Geology Reference
In-Depth Information
If the pressure and temperatures exceed gneiss-forming conditions, the gneiss begins to
melt on its way to becoming magma. When a rock forms from these conditions, it is a
migmatite. Migmatites are gneisses that have partially melted and then solidified into
rock. The minerals are still differentiated into dark and light foliated layers, but they are
usually swirled or curvy from all the pressure that nearly melted them into magma. An
example of a migmatite is pictured in the color photo section of this topic.
Limestones do not proceed through the sequence of metamorphism that I describe for
shale. Instead, under conditions of high temperatures and pressure, limestone (and do-
lostone) minerals are compressed until all the space between crystal grains is squeezed
out. The result is a very hard, smooth rock called marble. The solid, smooth feature of
marble — where the crystals form one continuous body — makes it a great material for
sculpting.
Sandstone also creates a very hard metamorphic rock called quartzite. Similar to marble,
quartzite is formed by compressing all the space from between mineral grains until the
crystals are smashed together in one continuous body of mineral grains.
To illustrate how limestone and sandstone change through metamorphism,
imagine holding a handful of frozen blueberries in your hands. While they are
frozen, solid, and round, there is space between them. As you squeeze them to-
gether — applying pressure and creating heat with your hands — they begin to
soften a little, and you can press them closer together. They deform from their
round shape to fill all the spaces as the pressure increases. When they are a mass
of squished blueberries with no space, re-freeze them, and now they resemble
marble or quartzite.
Transforming igneous rocks
As basalts are exposed to high pressure (but still relatively low temperatures), the min-
erals transform and become foliated. Low pressures create minerals with a green color,
so the metamorphic rock is called greenschist (which has a foliated texture as well as the
green color). Exposed to higher levels of pressure, the green-colored minerals transform
into blue-colored minerals, creating blueschist. Under increasing temperature and pres-
sure, these schists transform into gneiss, as I describe for sedimentary rocks in the pre-
vious section. Intrusive igneous rocks such as granite transform into gneiss rocks as the
temperature and pressure force metamorphic differentiation of the minerals into dark
and light layers.
Creating hornfels
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