica, such minerals as olivine, augite, or hypersthene. Peridotite, pyroxenite and serpentinite are rocks with ultramafic rocks.

The mantle is ultramafic in composition and has a density of about 3. What is perhaps most important about mafic vs. The difference in density has an impact on isostasy of crust floating on the semi-fluid upper mantle asthenosphere , with continental crust about 2. How do different kinds of igneous rocks form, and where do they occur? Igneous rocks are subdivided into three different categories based on mineral composition: felsic , intermediate , mafic , and ultramafic.

These three categories are then further subdivided into dominant rock types based one crystal size and their origin. Intrusive Rock Fig. Granite —a common, coarse-grained crystalline , light-colored, hard plutonic intrusive igneous rock consisting chiefly of quartz, orthoclase or microcline feldspars , and mica. Granite is found in plutonic rocks that have been exposed by erosion.

In North America, granite is abundant in the core of mountain ranges exposed throughout the Rocky Mountain region and the Canadian Shield. Rhyolite —a pale fine-grained volcanic extrusive igneous rock of granitic composition. Rhyolite is common in continental volcanic regions with notable deposits around Yellowstone and volcanic centers throughout the Great Basin region extending from Nevada to New Mexico.

Extrusive Rock Fig. Rhyolite a fine-grained felsic rock is exposed in the Grand Canyon of the Yellowstone, Wyoming. Intermediate Rocks Intermediate refers to rocks that are in-between felsic and mafic in mineral composition. Intermediate rocks tend to be pale to medium gray in color and subdivided into two groups, dacitic rocks and andesitic rocks. Dacitic rocks are most commonly found in volcanic arc regions along continental margins, such as in the eastern Cascades volcanic region of Washington. Oregon and northern California. Andesitic rocks are slightly more enriched in mafic minerals, giving them a darker gray appearance.

Introduction to Igneous Rocks

Andesitic rocks are associated with oceanic island arc volcanoes associated with subduction zones, such as those found in Japan or the Aleutian Islands volcanic chain. Dacitic Rocks: Granodiorite —a coarse-grained crystalline plutonic igneous containing quartz and plagioclase, intermediate between granite and diorite in composition. Dacite —an extrusive igneous volcanic rock with an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. Lassen Volcano in northern California and many volcanoes in the Cascade Range are dacitic in composition.

Chaos Crags on Lassen Volcano in northern California consists mostly of dacite. Origin of diorite a plutonic rock. If magma of the same composition erupts on the surface it will form andesite. Andesitic Rocks: Diorite —a crystalline intrusive igneous rock intermediate in composition between granite and gabbro, consisting essentially of plagioclase and hornblende or other mafic minerals; having a "salt and pepper"-like appearance.

Diorite is found in mountain ranges throughout the Pacific Northwest Andesite —A fine-grained, brown or grayish volcanic rock that is intermediate in composition between rhyolite and basalt, dominantly composed of plagioclase feldspar. Andesite is the most abundant rock found in the volcanic rocks of the Cascade Range extending from northern California into British Columbia.

Andesite is the common rock found in volcanic arc island chains throughout the Pacific Ring of Fire. Mount St. Helens has an andesite composition. This image shows the massive eruption of May 18, The volcano continues to erupt intermittently. Mafic Rocks The most abundant volcanic rocks are basalt or basaltic rocks.

Basaltic rocks form from very hot molten material and are very enriched in high-temperature mafic minerals. Basaltic rocks are found in association with rift zones, hot spots like Hawaii , spreading centers like exposed on Iceland , and are the dominant rock exposed along mid-ocean ridges and makes up the basement bedrock on the seafloor. Gabbro cliffs in the Black Canyon of the Gunnison, Colorado. Gabbro —dark-colored, crystalline intrusive igneous rock composed principally of calcic-plagioclase minerals labradorite or bytonite and augite, and with or without olivine and orthopyroxene.

It is the approximate intrusive equivalent of basalt. Gabbro is associated with silvers terranes of ancient oceanic crustal rocks that are preserved in within continental rocks, locally in California and elsewhere. Basalt — A dark-colored igneous rock, commonly extrusive from volcanic eruptions and composed primarily of the minerals of calcic plagioclase and pyroxene, and sometimes olivine.

Basalt is the fine-grained equivalent of gabbro.

Basalt is associated with areas associated with crustal extension, such as in the Great Basin Region. All volcanic rocks on Hawaii are basalt in composition. Basalt lava flows in Hawaii Volcanoes National Park. Basalt on Hawaiian volcanoes can be mafic to ultramafic in composition. Ultramafic Rocks Ultramafic rocks are highly enriched in magnesium and iron. Where they occur they are mostly intrusive igneous rocks associated with magma derived from the mantle.

Rocks of ultramafic composition are thought to be very similar to rocks found in the Earth's upper mantle. Material of ultramafic composition is carried to the surface in some magmas of deep origin. Peridotite —a dense, coarse-grained plutonic rock containing a large amount of olivine, considered to be the main constituent of the earth's mantle.

Pyroxenite —a dark gray or greenish, granular intrusive igneous rock consisting chiefly of pyroxenes and olivine; a dominant rock type found in intrusive igneous rocks associated with oceanic crust. Peridotite from Hawaii. The green mineral is olivine gem mineral is called peridote. The black mineral is pyroxene. Volcanic rocks with unusual textures and special names Materials ejected from volcanic eruptions have some unique characteristics.

Some are of interest to the gem community, not that they are gems or considered precious, but they can be cut or shaped into interesting variety of uses for jewelry and art. Pahoehoe has a ropey fluid texture formed when hot basaltic lava cools quickly. A'a is lava rock with a rough, blocky surface when a lava flow continues to move slowly as it cools, and congealed rock breaks into rough pieces. People who walk on it barefoot frequently yell "Ah! Vesicular lava rock is any igneous rock that has gas bubbles trapped in a fine-grained volcanic rock.

Scoria is volcanic rock with a light, frothy consistency due to the high volume of gas bubbles trapped in the rock as it cools as lava is ejected from a volcano Figure If the rock is so frothy from trapped gas inside that it will float it s called pumice. Huge mats of pumice have been observed floating on the ocean after massive volcanic eruptions. Tuff is a volcanic rock that contains an abundance of visible fragments of volcanic rock that have been crushed or welded together by the heat released during an explosive volcanic eruption Figure Flow-banded lava rock is a volcanic rock that has a layered appearance due to flowing or stretching like taffy candy that formed as the lava was still flowing as it cooled Figure Obsidian is a dark, glass-like volcanic rock formed by the rapid solidification of lava without crystallization natural glass Figure Obsidian breaks with a conchoidal fracture like glass.

Bubbles in volcanic rocks can fill with minerals, including gem minerals. Snowflake obsidian is very attractive when tumbled or polished. The snowflakes in the obsidian are crystals phenocrysts of feldspar. Pahoehoe lava has a ropey texture Hawaii. A'a lava has a rough, blocky texture Hawaii.

Flow-banded lava rock display lines where the partly molten lava stretched like pulling taffy. Obsidian natural glass is usually black but can occur in a variety of colors from Glass Mountain, CA. Xenolith —A rock fragment foreign to the igneous mass in which it occurs. Xenoliths are commonly composed of rock derived from the sides or roof of a magma chamber.

The rocks sink into the magma chamber but escape melting as the magma cools to stone. Xenoliths stand out in appearance from the surrounding magma that cooled into stone around them. One of the basic geologic principles, the Law of Inclusions , dictates that the inclusion xenolith is older than the intrusion itself.

Dark xenolith in granite in Joshua Tree National Park. Xenoliths tend to survive sinking into a pluton magma chamber if it is composed mostly of high-temperature mafic minerals. Porphyry —a hard igneous rock containing visible crystals phenocrysts , usually of feldspar, in a fine-grained microcrystalline mineral matrix—typically a dark gray, reddish, or purplish ground mass.

Phenocryst —a large or conspicuous crystal in a porphyritic volcanic or igneous rock, distinct from a more fine-grained ground mass mineral matrix. Phenocrysts crystals form in magma at depth before it reaches the surface where the magma or lava cools quickly for form the fine-grained matrix. Feldspar phenocrysts in andesite porphyry Pink feldspar K-spar phenocrysts in andesite porphyry. Pegmatite —a coarsely crystalline granite or other igneous rock with crystals several centimeters in length, and sometimes containing rare minerals rich in rare elements such as uranium, tungsten, beryllium and tantalum.

Fluids water, CO2, etc.