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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.
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. Lava Domes or Volcanic Domes - result from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava.
Since the viscosity is so high, the lava does not flow away from the vent, but instead piles up over the vent. Blocks of nearly solid lava break off the outer surface of the dome and roll down its flanks to form a breccia around the margins of domes.
The surface of volcanic domes are generally very rough, with numerous spines that have been pushed up by the magma from below. Explosive eruptions are favored by high gas content and high viscosity andesitic to rhyolitic magmas.
Explosive bursting of bubbles will fragment the magma into clots of liquid that will cool as they fall through the air. These solid particles become pyroclasts meaning - hot fragments and tephra or volcanic ash, which refer to sand- sized or smaller fragments. If the gas pressure inside the magma is directed outward instead of upward, a lateral blast can occur.
Directed blasts often result from sudden exposure of the magma by a landslide or collapse of a lava dome. Pyroclastic Deposits.
Pyroclastic material ejected explosively from volcanoes becomes deposited on the land surface. The process of deposition leaves clues that allow geologists to interpret the mode of ejection from the volcano. Pyroclastic flows are also sometimes called pyroclastic density currents PDCs. They can range from surges which can have a range of clast densities from low to high with generally low concentration of of solid clasts high amonts of gases to high clast concentration clouds of ash and gas pyroclastic flows.
As defined above, block and ash flows consist of an unsorted mixture of blocks and ash with the blocks being mostly rock fragments. Surges tend to hug the ground as they flow over the surface and thus tend to produce thicker deposits in valleys with thinner deposits over ridges. This helps to distinguish surge deposits from flow deposits and fall deposits.
Volcanic eruptions, especially explosive ones, are very dynamic phenomena. That is the behavior of the eruption is continually changing throughout the course of the eruption. This makes it very difficult to classify volcanic eruptions. Nevertheless they can be classified according to the principal types of behavior that they exhibit.
An important point to remember, however, is that during a given eruption the type of eruption may change between several different types. Hawaiian - These are eruptions of low viscosity basaltic magma. Gas discharge produces a fire fountain that shoots incandescent lava up to 1 km above the vent. The lava, still molten when it returns to the surface flows away down slope as a lava flow. Hawaiian Eruptions are considered non-explosive eruptions.
Very little pyroclastic material is produced. Teach your students about volcanoes with this collection of engaging material. Magma is a mixture of molten and semi-molten rock found beneath the surface of the Earth. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Image lava flow Lava magma that has erupted onto the Earth's surface is visually mesmerizing — as the molten rock flows downhill, lava exposed to the air cools to a deep black color, while the molten rock beneath glows bright orange.
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Interactives Any interactives on this page can only be played while you are visiting our website. Related Resources. View Collection. Magma's Role in the Rock Cycle. View Article. Magma on Hawaiian Coast.
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