The Rock Cycle

The surface of the Earth is in constant movement, though, most of it we do not see as it happens so slowly. The movement of the tectonic plates is only about a few centimetres per year. Some move slower and some move faster. Where the plates are moving apart magma comes up to create new crust. These are divergent boundaries, and most are on ocean floors. If it occurs on a continent it creates a rift valley, such as the East African Rift. Rock that is formed from magma is called igneous rock. Crystals form as the magma cools. The crystals are minerals and minerals are made up of chemical elements and combinations of chemical elements called compounds. Feldspar and quartz are lighter coloured minerals and hornblende or mica are dark coloured minerals and the combination makes up, for example, granite rock. Igneous rock that is rich in iron minerals are mostly black or dark in colour, such as, basalt. Basalt may cool into four, five or six sided columnar shapes and can be seen along some of BC’s highways. The size of the crystals in the igneous rock depends on how fast or slow the magma cooled. The longer it takes to cool the more time the crystals have to grow. The faster cooling rock is extrusive igneous rock and the minerals are generally too small to see. These rocks are formed where the magma is released relatively quickly from the Earth, such as a volcano with lava flow or at a divergent boundary. Magma that cools very slowly deep beneath the earth’s surface and can take millions of years to solidify, can grow large mineral crystals that we can easily see. An example is granite. These are intrusive or plutonic igneous rock and a mass of this type of rock is called a batholith. A lot of the Coast Mountain range is made up of this type of rock that was pushed up during plate movement, and is mainly granite. Basalt it the most common rock of the ocean floor, and granite is one of the most common rocks of the continents. Ocean floors do get pushed up and become part of the continents and many of these areas contain sedimentary rock. Black Tusk in Garibaldi Provincial Park is a stratovocano and made up of volcanic rock and is also part of the Coast Mountain range in BC.

All rock started off as magma and at some point returns down beneath the crust to become magma again. Where two plate push towards each other, one is generally forced down under the other and turned back into magma. Often the top part of the subducting plate gets pushed up onto the other plate creating new land and mountains. This is a subduction zone. The heavier and usually ocean plate moves down below the lighter continental plate. Sometimes, two continental plates push together also forming mountains. Another way new rock is formed is by volcanoes releasing magma (lava) and building up as the magma cools. British Columbia is the youngest part of Canada, as much of it has been added to the North American Plate due to plate tectonic movement over the last 200 million years. This is part of the rock cycle; basically the beginning and the end of the cycle as new igneous rock is formed and various forms of rock return to the mantle as one plate is pushed under another plate. In between this time many other geologic processes are occurring.

Metamorphic rock is created when igneous rock or sedimentary rock is heated and or put under extreme pressure in the movement of the plates. Where plates are pushed together such as in the western edge of the Kootenays where the Intermontane Belt was pushed into the Foreland Belt, the Omenica Belt was created under extreme pressure and heat creating metamorphic rock. It can also be created by chemical infusion from nearby magma. The rock changes chemically and in shape without melting. Marble is created by the metamorphosis of limestone. Metamorphic rock can also be found in parts of the Coast Mountains, along with granite (intrusive igneous rock) and basalt (extrusive igneous rock). The Coast Mountains also has sedimentary rock, such as Mount Fissile behind Blackcomb Mountain that contains seashell fossils over 2,000 meters above sea level.

Sedimentary rock is created from the weathering of rock over time by water and ice, weather, and by biological processes, such as plant roots or micros. The sediments that are washed down rivers and end up on ocean floors, as well as, the shells of marine animals and the bodies of dead animals that float down to the ocean floors eventually become new rock. Sediments that don’t reach the oceans can also over time become rock again. Minerals can also dissolve in water creating “hard” water. Through compaction, heating, and the dissolved minerals precipitating into the sediment accumulating on the ocean floor create sedimentary rock over millions of years and can be pushed up onto continental plates as the ocean plates are pushed back down into the mantle below. Much of the Interior Plateau of BC is made up of sedimentary rock. Common sedimentary rocks are sandstone made up of sand size materials, limestone made up of mostly shells and tiny fossils, and shale made up mostly of fine particles of clay.

So the rock cycle starts off as magma becoming igneous rock that gets weathered down and these sediments wash away and accumulate to be turned into sedimentary rock. Both igneous rock and sedimentary rock can be turned into metamorphic rock by extreme pressure and heat. Metamorphic rock also eventually becomes sedimentary rock through weathering. Eventually over millions of years mountains get eroded away as new mountains form in new locations, and most rock generally gets pushed back down into the mantle to become magma and start the cycle over again. “The Canadian Shield is some of the oldest rock on Earth being part of the ancient core of the North American plate with some rock being dated about 2 to 3 billion years old.” Britannia.com

References: Geography of British Columbia – People and Landscapes in Transition 3rd Ed. by Brett McGillivray and Geology of British Columbia – A Journey Through Time by S. Cannings, J. Nelson, and R. Cannings