Geology Reference
In-Depth Information
Plate friction: Melting rock beneath the earth's
crust
Everyone is familiar with the image of a volcano spewing (or oozing) hot lava.
Lava
is
melted rock from deep in the earth's crust. Before it reaches the surface the melted rock
is called
magma;
only after the volcano erupts is the melted rock called
lava.
Melting rocks to create magma requires a certain amount of heat relatively close to the
earth's surface. One place where this much heat can be created is where one plate sub-
ducts beneath another (a subduction zone). Even though the plates are at the surface of
the earth and therefore cooler than materials deep in the earth, the friction or rubbing
between the two plates creates extra heat — enough heat that water, carbon dioxide
gas, and other elements are squeezed out of the subducted crustal rocks into the mantle
surrounding them. This helps change the mantle rock to liquid magma. The newly
formed magma then moves upward through the crust. On its way to the surface, it melts
some of the minerals in the rocks it touches and adds those elements to its
melt,
or li-
quid. Because different minerals have different melting temperatures, the magma is able
to melt only certain minerals (not all of them).
This incomplete melting of rocks — where only some minerals are added to
the melt, while others remain solid rock — is called
partial melting.
The composi-
tion of a magma is determined by which minerals are partially melted by it as it
moves upward toward the surface.
Silicate minerals (such as quartz, feldspar, and others I describe in Chapter 5) melt at
lower temperatures than other minerals, so partial melting in subduction zones creates
magmas with high amounts of silica. When a magma containing a lot of silica cools, silic-
ate minerals are re-formed into igneous rocks on or in the earth's crust. These rocks
may, in the distant future, be subducted again (see the discussion of the rock cycle in
Chapter 7), thus continually concentrating the elements that form silicate minerals and
keeping them in the earth's crust.
Creating volcanic arcs and hotspots
As magma moves upward through the crust, it can cool and become an igneous rock
(see Chapter 7) or it can erupt onto the surface, in which case a volcano is born! Volca-
noes typically occur in two settings: 1) along the edge of a subducting plate as it par-
tially melts, or 2) as a hotspot in the middle of a plate. I explain both settings here.
