Download Metamorphic rocks - Wando High School

1.3.1 - Rocks!
Background: Rocks are naturally occurring solid combination of two or more minerals. Think of a rock as
being like a chocolate chip cookie. The cookie is made of flour, butter, sugar & chocolate. The cookie is like
a rock and the flour, butter, sugar & chocolate are like minerals that make up the rock.
Rocks tell the history of Earth’s surface, and will also comprise the material that will be broken down and
create the future Earth’s surface. This is because each rock is formed in a different way, and with a
different composition of minerals. By finding a certain rock in a place, scientists can determine what the
conditions were in that are when the rock was formed. Geologists must analyze mineral composition,
evidence of type of formation, and size & arrangement of minerals to determine the classification of rocks.
While there are 100s of different types of rocks, there are three major classifications of rock based on how
the rock was formed: sedimentary, metamorphic, and igneous.
Igneous rocks
Igneous rocks form deep underground when molten rock, called magma, cools and hardens. There are
over 700 different types of igneous rocks, but they are broken down into two major classifications based on
how quickly, and where, the magma cooled: intrusive and extrusive.
Intrusive igneous rocks formed when magma cooled slowly beneath the surface. Below the surface,
magma is going to cool slowly because rock around it is insulating the heat, not allowing any of that
heat to escape, meaning that magma was going to take a lot longer to cool down. Because they cool
slowly, the grains that make up the rock are much larger because they had
a long time for crystals to form. Granite is the most recognizable intrusive
igneous rock.
Extrusive igneous rocks form when the magma reaches the surface (which
means it is no longer magma and now it is lava) and quickly cools on the
surface. The lava cools quickly because being on the surface, there is
nothing insulating the heat from the lava, and it cools much more quickly
relatively. Because extrusive rocks cool quickly, their grains are very fine in size because they don’t
have much time for crystals to grow. Examples of extrusive igneous rocks include basalt, pumice,
rhyolite, and obsidian.
The difference between every type of igneous rock is simply what type of minerals were present in the
magma when it is cooling, and where it cools. As you can see in the image to the right, the only difference
between granite and rhyolite is that granite
cooled underground for long periods of
time, while rhyolite cooled quickly on the
surface; they are both composed of the
same mineral make-up (feldspar, quartz,
sodium-plagioclase, mica, and amphibole).
65% of Earth’s surface (land and ocean
floor) is made of igneous rock. Land is
called continental crust, while the ocean
floor is oceanic crust. 99% of the oceanic
crust is made of basalt, while granite is the
most common rock found in continental
crust. This is important, because the
difference in these rock types is going to
dictate how continental and oceanic crust
interact.
Sedimentary rocks
Sedimentary rocks are formed by deposition of and subsequent
cementation of that material within relatively shallow bodies of water.
The type of sedimentary rock formed is based on the size of the material
being deposited (sand, silt, and clay in order from largest to smallest, as
you can see in the image to the right) as well as the type of material
being deposited. The material can weathered pieces of other rocks,
mineral grains, or pieces of marine shells. Regardless of the materials
or side, the image below should give a basic idea of the process. Essentially, material is deposited
somewhere. Over long periods of time, other deposits will be
made on top of it. At some point, enough material has been
built up on top of that initial deposit that pressure starts to
increase and the sediment becomes cemented together with
minerals. The image to the right
should show how minerals hold
sediments together. Looking back
to the image to the left, you can see
that most sedimentary rock is
formed in shallow waters over
millions of years. As ocean levels change, new sediment is
deposited and new sedimentary rock is made. Sedimentary
rocks tend to form layers, called stratifications, as you can see in the image to the right. There isn’t always
just sedimentary rock in these layers, as you
can find layers of igneous or metamorphic
rock too. When you can see stratifications or
rock formations on the surface, this is called
an outcrop, again, easily seen to the right.
There are three basic classifications of
sedimentary rock that are based on the type of sediment particles and the grain size (coarse-grained,
medium-grained, fine-grained).
Clastic sedimentary rocks are composed of other rock fragments that were cemented together by
silicate minerals. Examples include conglomerates (generally coarse gravel sized sediment),
sandstones (made of quartz, feldspar, and lithic grains), and mudrocks (made of +50% silt/clay
sized particles. Shale and siltstone are both mudstones). Sedimentary rocks make up 8% of the
Earth’s surface, with upwards of 80% of that 8% being shale.
The particle shape that makes up sedimentary rocks also tells scientists what weathered the parent
rock. Rounded sediments indicate water weathered and transported materials, while angular
shapes indicate wind weathering and transport.
Organic sedimentary rocks contain sedimentation of organisms that were once alive. For instance,
most limestone is formed from the calcite skeletons of corals and mollusks. Coal is formed from
plants. Chalk is a type of limestone formed from minute calcitre shells of coccoliths. Chert is
formed from the sedimentation of diatoms.
Chemical based sedimentary rocks form when minerals in a solution are left behind as water
evaporates. Rock salt and gypsum are two major examples.
Metamorphic rocks
Metamorphic rocks form when one rock is transformed into another rock due to extreme heat and
pressure. Heat and pressure will change the minerals within rocks, reclassifying the rock. This process is
called recrystallization. The grade of a metamorphic rock is dependent upon the pressure on the rocks, the
temperature, and the depth below the surface where the rocks are forming. Metamorphic rocks are
separated into two major groups based on if there is banding or not: foliate and non-foliated.
Foliated metamorphic rocks have layers or banding. This is due to some complex chemistry caused
by an uneven distribution of pressure on the metamorphic rock
while it was forming. Examples of foliated metamorphic rock
include schist, gneiss, and slate.
Non-foliated metamorphic rocks were exposed to even,
uniform pressure and therefore do not have layers. Examples
of non-foliated rock include marble, quartzite, and soapstone.
Specific metamorphic rocks are formed based on what rock is being
exposed, called the parent rock, as well as how much pressure and
temperature it is exposed it. Limestone will form into Marble.
Sandstone will form into Quartzite. Shale will form into Slate under
relatively low metamorphic activity. Mudstone and shale will form
into Schist under medium metamorphic activity. Granite will form
into Gneiss under relatively high metamorphic activity. The chart to
the left should help put all this together. This helps us know what
conditions were present when the rock formed, because if we find
schist or gneiss, we know that pressure and temperatures had to be
pretty high when the rock formed.
Review Questions
1. Which rocks do you think are capable of containing fossils? Why do you think that is?
2. List the 3 rock types and explain how each one forms in a quick statement.
3. List 2 types of igneous rocks, where they form, how fast magma cools, the size of their crystals, and
2 examples of each. (hint: this would make a good table.)
4. What percentage of Earth’s surface is made of igneous rocks?
5. What specific type of rock makes up most of the oceanic crust?
6. What specific type of rock makes up much of the continental crust?
7. List the 3 types of sedimentary rocks, explain what each is, and list 2-3 examples of each. (hint: this
would make a good table.)
8. What percentage of Earth’s surface is made of sedimentary rocks?
9. List the size of particles from largest to smallest, starting with gravel.
10. What does stratification have to do with sedimentary rocks?
11. List the 2 major types of igneous rocks, explain what foliated vs non-foliated means, the difference
in how they formed, and 3 examples of each type. (hint: this would make a good table.)
12. What is the parent rock for: Marble? Gneiss?
13. What does finding a large outcrop of gneiss tell us?