Geology Reference
In-Depth Information
verse order in which they melted — in other words, the stuff that melted first will form
last.
The first item to reform is the walnuts because it was the last to melt (it has the highest
melting point). As the temperature cools back to below that melting point, the walnuts
reshape. After the walnuts are formed, the elements that make up the walnuts are taken
out of the soupy mix. The next items to form must be created from what remains. The
chocolate chips mineralize out of the cooling liquid next. Now all that is left in the liquid
is the stuff to make peanut butter and marshmallows. After the peanut butter is resolidi-
fied, the only elements left in the soup are the ones that create marshmallows.
Magmas cool in this same way: As the minerals with the highest melting
points are cooled and crystallized, they remove elements from the magma so that
only certain elements are left, and these elements can create only certain minerals
when they bond together. This process is called fractional crystallization.
Fractional crystallization explains how a single magma can produce rocks that can be ul-
tramafic, mafic, intermediate, and felsic. For example, silicate minerals (felsic minerals)
are the last to crystallize, so the rocks formed at the end of the cooling (such as granite)
are chock full of silica minerals. That's because most of the other elements (such as
iron, magnesium, and calcite) were crystallized and removed previously (creating more
mafic minerals).
Evolving magmas
As a magma cools and minerals form according to the sequence of Bowen's reaction
series, the magma evolves, or changes, each time some of the elements are removed by
mineralization. This changing of a magma that produces different igneous rocks is called
magmatic differentiation.
As the magma evolves, it can also be mixed with another magma at a different stage of
differentiation. For example, one of the magmas may move upward through the sur-
rounding rocks, intruding into the other magma.
In reality, magmas don't always start from completely melted rock materials and then
cool in sequence of their melting point as Bowen's reaction series describes. Reality is
messy! Realistically, as a magma liquid is created, the minerals that melt first are the
ones with high silica content (such as quartz and orthoclase), but the magma may not
get hot enough for the mafic minerals to melt, so they remain in rock form. This process
is called partial melting, which is how most magmas are actually created.
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