Text on this page is printable and can be used according to our Terms of Service. Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. The rock cycle is a web of processes that outlines how each of the three major rock types—igneous, metamorphic, and sedimentary—form and break down based on the different applications of heat and pressure over time.
For example, sedimentary rock shale becomes slate when heat and pressure are added. The more heat and pressure you add, the further the rock metamorphoses until it becomes gneiss. If it is heated further, the rock will melt completely and reform as an igneous rock.
Empower your students to learn about the rock cycle with this collection of resources. According to the United States Geologic Survey, there are approximately 1, potentially active volcanoes worldwide. Most are located around the Pacific Ocean in what is commonly called the Ring of Fire. A volcano is defined as an opening in the Earth's crust through which lava, ash, and gases erupt. The term also includes the cone-shaped landform built by repeated eruptions over time.
Teach your students about volcanoes with this collection of engaging material. The structure of the earth is divided into four major components: the crust, the mantle, the outer core, and the inner core. Each layer has a unique chemical composition, physical state, and can impact life on Earth's surface. Movement in the mantle caused by variations in heat from the core, cause the plates to shift, which can cause earthquakes and volcanic eruptions.
These natural hazards then change our landscape, and in some cases, threaten lives and property. Learn more about how the earth is constructed with these classroom resources.
Igneous rocks are one of three main types of rocks along with sedimentary and metamorphic , and they include both intrusive and extrusive rocks. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. Article Vocabulary. Friday, October 31, Magma is a molten and semi-molten rock mixture found under the surface of the Earth.
This mixture is usually made up of four parts: a hot liquid base, called the melt ; mineral s crystal lized by the melt; solid rock s incorporate d into the melt from the surrounding confine s; and dissolve d gas es. When magma is eject ed by a volcano or other vent , the material is called lava. Magma that has cooled into a solid is called igneous rock.
This heat makes magma a very fluid and dynamic substance, able to create new landform s and engage physical and chemical transform ations in a variety of different environment s. Earth is divided into three general layers. The core is the superheated center, the mantle is the thick, middle layer, and the crust is the top layer on which we live.
Most of the mantle and crust are solid, so the presence of magma is crucial to understanding the geology and morphology of the mantle. Differences in temperature , pressure , and structural formations in the mantle and crust cause magma to form in different ways.
Decompression melting involves the upward movement of Earth's mostly-solid mantle. This hot material rises to an area of lower pressure through the process of convection. Areas of lower pressure always have a lower melting point than areas of high pressure.
This reduction in overlying pressure, or decompression, enables the mantle rock to melt and form magma. Decompression melting often occurs at divergent boundaries, where tectonic plate s separate. The rift ing movement causes the buoyant magma below to rise and fill the space of lower pressure. The rock then cools into new crust. When located beneath the ocean, these plumes, also known as hot spot s, push magma onto the seafloor.
These volcanic mounds can grow into volcanic island s over millions of years of activity. As the liquid rock solidifies, it loses its heat to the surrounding crust. Transfer of heat often happens at convergent boundaries, where tectonic plates are crashing together. As the dense r tectonic plate subduct s, or sinks below, or the less-dense tectonic plate, hot rock from below can intrude into the cooler plate above.
This process transfers heat and creates magma. Under normal conditions, the geothermal gradient is not high enough to melt rocks, and thus with the exception of the outer core, most of the Earth is solid. Thus, magmas form only under special circumstances, and thus, volcanoes are only found on the Earth's surface in areas above where these special circumstances occur.
Volcanoes don't just occur anywhere, as we shall soon see. To understand this we must first look at how rocks and mineral melt. To understand this we must first look at how minerals and rocks melt.
As pressure increases in the Earth, the melting temperature changes as well. For pure minerals, there are two general cases. From the above we can conclude that in order to generate a magma in the solid part of the earth either the geothermal gradient must be raised in some way or the melting temperature of the rocks must be lowered in some way.
The geothermal gradient can be raised by upwelling of hot material from below either by uprise solid material decompression melting or by intrusion of magma heat transfer. Lowering the melting temperature can be achieved by adding water or Carbon Dioxide flux melting.
The Mantle is made of garnet peridotite a rock made up of olivine, pyroxene, and garnet -- evidence comes from pieces brought up by erupting volcanoes. In the laboratory we can determine the melting behavior of garnet peridotite.
Decompression Melting - Under normal conditions the temperature in the Earth, shown by the geothermal gradient, is lower than the beginning of melting of the mantle. Thus in order for the mantle to melt there has to be a mechanism to raise the geothermal gradient. Once such mechanism is convection, wherein hot mantle material rises to lower pressure or depth, carrying its heat with it. If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure, then a partial melt will form.
Liquid from this partial melt can be separated from the remaining crystals because, in general, liquids have a lower density than solids. Basaltic magmas appear to originate in this way. Upwelling mantle appears to occur beneath oceanic ridges, at hot spots, and beneath continental rift valleys. Thus, generation of magma in these three environments is likely caused by decompression melting.
Transfer of Heat - When magmas that were generated by some other mechanism intrude into cold crust, they bring with them heat. Upon solidification they lose this heat and transfer it to the surrounding crust. Repeated intrusions can transfer enough heat to increase the local geothermal gradient and cause melting of the surrounding rock to generate new magmas. Rhyolitic magma can also be produced by changing the chemical composition of basaltic magma as discussed later.
Transfer of heat by this mechanism may be responsible for generating some magmas in continental rift valleys, hot spots, and subduction related environments. Flux Melting - As we saw above, if water or carbon dioxide are added to rock, the melting temperature is lowered.
If the addition of water or carbon dioxide takes place deep in the earth where the temperature is already high, the lowering of melting temperature could cause the rock to partially melt to generate magma. One place where water could be introduced is at subduction zones. Here, water present in the pore spaces of the subducting sea floor or water present in minerals like hornblende, biotite, or clay minerals would be released by the rising temperature and then move in to the overlying mantle.
Introduction of this water in the mantle would then lower the melting temperature of the mantle to generate partial melts, which could then separate from the solid mantle and rise toward the surface. Chemical Composition of Magmas.
The chemical composition of magma can vary depending on the rock that initially melts the source rock , and process that occur during partial melting and transport. The initial composition of the magma is dictated by the composition of the source rock and the degree of partial melting. Melting of crustal sources yields more siliceous magmas.
In general more siliceous magmas form by low degrees of partial melting. As the degree of partial melting increases, less siliceous compositions can be generated. So, melting a mafic source thus yields a felsic or intermediate magma. Melting of ultramafic peridotite source yields a basaltic magma. But, processes that operate during transportation toward the surface or during storage in the crust can alter the chemical composition of the magma.
These processes are referred to as magmatic differentiation and include assimilation, mixing, and fractional crystallization. Now let's imagine I remove 1 MgO molecule by putting it into a crystal and removing the crystal from the magma.
Now what are the percentages of each molecule in the liquid? If we continue the process one more time by removing one more MgO molecule. Thus, composition of liquid can be changed. This process is called crystal fractionation. A mechanism by which a basaltic magma beneath a volcano could change to andesitic magma and eventually to rhyolitic magma through crystal fractionation, is provided by Bowen's reaction series, discussed next.
Bowen's Reaction Series Bowen found by experiment that the order in which minerals crystallize from a basaltic magma depends on temperature. As a basaltic magma is cooled Olivine and Ca-rich plagioclase crystallize first. Upon further cooling, Olivine reacts with the liquid to produce pyroxene and Ca-rich plagioclase react with the liquid to produce less Ca-rich plagioclase.
But, if the olivine and Ca-rich plagioclase are removed from the liquid by crystal fractionation, then the remaining liquid will be more SiO 2 rich. If the process continues, an original basaltic magma can change to first an andesite magma then a rhyolite magma with falling temperature.
In general, magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rocks. As they rise they may encounter a depth or pressure where the dissolved gas no longer can be held in solution in the magma, and the gas begins to form a separate phase i.
When a gas bubble forms, it will also continue to grow in size as pressure is reduced and more of the gas comes out of solution. In other words, the gas bubbles begin to expand. If the liquid part of the magma has a low viscosity, then the gas can expand relatively easily.
When the magma reaches the Earth's surface, the gas bubble will simply burst, the gas will easily expand to atmospheric pressure, and a non-explosive eruption will occur, usually as a lava flow Lava is the name we give to a magma when it on the surface of the Earth.
If the liquid part of the magma has a high viscosity, then the gas will not be able to expand very easily, and thus, pressure will build up inside of the gas bubble s. When this magma reaches the surface, the gas bubbles will have a high pressure inside, which will cause them to burst explosively on reaching atmospheric pressure. This will cause an explosive volcanic eruption.
Effusive Non-explosive Eruptions. Non explosive eruptions are favored by low gas content and low viscosity magmas basaltic to andesitic magmas.
If the viscosity is low, non-explosive eruptions usually begin with fire fountains due to release of dissolved gases. When magma reaches the surface of the earth, it is called lava. Since it its a liquid, it flows downhill in response to gravity as a lava flows. Different magma types behave differently as lava flows, depending on their temperature, viscosity, and gas content. Pahoehoe Flows - Basaltic lava flows with low viscosity start to cool when exposed to the low temperature of the atmosphere.
This causes a surface skin to form, although it is still very hot and behaves in a plastic fashion, capable of deformation. Such lava flows that initially have a smooth surface are called pahoehoe flows. Initially the surface skin is smooth, but often inflates with molten lava and expands to form pahoehoe toes or rolls to form ropey pahoehoe.
See figure 6. Pahoehoe flows tend to be thin and, because of their low viscosity travel long distances from the vent. A'A' Flows - Higher viscosity basaltic and andesitic lavas also initially develop a smooth surface skin, but this is quickly broken up by flow of the molten lava within and by gases that continue to escape from the lava.
This creates a rough, clinkery surface that is characteristic of an A'A' flow see figure 6. Pillow Lavas - When lava erupts on the sea floor or other body of water, the surface skin forms rapidly, and, like with pahoehoe toes inflates with molten lava.
Eventually these inflated balloons of magma drop off and stack up like a pile of pillows and are called pillow lavas. Ancient pillow lavas are readily recognizable because of their shape, their glassy margins and radial fractures that formed during cooling.
Although solid, the hotter mantle material will rise because of its greater heat. Convection is the process by which heated material rises and cooler material sinks. We see convection every day; for example, the currents that swirl in a pot of liquid on a stove or the warm air that rises over the surface of sun-heated lava.
Solid materials also convect, although at considerably slower rates-perhaps only a few centimeters per year in the case of the mantle. Within the Earth, heated blobs of mobile yet solid mantle rise within a solid cooler mantle. Though heat is being transferred by these rising blobs, no magma is created because nothing has melted.
Individual blobs probably don't traverse the entire mantle. As each one stalls, its heat is transferred to adjacent rock, provoking continued convection. It's a "Pony Express," in which the horses will traverse only one part of the mantle, yet the message, the heat, continues through. Heated rocks remain solid at the great pressures deep in the mantle.
Perhaps about km mi deep, however, the pressure is sufficiently low that melting can take place. Beads of magma sweat from the rocks and rise. Our convection system now comprises liquid rising within a solid mesh. The beads coalesce to create a braided stream that penetrates the crust of the Pacific plate.
Ultimately the magma accumulates in complex reservoirs lying km mi beneath each active volcano. In summary, a hot spot originates at relatively great depth, the magma at relatively shallow depth.
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