http://www.edu.pe.ca/southernkings/igneousjj.htm

The rock cycle is an ongoing process, beginning as rocks are pushed up by tectonic forces, and eroded by wind and rain. The eroded rocks travel by wind or moving water until they are deposited, settling into layers. Additional eroded rocks may bury these layers until heat and pressure change the underlying layers to metamorphic rock. More eroded rocks may squeeze and press the layers into sedimentary rocks. Rocks can also be sunk down into the lower layers of the earth by plate tectonic processes. Buried rocks may also melt and recrystallize into igneous rocks. Metamorphic, sedimentary, and igneous rocks may then be pushed up by tectonic forces, starting the rock cycle again.

Sedimentary Rock

Metamorphic Rock

Igneous Rock

Sedimentary Rock

The forces of weather break rocks into small pieces that are carried away and deposited elsewhere. These small pieces are often deposited in shallow seas or lakes as sediments. As the layers of deposits pile up, perhaps over millions of years, pressure from the weight of the sediments above turns the lower layers into solid rock. Sand may turn into sandstone; silt and clay become shale. Such rock, made of sediment, is called sedimentary rock.

Geologists believe that sedimentary rock was forming three and a half billion years ago. Sedimentary rock is formed when mineral matter of plants and animals settle out of water and, less commonly, air or ice. The most common materials for sedimentary rocks are fossils, formed when sediment covered dead plants and animals as the sediment changed into rock. The remains are outlines of the dead plants and animals. Some limestone is made entirely of fossils, microscopic sea life, and is deposited in oceans. Sedimentary rock covers about three fourths of the land area, and most of the ocean floor. Where the earth’s crust is deformed or eroded, large areas of buried sedimentary rock may be exposed. In some places, such as the mouths of rivers, the sedimentary rock is 12,000 meters thick.

After thousands of years, sedimentary rock is formed by many compact layers of rock building up pressure from water and the weight of other layers of overlying rock squeezing the rock until it molds together. Water that trickles slowly through layers of coarse sand and gravel, deposits mineral cement around these particles, cementing the layers together to form rock. The layers, which vary from one another in composition or texture, distinguish sedimentary rock from igneous and metamorphic rock. For sedimentary rock to bed or to form broad, flat layers from the collection of grains of clay, silt, or sand settling in river valleys, or on the bottoms of lakes and oceans, there has to be a parent rock for the sediment to form around. This parent rock can be any type of rock, igneous, metamorphic, or sedimentary.

Metamorphic Rock

Metamorphic rock is rock that has been altered by heat or by heat and pressure. ‘Metamorphic’ means ‘change of form’; heat and pressure can change the forms of many things, for example, a glassmaker uses heat to change a certain kind of sand into glass. Rocks change when mountain-building forces apply a great deal of pressure and heat to them. Rock is changed by heat produced by nearby molten igneous rock, that is, molten rock, or by both heat and pressure produced mainly by movements in the earth’s surface which are associated with the formation of mountains. These rocks formed from other rocks by essentially solid state changes in mineralogy and textures resulting from chemical or physical changes that occur in solid rock buried in the earth’s crust are called metamorphic rocks. They have been changed usually by heat and pressure from their original condition into rock with new minerals and structures; some of the minerals in rock are broken down and form new minerals. The grains that make up the rock may become larger. The mineral content of metamorphic rocks depends both on its protolith and the metamorphic conditions the rocks endured. The presence of some specific minerals in a metamorphic rock can indicate the degree of heat and pressure it sustained.

Metamorphic rocks are sedimentary or igneous rocks that have been modified or changed in form, that is, the size, shape and arrangement of the minerals in rocks, by heat or pressure. As they are derived from previously existing igneous, sedimentary or even metamorphic rock, their appearance varies from one to the other. Metamorphic rocks are identified by the types of minerals they contain and their texture. Examples of sedimentary rocks changing to metamorphic rock as a result of heat and pressure are limestone changing to marble, and shale changing to slate.

Igneous Rock

On the earth’s surface, there are three types of rocks, metamorphic, sedimentary, and igneous. In northern Canada, much of the hard rock found is igneous rock. Igneous rock is rock formed by the hardening and crystallization of molten material that originates deep within the earth. The word “igneous” comes from the Greek word for fire; some people call igneous rock a fire ball rock. Rocks that have hardened from magma, the hot liquid beneath the earth’s crust, are called igneous rocks.

The inside of the earth is very hot – hot enough to melt rocks. The deeper you get the hotter it gets. Below the surface, the molten rock is called magma; at the earth’s surface it becomes lava, nothing has changed only the name of the liquid. Igneous rock is made by fire. Small wonder the magma from which igneous rock is formed can reach temperatures close to 1200 degrees Celsius. As the super hot magma cools, it solidifies to form a rock and that rock is called igneous rock. When most people think of igneous rock they vision a volcano erupting pumice and lava. Igneous rocks form directly by crystallization from materials from a magma melt. Igneous rocks crystalize after the magma reaches the earth’s surface.

Igneous rock is divided into 2 groups, extrusive and intrusive. Extrusive rocks form when magma flows onto the surface of the earth or floor of the ocean through deep cracks or fissures and at volcanic vents, and then cools and hardens. Intrusive rock results when magma solidifies beneath the earth’s surface in mines and tunnels or at the surface where it has been exposed by geological uplifting and by erosion. In general, extrusive rocks have a finer grained texture than intrusive rocks; intrusive rocks vary from thin sheets to huge, irregular masses. Since magma that forms intrusive rocks solidifies slowly, most intrusive rocks have larger crystals than extrusive rocks.