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This article is available at 5 reading levels at https://newsela.com.
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
This article is available at 5 reading levels at https://newsela.com.
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
This article is available at 5 reading levels at https://newsela.com.
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.
This article is available at 5 reading levels at https://newsela.com.
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.
This article is available at 5 reading levels at https://newsela.com.
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
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