Ma gma's r ol e i n t h e r oc k c y c l e...Decompression Melting Decompression melting involves the upward movement of Earth's mostly-solid mantle. This hot material rises to an

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Magma's role in the rock cycle

Image 1. A lava breakout on Kilauea, Hawaii. Lava is magma that reaches Earth's surface through a volcano vent. Photo: Justinreznick viaGetty Images

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 degrees and 1,300 degrees Celsius (1,292 degrees and

2,372 degrees 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

By National Geographic on 10.31.19Word Count 1,518Level MAX


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 percent, 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 1000 degrees and 2000 degrees Celsius

(1832 degrees and 3632 degrees 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 percent) than mafic magma. This

results in a higher gas content and viscosity. Its mean temperature ranges from 800 degrees to

1000 degrees Celsius (1472 degrees to 1832 degrees 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 percent. As a

result, felsic magma also has the highest gas content and viscosity, and lowest mean temperatures,

between 650 degrees and 800 degrees Celsius (1202 degrees and 1472 degrees 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.


Quiz

1 Which of these statements would be MOST important to include in an objective summary of the article?

(A) Magma that has cooled into a solid is called igneous rock.

(B) Magma is extremely hot — between 700 degrees and 1,300 degrees Celsius (1,292 degrees and 2,372degrees Fahrenheit).

(C) Magma originates in the lower part of the Earth's crust and in the upper portion of the mantle.

(D) As the liquid rock solidifies, it loses its heat to the surrounding crust.

2 Which of the following details is MOST important to the development of the central idea?

(A) This mixture is usually made up of four parts: a hot liquid base, called the melt; minerals crystallized bythe melt; solid rocks incorporated into the melt from the surrounding confines; and dissolved gases.

(B) Most of the mantle and crust are solid, so the presence of magma is crucial to understanding thegeology and morphology of the mantle.

(C) Decompression melting also occurs at mantle plumes, columns of hot rock that rise from Earth's high-pressure core to its lower-pressure crust.

(D) n this case, water overlying the subducting seafloor would lower the melting temperature of the mantle,generating magma that rises to the surface.

3 Read the following selection from the section "Transfer Of Heat."

Much like hot fudge being poured over cold ice cream, this transfer of heat is able to melt thesurrounding rock (the "ice cream") into magma.

How does the analogy help the reader to understand how magma interacts with rock?

(A) It builds upon the scientific background knowledge of the reader.

(B) It provides a visual that builds upon the a shared experience.

(C) It explains the connection hot fudge and cold ice cream.

(D) It shares an anecdote about hot fudge that looks like magma.

4 Read the following sentence from the section "Mafic Magma."

This type of magma has a low gas content and low viscosity, or resistance to flow.

The author uses the word "content" to mean:

(A) everything that is in a container.

(B) the subjects covered in a book.

(C) significance or meaning.

(D) the portion of a substance contained

Ma gma's r ol e i n t h e r oc k c y c l e...Decompression Melting Decompression melting involves the upward movement of Earth's mostly-solid mantle. This hot material rises to an

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