School: Meadowbrook High School
Subject: Geography
Grade: 10
Topic: The Rock Cycle
Objectives: At the end of the lesson students should be able to:
1. List the three types of rocks.
2. Describe the formation of igneous rocks, sedimentary and metamorphic rocks.
3. Explain the rock cycle.
4. Sketch diagram to show the different processes of the rock cycle.
Rocks
A rock is any hard, naturally occurring substance which is composed of minerals and which
is formed by geological processes. A mineral is a substance which is normally crystalline and
is formed by geological processes. A geological process is any natural process which
modifies (changes) geological features. People who study rocks have found it useful to
classify them according to the processes by which they were formed. There are three major
ways in which rocks are formed and hence there are three major classes of rock; igneous,
sedimentary and metamorphic rocks.
Igneous Rocks
These rocks are formed when molten rock from deep within the Earth’s crust (magma) finds
its way into or onto the Earth’s crust where it cools and hardens. When magma cools and
hardens within the Earth’s crust the resulting rocks are called intrusive igneous rocks.
Because these rocks were formed by the slow cooling of magma, they often contain large
crystals, making them coarse grained. Examples of intrusive igneous rocks are granite and
gabbro. These are made up of large crystals which can be seen with the naked eye.
Diagram of Grabbo
Diagram of Granite
When magma reaches the Earth’s surface it is known as lava. This lava cools and hardens
quickly, forming what are known as extrusive igneous rocks. These rocks are usually fine
grained as the lava cools too quickly to allow large crystals to develop. Basalt and obsidian
are examples of extrusive igneous rocks. Obsidian (also known as volcanic glass) is formed
when lava cools so quickly that crystals are unable to form.
Basalt
Obsidian
Sedimentary rock
These rocks are formed when material which has been deposited by agents such as rivers is
compacted over time until it hardens into rock. This usually happens in bodies of water such
as the sea. The material is deposited in layers and these layers are often clearly visible in the
resulting rock. Examples of sedimentary rocks include limestone and sandstone.
Sandstone
Limestone
Metamorphic rock
When igneous or sedimentary rocks are subjected to intense pressure and heat over a long
period of time, their structure and mineral composition may change a great deal. These rocks
metamorphose into a completely different type of rock. The heat and pressure required to
bring about this change in the rock exists far below the earth’s surface. Examples of
metamorphic rocks are marble (which is metamorphosed limestone) and gnesis.
Marble
Gnesis
The Rock Cycle
After rocks have formed, processes such as weathering, erosion, plate tectonics and other
geological processes begin to act upon them. As a result of this the rocks begin to undergo
changes. Extreme heat and pressure may cause sedimentary and igneous rocks to become
metamorphic rock. However, this is only one possible change that rocks can go through.
Metamorphic and igneous rocks may be broken down into small fragments by a process
called weathering. These fragments may be carried by a river and deposited in the sea. Over
time, layers of these fragments may build up and become compacted forming sedimentary
rock.
Sedimentary and metamorphic rock may be forced down into the mantle where they melt.
They may eventually find their way into the crust and cool forming intrusive igneous rocks.
Alternatively, they may erupt out of a volcano as lava and cool forming extrusive igneous
rocks.
Rocks are constantly being formed, broken down and reformed in an endless cycle known as
the rock cycle.
Diagram of the rock cycle
