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. 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. What does a P-T diagram of the mantle show? A P-T diagram plots temperature against pressure, both of which increase with depth. 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. 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. Xenoliths are bits of country rock that are incorporated within a mass of igneous rock. 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. 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. Fractional crystallization by crystals settling out under gravity is one way that magmas can change in composition while cooling. 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.
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. The figure shows the main features of a typical stratovolcano : 1 magma chamber , 2 upper layers of lithosphere , 3 the conduit or narrow pipe through which the lava Liquid rock on the surface of the Earth. A parasitic cone is a small volcano located on the flank of a larger volcano such as Shastina on Mount Shasta.
Kilauea sitting on the flank of Mauna Loa is not considered a parasitic cone because it has its own separate magma chamber , 13 the vents of the parasite and the main volcano , 14 the rim of the crater, 15 clouds of ash Volcanic tephra that is less than 2 mm in diameter. The largest craters are called calderas , such as the Crater Lake Caldera in Oregon.
Many volcanic features are produced by viscosity , a basic property of a lava Liquid rock on the surface of the Earth.
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.
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