• 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.
    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 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, slow-moving 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.
    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 
    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 SiO2. These differences in chemical composition are directly related to differences in gas content, temperature, and viscosity
    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, between 1000o and 2000o Celsius (1832o and 3632o 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 ranges from 800o to 1000o Celsius (1472o to 1832o 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.  
    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 650o and 800o Celsius (1202o and 1472o Fahrenheit). 
    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. 
    Magma's Role in the Rock Cycle
    Magma exists as pockets and plumes beneath the surface of the Earth.

    Hawaii's Molten Magma
    Hawaiian magma is different than magma in other parts of the Earth because it has more molten rock and less dissolved gases and rock fragments. The Hawaiian volcano Kilauea, on the "Big Island" of Hawaii, has erupted with enough molten lava to pave a road around the Earth three times.

    Tapping into Magma’s Potential 
    In 2009, the Icelandic Deep Drilling Project created a well that uses magma to generate geothermal energy. Normally, geothermal energy is created by pumping water into hot volcanic bedrock, creating steam that is then harnessed to generate electricity. While normal geothermal sources average around 60° to 80° Celsius (140° to 176° Fahrenheit), the magma well’s steam reached a record-breaking temperature of 450° Celsius (842° Fahrenheit)! This huge increase in temperature allowed just one magma well to generate roughly 36 megawatts of electricity, powering 36,000 homes. In comparison, one single wind turbine generates between 1 to 3 megawatts. 
    Mysterious Magma
    Magma is usually studied as lava or igneous rock. But most magma remains molten or partly molten beneath Earth’s surface. In fact, magma has only been discovered three times in its “natural” habitat deep in Earth’s crust. Geothermal drilling projects discovered two magma sites on the “Big Island” of Hawaii, and one on Iceland.
  • Term Part of Speech Definition Encyclopedic Entry
    abundant Adjective

    in large amounts.

    andesite Noun

    dark-colored volcanic rock.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    basalt Noun

    type of dark volcanic rock.

    buoyant Adjective

    capable of floating.

    caldera Noun

    large depression resulting from the collapse of the center of a volcano.

    Encyclopedic Entry: Types of Calderas
    confine Noun

    boundary or limit.

    continent Noun

    one of the seven main land masses on Earth.

    Encyclopedic Entry: continent
    convection Noun

    transfer of heat by the movement of the heated parts of a liquid or gas.

    convergent plate boundary Noun

    area where two or more tectonic plates bump into each other. Also called a collision zone.

    core Noun

    the extremely hot center of Earth, another planet, or a star.

    Encyclopedic Entry: core
    crucial Adjective

    very important.

    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    crystal Noun

    type of mineral that is clear and, when viewed under a microscope, has a repeating pattern of atoms and molecules.

    dacite Noun

    fine-grained volcanic rock.

    decompression melting Noun


    upward movement of Earth's mantle to an area of lower pressure, allowing mantle rock to melt, leading to magma formation.

    dense Adjective

    having parts or molecules that are packed closely together.

    destructive Adjective


    dike Noun

    a barrier, usually a natural or artificial wall used to regulate water levels.

    Encyclopedic Entry: dike
    dissolve Verb

    to break up or disintegrate.

    divergent boundary Noun

    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 Adjective

    always changing or in motion.

    eject Verb

    to get rid of or throw out.

    element Noun

    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.

    enormous Adjective

    very large.

    environment Noun

    conditions that surround and influence an organism or community.

    eruption Noun

    release of material from an opening in the Earth's crust.

    explosion Noun

    violent outburst; rejection, usually of gases or fuel

    extrude Verb

    to force or push out.

    felsic Adjective

    having to do with igneous rocks that contain mostly feldspars and quartz.

    fissure Noun

    narrow opening or crack.

    fluid Noun

    material that is able to flow and change shape.

    flux melting Noun

    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 Noun

    state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

    geology Noun

    study of the physical history of the Earth, its composition, its structure, and the processes that form and change it.

    hot spot Noun

    intensely hot region deep within the Earth that rises to just underneath the surface. Some hot spots produce volcanoes.

    Encyclopedic Entry: Hot Spot Volcanism
    igneous rock Noun

    rock formed by the cooling of magma or lava.

    incorporate Verb

    to blend or bring together.

    intrude Verb

    to thrust or bring into.

    landform Noun

    specific natural feature on the Earth's surface.

    Encyclopedic Entry: landform
    lava Noun

    molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.

    lava fountain Noun

    phenomenon where lava is forcefully but not violently ejected from a volcano through a fissure or vent.

    liquid Noun

    state of matter with no fixed shape and molecules that remain loosely bound with each other.

    mafic Adjective

    having to do with igneous rocks that contain large amounts of iron and magnesium.

    magma Noun

    molten, or partially melted, rock beneath the Earth's surface.

    Encyclopedic Entry: Magma's Role in the Rock Cycle
    magma chamber Noun

    underground reservoir that holds molten rock.

    mantle Noun

    middle layer of the Earth, made of mostly solid rock.

    Encyclopedic Entry: mantle
    mean Noun

    mathematical value between the two extremes of a set of numbers. Also called the average.

    melt Noun

    liquid part of magma.

    melting point Noun

    temperature at which a solid turns to liquid.

    mineral Noun

    inorganic material that has a characteristic chemical composition and specific crystal structure.

    molten Adjective

    solid material turned to liquid by heat.

    morphology Noun

    study of the form and structure of organisms or materials.

    originate Verb

    to begin or start.

    pluton Noun

    igneous rock that has solidified beneath the Earth's surface.

    pressure Noun

    force pressed on an object by another object or condition, such as gravity.

    pumice Noun

    type of igneous rock with many pores.

    reduce Verb

    to lower or lessen.

    rhyolite Noun

    fine-grained igneous rock containing large amounts of silica.

    rift Noun

    break in the Earth's crust created by it spreading or splitting apart.

    rock Noun

    natural substance composed of solid mineral matter.

    seafloor Noun

    surface layer of the bottom of the ocean.

    sedimentary rock Noun

    rock formed from fragments of other rocks or the remains of plants or animals.

    silica Noun

    chemical compound (SiO2) that makes up most of the Earth's rocks.

    stratovolcano Noun

    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 Noun

    area where one tectonic plate slides under another.

    tectonic plate Noun

    massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.

    temperature Noun

    degree of hotness or coldness measured by a thermometer with a numerical scale.

    Encyclopedic Entry: temperature
    tephra Noun

    solid material ejected from a volcano during an eruption.

    transform Verb

    to change from one form into another.

    vent Noun

    crack in the Earth's crust that spews hot gases and mineral-rich water.

    viscosity Noun

    measure of the resistance of a fluid to a force or disturbance.

    viscous Adjective

    liquid that is thick and sticky.

    volcanic arc Noun

    chain of volcanoes formed at a subduction zone.

    volcanic ash Noun

    fragments of lava less than 2 millimeters across.

    Encyclopedic Entry: Human and Environmental Impacts of Volcanic Ash
    volcanic glass Noun

    hard, brittle substance produced by lava cooling very quickly.

    volcanic island Noun

    land formed by a volcano rising from the ocean floor.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: Plate Tectonics and Volcanic Activity
    xenolith Noun

    piece of rock embedded in another type of rock, usually igneous.

    Encyclopedic Entry: xenolith