The Rock Cycle
Rocks are constantly being formed, worn down and then formed again. This is
known as the Rock Cycle. It is like the water cycle but it takes a lot longer. It
takes thousands and millions of years for rocks to change. Rocks are divided into
3 Types. They are classified by how they were formed.
• Igneous
• Sedimentary
• Metamorphic
Igneous Rock
Igneous means made from fire or heat. When volcanoes erupt and the liquid
rock comes up to the earth's surface, then new igneous rock is made. When the
rock is liquid & inside the earth, it is called magma. When the magma gets hard
inside the crust, it turns into granite. Most mountains are made of granite. It
cools very slowly and is very hard.
When the magma gets up to the surface and flows out, like what happens when a
volcano erupts, then the liquid is called lava. Lava flows down the sides of the
volcano. When it cools & turns hard it is called obsidian, lava rock or pumice depending on what it looks like.
Igneous rocks form when molten lava (magma) cools and turn to solid rock. The
magma comes from the Earth’s core which is molten rock. The core makes up
about 30% of the Total Earth Mass (31.5%)
• Obsidian is nature’s glass. It forms when lava cools quickly on the surface.
It is glassy and smooth.
• Pumice is full of air pockets that were trapped when the lava cooled when
it frothed out onto the surface. It is the only rock that floats.
• There are 5 kinds of igneous rocks, depending on the mix of minerals in
the rocks.
 Granite contains quartz, feldspar & mica
 Diorite contains feldspar & one or more dark mineral. Feldspar is
dominant.
 Gabbro contains feldspar & one or more dark mineral. The dark
minerals are dominant.
 Periodotite contains iron and is black or dark.
 Pegmatite is a coarse-grained granite with large crystals of quartz,
feldspar and mica.
Sedimentary Rock
When mountains are first formed, they are tall and jagged like the Rocky
Mountains on the west coast of North America. Over time (millions of years)
mountains become old mountains like the Appalachian Mountains on the east
coast of Canada and the United States. When mountains are old, they are
rounded and much lower. What happens in the meantime is that lots of rock
gets worn away due to erosion. Rain, freeze/thaw cycle, wind and running
water cause the big mountains to crumble a little bit at a time.
Eventually most of the broken bits of the rock end up in the streams & rivers that
flow down from the mountains. These little bits of rock & sand are called
sediments. When the water slows down enough, these sediments settle to the
bottom of the lake or oceans they run into. Over many years, layers of different
rock bits settle at the bottom of lakes and oceans.
Think of each layer as a page in a book. One piece of paper is not heavy. But a
stack of telephone books is very heavy & would squish anything that was
underneath. Over time the layers of sand and mud at the bottom of lakes &
oceans turned into rocks. These are called sedimentary rocks.
Some examples of sedimentary rocks are sandstone and shale.
Sedimentary rocks often have fossils in them.
Plants & animals that have died
get covered up by new layers of sediment and are turned into stone. Most of
the fossils we find are of plants & animals that lived in the sea. They just settled
to the bottom. Other plants & animals died in swamps, marshes or at the edge
of lakes. They were covered with sediments when the size of the lake got
bigger.
When large amounts of plants are deposited in sedimentary rocks, then they turn
into carbon. This gives us our coal, oil, natural gas and petroleum. A large sea
once covered the central part of Canada and the climate was very tropical. In
time, sedimentary rocks formed there. That is why we find dinosaur fossils in
Alberta and the area is a good source of natural fuels.
• Sedimentary rocks cover 75% of the earth’s surface. Most of the rocks
found on the Earth’s surface is sedimentary even though sedimentary rocks
only make up less than 5% of all the rocks that make up Earth.
• When rocks are exposed to the elements – air, rain, sun, freeze/thaw cycle,
plants – erosion occurs and the little bits of rock worn away get deposited
as sediments. Over time, these sediments harden as they get buried by
more sediments and turn into sedimentary rocks.
• Sedimentary rocks are usually formed in layers called strata.
• There are 6 main kinds of sedimentary rocks depending on the appearance
of the rock.
 Conglomerate rock has rounded rocks (pebbles, boulders) cemented
together in a matrix.
 Sandstone is a soft stone that is made when sand grains cement
together. Sometimes the sandstone is deposited in layers of
different colored sand.
 Shale is clay that has been hardened and turned into rock. It often
breaks apart in large flat sections.
 Limestone is a rock that contains many fossils and is made of calcium
carbonate &/or microscopic shells.
 Gypsum, common salt or Epsom salt is found where sea water
precipitates the salt as the water evaporates.
 Breccia has jagged bits of rock cemented together in a matrix.
You may have a difficult time imagining something solid like rocks wearing down
over time - but everything does. If you take a look in the lint trap of your dryer,
you will see that your clothes are being worn away as they tumble in the dryer. In
fact if there is enough lint - you will see how these bits have been laid down into
layers - just like sediments at the bottom of the lake. You will see layers of
different colors because the clothes you dried were different - just like you will
see different layers of rocks in sedimentary rocks. What is even more interesting
is that if you scrunch up the lint a bit like in the picture here, you can see the
layers of lint bending - just like the layers of rock are bent. Look carefully at the
rocks in road cuts and you sill see layers of rocks that have been folded just lhe
lint in your dryer.
Metamorphic Rock
Metamorphic rocks are rocks that have changed. The word comes from the Greek
"meta" and "morph" which means to change form. Metamorphic rocks were
originally igneous or sedimentary, but due to movement of the earth's crust, were
changed.
If you squeeze your hands together very hard, you will feel heat and pressure. When the earth's crust moves, it causes rocks to get squeezed so hard that the
heat causes the rock to change. Marble is an example of a sedimentary rock that
has been changed into a metamorphic rock.

Metamorphic rocks are the least common of the 3 kinds of rocks. Metamorphic
rocks are igneous or sedimentary rocks that have been transformed by great
heat or pressure.
o Slate is transformed shale. It splits into smooth slabs.
o Schist is the most common metamorphic rock.
o Marble is transformed limestone.