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 and METAMORPHIC
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. Igneous rocks form when molten lava (magma) cools and turn to solid rock.
When the rock is liquid & inside the earth, it is called magma. 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%)
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.
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.
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.
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.
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. Metamorphic rocks are rocks that have been transformed by great heat or pressure. Marble is an example of a sedimentary rock that has been changed into a metamorphic rock.
If you squeeze your hands together very hard, you will feel heat and pressure. If you have every aqueezed a piece of bread together and made it into a ball, it is changed and cannot be changed back. That is what happens when metamorphic rock is made. When the earth's crust moves, it causes rocks to get squeezed so hard that the heat and pressure causes the rock to change.
Metamorphic rocks are the least common of the 3 kinds of rocks. Foliated metamorphic rocks have layers, or banding. Non-foliated metamorphic rocks are not layered.
- SLATE is transformed shale. It splits into smooth slabs.
- SCHIST is the most common metamorphic rock. Mica is the most common mineral.
- GNEISS has a streaky look because of alternating layers of minerals.
- MARBLE is transformed limestone.
- QUARTZITE is very hard.