*The following words and information are from rocksforkids.com
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:
(1) Igneous
(2) Sedimentary
(3) Metamorphic

What category a rock is in depends on how the rock was formed.

Igneous Rocks

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 and 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.

• Igneous rocks form when molten lava or 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%)
   
• 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
   
• 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.

Sedimentary Rocks 

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 the United States. When they 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.

When large amounts of plants are deposited in sedimentary rocks, then they turn into carbon, which 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.

Metamorphic Rocks 

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.

Back to Top