Name:
Date:
TOC #
The Rock Cycle
CGC1P Physical Diversity
When we say something is “solid as a rock,” we usually mean that it is unchanging and reliable. But rocks actually do
change over time. In fact, the Earth is like a giant recycling plant for rocks!
Geologists call this endless process the rock cycle. Magma from inside the Earth crystallizes, hardens, and then
gradually wears away, creating layers of sediment. These layers are slowly pressed by their own weight into new rocks.
Then, heat and pressure change the rocks into new forms.
Geologists know how to identify one rock from another and what type of minerals they may contain. With this
knowledge, they can advise companies where to search for specific mineral resources. For example, on the geologist’s
advice, an oil company will search only in areas where the type of rock and the structure within that rock promises oil,
gas, coal or even diamonds! Without the help of the geologist’s specialized knowledge, mining companies could waste
time and money searching for minerals in the wrong places.
Let’s look at the three types of rock that come from this process:
Igneous Rock
The oldest rock material on Earth is igneous rock. Igneous means “fire formed.” That’s
because this type of rock is created either by volcanic eruptions or by molten magma flowing in
the Earth’s crust. Most igneous rock contains clearly identifiable particles called crystals.
These crystals are minerals that get mixed together when the igneous rock is formed.
Large areas of the Canadian Shield in Northern Ontario are covered by granite, an igneous
rock with a mixture of the pink, black, and white crystals from four different minerals.
Igneous rock contains metallic minerals such as nickel, copper, gold and silver. The
Canadian Shield is partially composed of igneous rock and consequently, contains large
amounts of these minerals. In fact, the Shield is often referred to as Canada’s “storehouse
of metallic minerals.”
Sedimentary Rock
Sedimentary rock is formed from igneous rock that was eroded by wind, water and ice. Tiny
pieces of igneous rock were eroded and carried away by running water to rivers into shallow
seas, and settled onto the sea floors, gradually building up layers. As the layers multiplied,
their weight squeezed the silt below, forming sedimentary rock such as limestone. Another
sedimentary rock, shale, formed from layers of mud. Sandstone, as its name implies, formed
from eroded rock crystals (sand).
Fossils are found in some sedimentary rocks. Long ago, as some living things died, their remains floated to
the sea floors and were buried by silt. Many of the grey skipping stones that you find along the shores of the
Great Lakes contain fossils that are 400 million years old.
Metamorphic Rock
As time passes, both igneous and sedimentary rocks erode, and are
warped and changed by forces within the Earth’s crust. Metamorphic
rocks are rocks that have been changed by heat and pressure. For example,
when the sedimentary rock limestone is heated in this way, it turns into
beautiful swirled marble, which is used decoratively on walls, floors,
tabletops, and other hard surfaces.
Pressure
or
Squeezing