>>In this video we will discuss why there
are different igneous rocks. There are different
sources for these igneous rocks. For example,
the mantle is mostly composed of mafic material,
and as magma comes up at mid-ocean ridges,
it produces basalt and gabbro. In contrast,
the crust is mostly composed of granite. There's
something called magmatic differentiation,
which is very important in forming different
types of igneous rocks. For example, assume
a magma is composed of 50% mafic minerals
and 50% felsic minerals. It would be called
a diorite or an andesite. However, recall
that when we discussed formation of mafic
and felsic minerals, it's the mafic minerals
that form first. Assume that mafic minerals
form first from that magma, and they settle
out. They will produce perhaps a gabbro, but
what's going to happen? The remaining liquid
will form a granite, which is composed mostly
of felsic minerals because those minerals
form a lower temperatures. Now we'll discuss
how igneous rocks are related to plate tectonics.
We've already mentioned some of these points.
For example, the oceans--at mid-ocean ridges
you produce basalt at divergent plate boundaries.
Granites form when continents collide. That
causes continental rock to heat up. You melt
continental rock. You melt granite. You produce
granite. What happens at subduction zones
where you have basalt flowing underneath a
granite continent? And what's going to happen:
you will melt some of that basalt. It will
rise up, and it will produce a rock that's
half felsic and half mafic. It's called an
andesite, and you get volcanoes above subduction
zones.
