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M
Magma
Any mixture of liquid, gases,
and chunks of solid rock beneath the Earth’s surface. Magma is
the source of all of the Earth’s igneous rocks. It erupts onto the Earth’s surface as lava, pushes into fractures in underground rocks
as sills and dikes and solidifies underground to form batholiths.
Many volcanoes are fed by vast underground caverns filled with magma. They are called magma chambers. A magma chamber may be hundreds of kilometers long and feed many volcanoes.
The kind of rock that forms from magma depends on the composition of the magma and
(a) Active magma chamber
Active
on how it cools. The composition depends, in turn, on the materials from which the magma is made. Magmas are partly molten material from the mantle and partly rock from the crust. Some magmas— called acid magmas—contain a large proportion of silica; they
are acid and very sticky. They
are magmas that produce explosive eruptions. By contrast, some magmas—called basic magmas— contain little crustal material, are very runny, and produce hardly any explosions at all.
Hot, watery liquids called hydrothermal fluids may also rise from the magma and flow into fissures in the crust. When they cool and become solid, they provide rich deposits of metals. They are called mineral veins.
Magma chamber
A large cavity melted in the Earth’s crust that is filled with magma. Many magma chambers formed in the shape of plumes of magma that have melted and risen from the mantle to the upper part of the crust. When magma stops rising into the chamber, any volcanoes
it is supplying become extinct, and the magma remaining in the chamber solidifies to form a granite batholith.
Magnitude
A number that describes the strength of an earthquake. Magnitude is based on the maximum motion recorded by a seismograph. The most commonly used scale is the Richter scale.
Main shock
The largest earthquake that occurs during a ground rupture. Smaller foreshocks may go before it, and aftershocks may follow
it. The main shock is the one
used to find the magnitude on the Richter scale.
Mantle
The layer of the Earth between the crust and the core. It is approximately 2,900km thick and is the largest of the Earth’s major layers by volume. The mantle is divided into a number of zones. The part of the mantle just below the crust is of most importance
in causing volcanoes and earthquakes (see: Asthenosphere and Lithosphere). Here, parts of the mantle are molten, and most
of it can move very slowly. This region moves with a churning motion called convection. In convection the hot material rises, while the cooler material sinks. The heat for convection is produced deep within the Earth. The
(b) Slowly filling magma chamber
Dormant
(c) Inactive magma chamber
Extinct
Mantle
Magma chamber
Flow of new magma
Slow flow of magma
Magma solidified
 Magma chamber—A magma chamber is the soruce of a volcano. Maintaining the cycle of a volcanic eruption depends on what happens inside the magma chamber. (a) If the magma chamber fills and increases pressure on the plug blocking the vent of the volcano until the volcano erupts, that produces an active volcano. (b) The magma chamber begins to refill. If the refilling happens slowly, the volcano appears dormant. (c) The volcano ceases to erupt because no new magma reaches the chamber.
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