Your web browser (Safari 7) is out of date. For more security, comfort and the best experience on this site: Update your browser Ignore MAGMA For the complete encyclopedic entry with media resources, visit: http://www.nationalgeographic.org/encyclopedia/magma/ 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; minerals crystallized by the melt; solid rocks incorporated into the melt from the surrounding confines; and dissolved gases. When magma is ejected by a volcano or other vent, the material is called lava. Magma that has cooled into a solid is called igneous rock. Magma is extremely hot between 700 and 1,300 Celsius (1,292 and 2,372 Fahrenheit). This heat makes magma a very fluid and dynamic substance, able to create new landforms and engage physical and chemical transformations in a variety of different environments. How Magma Forms 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. Magma originates in the lower part of the Earth s crust and in the upper portion of the mantle. 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. 1 of 11
Decompression Melting 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 plates separate. The rifting movement causes the buoyant magma below to rise and fill the space of lower pressure. The rock then cools into new crust. Decompression melting also occurs at mantle plumes, columns of hot rock that rise from Earth s high-pressure core to its lower-pressure crust. When located beneath the ocean, these plumes, also known as hot spots, push magma onto the seafloor. These volcanic mounds can grow into volcanic islands over millions of years of activity. Transfer of Heat Magma can also be created when hot, liquid rock intrudes into Earth s cold crust. As the liquid rock solidifies, it loses its heat to the surrounding crust. Much like hot fudge being poured over cold ice cream, this transfer of heat is able to melt the surrounding rock (the ice cream ) into magma. Transfer of heat often happens at convergent boundaries, where tectonic plates are crashing together. As the denser tectonic plate subducts, 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. Over millions of years, the magma in this subduction zone can create a series of active volcanoes known as a volcanic arc. Flux Melting Flux melting occurs when water or carbon dioxide are added to rock. These compounds cause the rock to melt at lower temperatures. This creates magma 2 of 11
in places where it originally maintained a solid structure. Much like heat transfer, flux melting also occurs around subduction zones. In this case, water overlying the subducting seafloor would lower the melting temperature of the mantle, generating magma that rises to the surface. Magma Escape Routes Magma leaves the confines of the upper mantle and crust in two major ways: as an intrusion or as an extrusion. An intrusion can form features such as dikes and xenoliths. An extrusion could include lava and volcanic rock. Magma can intrude into a low-density area of another geologic formation, such as a sedimentary rock structure. When it cools to solid rock, this intrusion is often called a pluton. A pluton is an intrusion of magma that wells up from below the surface. Plutons can include dikes and xenoliths. A magmatic dike is simply a large slab of magmatic material that has intruded into another rock body. A xenolith is a piece of rock trapped in another type of rock. Many xenoliths are crystals torn from inside the Earth and embedded in magma while the magma was cooling. The most familiar way for magma to escape, or extrude, to Earth s surface is through lava. Lava eruptions can be fire fountains of liquid rock or thick, slowmoving rivers of molten material. Lava cools to form volcanic rock as well as volcanic glass. Magma can also extrude into Earth s atmosphere as part of a violent volcanic explosion. This magma solidifies in the air to form volcanic rock called tephra. In the atmosphere, tephra is more often called volcanic ash. As it falls to Earth, tephra includes rocks such as pumice. 3 of 11
Magma Chamber In areas where temperature, pressure, and structural formation allow, magma can collect in magma chambers. Most magma chambers sit far beneath the surface of the Earth. The pool of magma in a magma chamber is layered. The least-dense magma rises to the top. The densest magma sinks near the bottom of the chamber. Over millions of years, many magma chambers simply cool to form a pluton or large igneous intrusion. If a magma chamber encounters an enormous amount of pressure, however, it may fracture the rock around it. The cracks, called fissures or vents, are tell-tale signs of a volcano. Many volcanoes sit over magma chambers. As a volcano s magma chamber experiences greater pressure, often due to more magma seeping into the chamber, the volcano may undergo an eruption. An eruption reduces the pressure inside the magma chamber. As long as more magma pools into a volcano s magma chamber, there is the possibility of an eruption and the volcano will remain active. Large eruptions can nearly empty the magma chamber. The layers of magma may be documented by the type of eruption material the volcano emits. Gases, ash, and light-colored rock are emitted first, from the least-dense, top layer of the magma chamber. Dark, dense volcanic rock from the lower part of the magma chamber may be released later. In violent eruptions, the volume of magma shrinks so much that the entire magma chamber collapses and forms a caldera. Types of Magma 4 of 11
All magma contains gases and a mixture of simple elements. Being that oxygen and silicon are the most abundant elements in magma, geologists define magma types in terms of their silica content, expressed as SiO. These differences in chemical composition are directly related to differences in gas content, temperature, and viscosity. 2 Mafic Magma Mafic magma has relatively low silica content, roughly 50%, and higher contents in iron and magnesium. This type of magma has a low gas content and low viscosity, or resistance to flow. Mafic magma also has high mean temperatures, o o o o between 1000 and 2000 Celsius (1832 and 3632 Fahrenheit), which contributes to its lower viscosity. Low viscosity means that mafic magma is the most fluid of magma types. It erupts non-explosively and moves very quickly when it reaches Earth s surface as lava. This lava cools into basalt, a rock that is heavy and dark in color due to its higher iron and magnesium levels. Basalt is one of the most common rocks in Earth s crust as well as the volcanic islands created by hot spots. The Hawaiian Islands are a direct result of mafic magma eruptions. Steady and relatively calm lava fountains continue to change and expand the Big Island of Hawaii. Intermediate Magma Intermediate magma has higher silica content (roughly 60%) than mafic magma. This results in a higher gas content and viscosity. Its mean temperature o o o o ranges from 800 to 1000 Celsius (1472 to 1832 Fahrenheit). As a result of its higher viscosity and gas content, intermediate magma builds up pressure below the Earth s surface before it can be released as lava. This more gaseous and sticky lava tends to explode violently and cools as andesite rock. Intermediate magma most commonly transforms into andesite due to the transfer of heat at convergent plate boundaries. Andesitic rocks are often found at continental volcanic arcs, such as the Andes Mountains in South America, after which they are named. 5 of 11
Felsic Magma Felsic magma has the highest silica content of all magma types, between 65-70%. As a result, felsic magma also has the highest gas content and viscosity, and lowest mean temperatures, between 650 and 800 Celsius (1202 and o 1472 Fahrenheit). o o o Thick, viscous felsic magma can trap gas bubbles in a volcano s magma chamber. These trapped bubbles can cause explosive and destructive eruptions. These eruptions eject lava violently into the air, which cools into dacite and rhyolite rock. Much like intermediate magma, felsic magma may be most commonly found at convergent plate boundaries where transfer of heat and flux melting create large stratovolcanoes. Vocabulary Term abundant adjectivein large amounts. andesite dark-colored volcanic rock. atmosphere layers of gases surrounding a planet or other celestial body. basalt type of dark volcanic rock. buoyant caldera adjectivecapable of floating. large depression resulting from the collapse of the center of a volcano. confine boundary or limit. continent one of the seven main land masses on Earth. convection convergent plate boundary transfer of heat by the movement of the heated parts of a liquid or gas. area where two or more tectonic plates bump into each other. Also called a collision zone. 6 of 11
Term core the extremely hot center of Earth, another planet, or a star. crucial adjectivevery important. crust rocky outermost layer of Earth or other planet. crystal type of mineral that is clear and, when viewed under a microscope, has a repeating pattern of atoms and molecules. dacite fine-grained volcanic rock. decompression melting dense destructive dike upward movement of Earth's mantle to an area of lower pressure, allowing mantle rock to melt, leading to magma formation. having parts or molecules that are packed closely adjective together. adjectiveharmful. a barrier, usually a natural or artificial wall used to regulate water levels. dissolve verb to break up or disintegrate. divergent boundary area where two or more tectonic plates are moving away from each other. Also called an extensional boundary. document verb to keep track of. dynamic adjectivealways changing or in motion. eject verb to get rid of or throw out. element chemical that cannot be separated into simpler substances. embed verb to attach firmly to a surrounding substance. emit verb to give off or send out. encounter verb to meet, especially unexpectedly. engage verb to interact with. 7 of 11
Term enormous adjective very large. environment conditions that surround and influence an organism or community. eruption release of material from an opening in the Earth's crust. explosion violent outburst; rejection, usually of gases or fuel extrude verb to force or push out. felsic having to do with igneous rocks that contain mostly adjective feldspars and quartz. fissure narrow opening or crack. fluid material that is able to flow and change shape. flux melting process that occurs when water is added to hot, solid rock, lowering its melting point and allowing the creation of magma. fracture verb to break. gas geology hot spot state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion. study of the physical history of the Earth, its composition, its structure, and the processes that form and change it. intensely hot region deep within the Earth that rises to just underneath the surface. Some hot spots produce volcanoes. igneous rock rock formed by the cooling of magma or lava. incorporate verb to blend or bring together. intrude verb to thrust or bring into. landform specific natural feature on the Earth's surface. lava molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface. 8 of 11
lava fountain liquid mafic magma magma chamber phenomenon where lava is forcefully but not violently ejected from a volcano through a fissure or vent. state of matter with no fixed shape and molecules that remain loosely bound with each other. having to do with igneous rocks that contain large adjective amounts of iron and magnesium. molten, or partially melted, rock beneath the Earth's surface. underground reservoir that holds molten rock. mantle middle layer of the Earth, made of mostly solid rock. mean mathematical value between the two extremes of a set of numbers. Also called the average. melt liquid part of magma. melting point temperature at which a solid turns to liquid. mineral molten morphology inorganic material that has a characteristic chemical composition and specific crystal structure. adjectivesolid material turned to liquid by heat. study of the form and structure of organisms or materials. originate verb to begin or start. pluton Term pressure igneous rock that has solidified beneath the Earth's surface. force pressed on an object by another object or condition, such as gravity. pumice type of igneous rock with many pores. reduce verb to lower or lessen. rhyolite fine-grained igneous rock containing large amounts of silica. 9 of 11
Term rift break in the Earth's crust created by it spreading or splitting apart. rock natural substance composed of solid mineral matter. seafloor surface layer of the bottom of the ocean. sedimentary rock formed from fragments of other rocks or the rock remains of plants or animals. silica stratovolcano chemical compound (SiO ) that makes up most of the Earth's rocks. steep volcano made of hardened lava, rock, and ash. Also known as a composite volcano. subduct verb to pull downward or beneath something. subduction zone area where one tectonic plate slides under another. massive slab of solid rock made up of Earth's tectonic plate lithosphere (crust and upper mantle). Also called lithospheric plate. temperature degree of hotness or coldness measured by a thermometer with a numerical scale. tephra solid material ejected from a volcano during an eruption. transform verb to change from one form into another. vent crack in the Earth's crust that spews hot gases and mineral-rich water. viscosity measure of the resistance of a fluid to a force or disturbance. viscous adjectiveliquid that is thick and sticky. volcanic arc chain of volcanoes formed at a subduction zone. volcanic ash fragments of lava less than 2 millimeters across. 2 volcanic glass hard, brittle substance produced by lava cooling very quickly. 10 of 11
Term volcanic island land formed by a volcano rising from the ocean floor. volcano an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions. xenolith piece of rock embedded in another type of rock, usually igneous. Articles & Profiles National Geographic News: New Magma Layer Found Deep in Earth's Mantle? National Geographic News: Yellowstone Has Bulged as Magma Pocket Swells Tulane University: Volcanoes, Magma, and Volcanic Eruptions National Science Foundation: Magma in Earth's Mantle Forms Deeper Than Once Thought Websites San Diego State University: How Volcanoes Work 1996 2017 National Geographic Society. All rights reserved. 11 of 11