Geology Reference
In-Depth Information
Hornfels
are metamorphic rocks created through contact metamorphism. When magma
first moves into a rock close to the surface, it increases temperatures enough to change
the mineral composition and texture of the surrounding rocks. However, because no
pressure is applied, hornfels are not foliated. Also, they have very small mineral grains
because the heating by the magma occurs only for a short time; the minerals don't have
time to grow very large before the rock cools again.
Moving through the Rock Cycle: How
Rocks Change from One Type to Another
If you look back through the previous sections, you may notice that each rock type
forms from the remains of a previously existing rock. Each rock is melted and cooled,
weathered, or compressed into something new. There is no beginning or end to this
rock
cycle;
it just goes round and round as rocks, sediment, and magma move around the
earth's lithosphere.
In earth's early history (see Chapter 18), molten mantle rock began erupting to the sur-
face, forming the earliest surface rocks. On the surface, these rocks were subjected to
weathering. The sediments formed by weathering were deposited in oceans, creating
the first sedimentary rocks. These rocks on the ocean floor were then subducted back
beneath the surface and exposed to the heat and pressure that creates metamorphic
rocks, eventually perhaps being heated enough to melt back into magma. The cycle
seems straightforward, but it's not.
Figure 7-17 illustrates the major components of the rock cycle: the rock types and the
processes that transform rocks from one type to another.
The rock cycle combines all the processes that create, transform, and destroy
the different rock types and shows you how the earth materials that compose
these rocks are constantly transitioning from one type to the next.
At each stage of the rock cycle, any rock has multiple paths it may take. For example, a
sedimentary rock may be buried, compressed, and transformed into a metamorphic
rock; or weathered into sediments; or buried deep in the crust where it is heated and
melted into magma and then erupted onto the surface as an igneous rock. As Figure 7-17
illustrates, there are shortcuts in the cycle, and how a rock is transformed depends on
where it is on the planet and what forces are acting on it.
