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.
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.
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. 

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.

abundant
Adjective

in large amounts.

andesite
Noun

dark-colored volcanic rock.

Noun

layers of gases surrounding a planet or other celestial body.

basalt
Noun

type of dark volcanic rock.

buoyant
Adjective

capable of floating.

Noun

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

confine
Noun

boundary or limit.

Noun

one of the seven main land masses on Earth.

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.

Noun

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

crucial
Adjective

very important.

Noun

rocky outermost layer of Earth or other planet.

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

harmful.

Noun

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

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.

Noun

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

Noun

rock formed by the cooling of magma or lava.

incorporate
Verb

to blend or bring together.

intrude
Verb

to thrust or bring into.

Noun

specific natural feature on the Earth's surface.

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.

Noun

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

magma chamber
Noun

underground reservoir that holds molten rock.

Noun

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

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.

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.

Noun

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

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.

Noun

fragments of lava less than 2 millimeters across.

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.

Noun

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

Noun

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