Volcanic eruptions create what kind of rocks

Huge pumice blocks have been seen floating on the ocean after large eruptions. Some lava blocks are large enough to carry small animals. Pumice is ground up and used today in soaps, abrasive cleansers, and also in polishes. Rhyolite is very closely related to granite. The difference is rhyolite has much finer crystals. These crystals are so small that they can not be seen by the naked eye. Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite giving it a glassy appearance.

The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende. Gabbros are dark-colored, coarse-grained intrusive igneous rocks. They are very similar to basalts in their mineral composition. They are composed mostly of the mineral plagioclase feldspar with smaller amounts of pyroxene and olivine.

Obsidian is a very shiny natural volcanic glass. When obsidian breaks it fractures with a distinct conchoidal fracture. Notice in the photo to the left how it fractures. Obsidian is produced when lava cools very quickly. The lava cools so quickly that no crystals can form.

When people make glass they melt silica rocks like sand and quartz then cool it rapidly by placing it in water. Obsidian is produced in nature in a similar way. Obsidian is usually black or a very dark green, but it can also be found in an almost clear form.

Ancient people throughout the world have used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds.

Today obsidian is used as a scalpel by doctors in very sensitive eye operations. Intrusive rocks are formed from magma that cools and solidifies within the crust of the planet.

When lava comes out of a volcano and solidifies into extrusive igneous rock, also called volcanic, the rock cools very quickly. Crystals inside solid volcanic rocks are small because they do not have much time to form until the rock cools all the way, which stops the crystal growth.

If lava cools almost instantly, the rocks that form are glassy with no individual crystals, like obsidian. There are many other kinds of extrusive igneous rocks. Intrusive rocks, also called plutonic rocks, cool slowly without ever reaching the surface.

They have large crystals that are usually visible without a microscope. This surface is known as a phaneritic texture. Perhaps the best-known phaneritic rock is granite. One extreme type of phaneritic rock is called pegmatite , found often in the U. Pegmatite can have a huge variety of crystal shapes and sizes, including some larger than a human hand. Rock texture with crystals that are invisible without magnification.

Common minerals include quartz, feldspar, mica, amphibole, olivine, and calcite. A rock is an aggregate of one or more minerals, or a body of undifferentiated mineral What are metamorphic rocks?

Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous , sedimentary , or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors.

Conditions like these are Filter Total Items: 2. View Citation. Date published: April 4, Date published: September 29, Filter Total Items: List Grid. May 25, July 20, The project is funded by the USGS Mineral Resources Program that is focused on investigating the bedrock geology of the national park and surrounding areas through geologic mapping and supporting analytical work such as geochemistry and March 23, November 24, October 18, July 7, July 4, April 22, 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.

What is the process by which decompression melting produces magma at divergent plate boundaries? Decompression melting takes place when pressure is reduced on rising asthenospheric material which remains at the same temperature at divergent boundaries. What does a P-T diagram of the mantle show? A P-T diagram plots temperature against pressure, both of which increase with depth. If volatiles such as water vapor and carbon dioxide are added to a rock, what will happen to the melting temperature?

Volatiles added to hot rock act as a flux reducing the melting point and causing the rock to melt at that same temperature. Even though all magmas originate from similar mantle rocks, and start out as similar magma , other things, like partial melting and crystallization processes like magmatic differentiation , can change the chemistry of the magma.

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. Fractional crystallization by crystals settling out under gravity is one way that magmas can change in composition while cooling. As magma travels up from the asthenosphere through the lithosphere into continental crust , how will fractional crystallization change the chemistry of an ultramafic magma?

Referring to the Bowen diagram and fractional crystallization , the ultramafic magma will become more mafic. Xenoliths are bits of country rock that are incorporated within a mass of igneous rock.

Partial melting produces a magma a step lower on the Bowen diagram than the rock from which it melts. Assimilation is the process by which rising magma incorporates country rock and composition changes. 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. This section describes volcano location, type, hazards, and monitoring. Most volcanoes are interplate volcanoes. 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.

Rifts can be narrow focused in one place or broad spread out over a large area with many faults. 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.