Geology Reference
In-Depth Information
When molten rock exists below the surface of the earth, it is called magma.
But after it erupts onto the surface of the earth (such as through a volcano), that
same molten rock is called lava.
Magmas are created when the conditions of heat and pressure are just right for melting
rocks into liquid. These conditions most commonly exist beneath the earth's surface
near the boundary of the crust and mantle. (See Chapter 4 for a description of earth's
layers.) A magma created here, from primarily mantle rock material, has low amounts of
silica (and felsic minerals) and high amounts of other (mafic) minerals because earth's
mantle is composed of relatively low silica. Magma with this composition is called a
maic or basalt magma.
Magmas are also created when one crustal plate is subducting below another. (I explain
subduction in Chapter 10.) The magma created under these conditions has high
amounts of silica because the crustal rock that melts is composed of a large percentage
of silica minerals. This type of magma is called a rhyolite or felsic magma.
Reacting in sequence: Bowen's reaction series
When molten rock in a magma (or lava) begins to form mineral crystals, it does so in a
particular order, determined by the temperature at which different minerals form as the
magma cools. This sequence of mineralization (formation of minerals) is called Bowen's
reaction series, named after the scientist who first explained it from laboratory experi-
ments.
To visualize mineralization through Bowen's reaction series, imagine a soup
pot full of chocolate chips, marshmallows, peanut butter, and walnuts. As this
soup pot heats up, the “particles” begin to melt. Each item has a different melting
point: The marshmallow melts first, then the peanut butter, then the chocolate
chips, and finally the walnuts. (It has to be really hot to turn walnuts into liquid!)
When the heat is really high, all these items melt into a liquid with no chunks —
just a smooth, flowing liquid mix of chocolate, marshmallow, peanut butter, and
walnuts. This liquid is like a magma: a fluid mix of melted rock materials.
When you turn the stove off, the pot begins to cool down. Imagine that as it cools, the
stuff in the soup pot reforms its original shape. These items will be created in the re-
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