Video Transcript
In this video, we will learn how to
identify, name, and describe igneous, sedimentary, and metamorphic rocks and explain
the formation of each.
The Earth consists of four main
layers. The outermost layer called the
crust is the thinnest layer. At the very surface of the crust,
we find layers of soil sitting upon the rocks below. Soil is a substance that consists
of decayed organic matter, mineralogical substances, plant roots, water, and
air.
Rock is a naturally occurring solid
material substance formed from one or more minerals. There are many different rocks,
including marble, sandstone, pumice, obsidian, slate, and limestone just to name a
few. While each of these rocks may
appear different, they can be grouped according to how they are formed into one of
the three main classifications of rock: igneous, sedimentary, or metamorphic. Let’s take a look at each of these
classifications of rock one at a time.
The word igneous comes from the
Latin meaning fiery. Igneous rocks are formed when hot,
viscous, molten material found below the Earth’s crust cools and solidifies. When this hot, molten material is
below the surface of the Earth, it’s known as magma. But when magma flows to the surface
of the Earth, it’s then referred to as lava. While magma and lava consist of the
same hot, molten material, they are under different conditions and therefore produce
different types of igneous rocks.
Magma can be found in the lower
part of the Earth’s crust and upper part of the mantle where there is a high ambient
temperature. This means that when magma cools,
it does so very slowly. The rocks formed when magma cools
consist of large crystals. This is because the slow cooling of
magma allows time for the different substances in the rock to arrange themselves,
causing larger crystals to grow. We call the type of igneous rock
that forms when magma cools slowly plutonic rock.
Lava above the Earth’s surface
cools much more quickly than magma deep below. Because lava cools quickly, there
isn’t sufficient time for large crystals to form, so these rocks consist of small
crystals. In addition, the rocks formed when
lava cools often contain small holes. These holes are formed when gases
in magma escape once the magma reaches the surface and becomes lava. We call the type of igneous rock
formed when lava quickly cools volcanic rock or surface rock.
Now that we’ve seen how igneous
rocks are formed, let’s take a look at a few examples of this type of rock.
Granite is one of the most common
examples of plutonic rock. Granite is hard, heavy, and
abrasion resistant, which makes it an excellent rock to use for the exterior of
buildings, floor tiles, kitchen counters, or stone monuments. Granite is typically a mixture of
colors, including pink, gray, white, and black. These colors are due to the three
main minerals that make up granite. Quartz is gray and glassy, feldspar
is usually pink, white, or light gray, and mica is often black.
Both basalt and pumice are examples
of volcanic rocks. Each of these volcanic rocks
contains small holes that were formed when gases escaped as the lava cooled. Basalt is a strong, dense,
dark-colored rock. Unlike with granite, the minerals
that form basalt — olivine, pyroxene, and feldspar — cannot be seen with the naked
eye. Basalt is the most abundant rock in
Earth’s crust. Due to its abundance and strength,
it is often used as the base of pavements and roads.
Pumice is typically a light-colored
rock that has a lot more holes than basalt. These holes allow pumice to float
on water. As with basalt, the minerals that
make up pumice cannot be seen with the naked eye. Pumice can be used to make
lightweight concrete, but it’s also found in some hand soaps as it is a mild
abrasive and skin exfoliant.
Now that we’ve looked at igneous
rocks, let’s take a look at another classification of rock.
The word “sedimentary” comes from
the Latin meaning a settling. Sedimentary rocks are formed from
the settling of sediment over time or more specifically the deposition,
accumulation, and solidification of small mineral or organic particles. There are a series of processes
which allows sedimentary rocks to form. We’ll use this diagram to help us
understand these processes. The first step is the process of
weathering. During weathering, existing rock is
fragmented and disintegrated into smaller particles of rock or sediment. The next process is transportation,
where the rock fragments are transported typically by air or water in rivers or
seas. The rock fragments are then
deposited in a layer.
The processes of weathering,
transportation, and deposition repeat, creating more and more layers of
sediment. As the layers increase, the
pressure on the bottom layer increases and the bottom layer compacts. At the same time, the increase in
pressure removes water from the bottom layer and cements the sediment together,
forming sedimentary rock. The compaction and cementation of
the bottom layer is sometimes referred to as a single process called
lithification. Over time, more sediment is
lithified, adding more layers to the sedimentary rock. Since the lowest layer lithifies
first, it will form the oldest layer of rock, with newer layers building on top.
Now that we know how sedimentary
rock is formed, let’s take a look at a few examples of sedimentary rocks.
Sedimentary rocks cover
approximately 75 percent of the Earth’s surface but only make up about five percent
of the volume of the crust. Sandstone and limestone are two
common examples of sedimentary rocks. Sandstone is formed when grains of
sand that are less than two millimeters in diameter cement together. Sandstone is typically yellow and
is very coarse. The composition of sandstone
depends on the region in which it is formed, but it’s mostly composed of quartz
sand. Sandstone is a common building
stone, and it can be ground down to produce man-made sand used on golf courses.
Limestone is a smooth, soft, white
rock that often contains fossils. Limestone contains the mineral
calcite, which is the crystal form of calcium carbonate. This mineral can react with dilute
hydrochloric acid to produce carbon dioxide gas. So, when dilute hydrochloric acid
is added to limestone, we’ll see visible effervescence or bubbles due to the carbon
dioxide gas being produced. Limestone is used in concrete and
can be ground up for use as an agricultural fertilizer.
Now let’s take a look at the final
classification of rock.
The word metamorphic comes from the
Greek for change of form. In fact, metamorphic rocks are
formed from the change or conversion of existing rock to a new type of rock. This conversion occurs when
existing rock is exposed to high temperature and high pressure. Metamorphic rocks are often formed
deep underground when magma comes into contact with existing rock. The magma permeates through the
rock. This increases the temperature of
the rock and causes the rock to undergo a physical change, producing metamorphic
rocks. The type of metamorphic rock
produced depends on the temperature of the magma, the pressure created by the mass
of the magma, and the identity of the original rock. Let’s look at an example of a
metamorphic rock.
We know that limestone is a
sedimentary rock that is smooth, soft, and white. When limestone is exposed to high
temperature and high pressure, it can be converted into marble, a metamorphic
rock. Marble has a rough texture and is
white when pure. Marble is harder than limestone,
but it’s still soft enough that it’s relatively easy to chisel and shape. Marble is also particularly
attractive when polished and is often used for monuments, kitchen countertops, and
interior decoration.
We’ve now learned about the three
main classifications of rock and have seen some examples of each. But before we wrap up this video,
let’s take a look at a few questions.
The image below shows a section of
sedimentary rock. Which label corresponds to the
oldest layer of the rock?
Sedimentary rocks are rocks formed
from the deposition, accumulation, and solidification of small mineral or organic
particles. During the formation of sedimentary
rocks, small bits of rock, sediment, or sand are deposited into a layer. Over time, more and more layers of
sediment or sand are deposited on one another. As the layers increase, the
pressure on the bottom layer increases. The increased pressure forces the
sediment in the bottom layer to compact together. The increased pressure also removes
water from the bottom layer. This cements the sediment together,
forming the first layer of sedimentary rock.
Compaction and cementation can
occur with each successive layer, building up the layers of sedimentary rock from
the bottom up. As the bottommost layer solidifies
first, this layer represents the oldest layer of rock. As the layers of sedimentary rock
solidify from the bottom up, the top most layers will be newer than the bottom.
In this question, we want to know
which label in the diagram corresponds to the oldest layer of rock. The bottommost layer, labeled E,
will have solidified before the layers above it. Therefore, the label that
corresponds to the oldest layer of the rock is E.
Which of the following rocks
contains small holes in its structure due to the extruding of gases during its
formation? (A) Granite, (B) sandstone, (C)
limestone, (D) basalt, (E) marble.
Rock is a naturally occurring solid
material substance formed from one or more minerals. Each type of rock can be grouped
into one of three classifications according to how it is formed.
Igneous rocks are formed when hot,
molten material cools. This hot, molten material is called
magma when it’s below the Earth’s surface and lava when it is above the surface. Magma has a high ambient
temperature and cools slowly, forming rocks with large crystals. The type of igneous rock formed
when magma cools slowly is called plutonic rock. Lava on Earth’s surface cools
quickly and produces rocks that contain small crystals. In addition, these rocks often
contain small holes caused by gases escaping or extruding from the lava. This type of igneous rock is called
volcanic or surface rock.
Sedimentary rock is formed when
layers of sediment or sand compact and cement together. When looking at sedimentary rock,
we can sometimes see the layers or even the individual grains of sand and sediment
that make up the rock.
Metamorphic rocks are often formed
when existing rocks deep underground come into contact with magma. The high temperature and high
pressure convert the existing rocks into metamorphic rocks.
In this question, we want to know
which of the rocks will contain small holes in its structure due to the extruding of
gases during its formation. This means that the rock is likely
a volcanic igneous rock. Let’s take a look at each of the
answer choices.
Granite is a coarse-grained
plutonic igneous rock that appears a combination of pink, white, gray, and
black.
Sandstone and limestone are
sedimentary rocks. Sandstone is a coarse yellow rock,
while limestone is a smooth, soft, white rock.
Basalt is a volcanic igneous
rock. It is primarily black and contains
lots of small holes.
Marble is a rough, white
metamorphic rock formed when limestone is exposed to high temperature and high
pressure.
We can see that the only rock that
is a volcanic igneous rock is basalt. Therefore, the rock that contains
small holes in its structure due to the extruding of gases during its formation is
answer choice (D), basalt.
Now let’s summarize what we’ve
learned with the key points. The Earth’s crust consists of a
thin layer of soil on top of a naturally occurring solid material substance called
rocks. Rocks are grouped into one of three
classifications according to how they are formed. The three main classifications are
igneous, sedimentary, and metamorphic.
Igneous rocks are formed when hot,
molten material found below the Earth’s crust cools and solidifies. There are two types of igneous
rocks. Plutonic igneous rocks are formed
when magma cools slowly, producing rocks with large crystals. Granite is an example of a plutonic
igneous rock. Volcanic or surface rocks are
igneous rocks formed when lava cools quickly, producing rocks with small
crystals. These rocks often have small holes
due to the extrusion of gases. Basalt is an example of a volcanic
igneous rock.
Sedimentary rocks are formed when
existing rocks are weathered. The sediment produced is
transported and deposited into layers that are then compacted and cemented
together. Sandstone and limestone are
examples of sedimentary rocks.
Metamorphic rocks are formed when
existing rock is exposed to high temperature and high pressure. Marble is an example of a
metamorphic rock. It is formed when limestone, a
sedimentary rock, is exposed to high temperature and high pressure.
