Week 3
Rocks and the Rock Cycle
Week 3
Rocks and the Rock Cycle
Rocks and the Rock Cycle
The Earth is composed of three different rock types: igneous, sedimentary, and metamorphic.
Igneous Rocks
- Igneous rocks form when a molten magma (melted rock) cools and crystallizes.The name igneous is derived from the Latin word “ignis”, which means from fire.
- Igneous rocks form at relatively high temperatures ranging from around 700-1200 0 C, as the magma cools.
- At certain characteristic temperatures each of the silicate minerals begins to crystallize. As the temperature of the cooling magma continues to drop the remaining silicate minerals found in the melt begin to crystallize until the rock is completely formed, at about 700 0 C.
- Igneous rocks are composed almost exclusively of the silicate minerals.
Extrusive Igneous Rocks
Igneous rocks form rapidly when magma is "extruded" from volcanoes onto the surface of the earth. After eruption the air cools the magma rapidly and many very small crystalls form quickly to form an "Extusive" or "Volcanic Igneous" rock.
The first phase is upwelling of the magma towards the surface.
The second phase is eruption or extusion of the magma onto the surface of the volcano.
The final phase is the rapid cooling and crystallization of the rock.
Igneous rocks may also be formed slowly beneath the surface of the earth.
Intrusive Igneous Rocks
Igneous rocks may also form by slow cooling beneath the surface of the Earth in a process called intrusion.
Intrusive igneous rocks rocks form when magma cools slowly beneath the surface of the Earth. As it clowly cools, lage crystals form and grow towards each other until a matrix of coarse grains is formed.
In the first phase the magma is upwelled and intruded into a void in the crust, beneath the surface.
Once the magma is intruded it begins to cool slowly and a few large crystals begin to form.
In the final phase crystals form in the void space and slowly grow together to fill the void with an interlocking matrix of coarse mineral grains.
Porphyritic Igneous Rocks with Mixed Cooling History
Igneous rocks may also have a mixed cooling history in which coarse grains form in a cooling magma before it is extruded. This creates a rock with a fine grained matrix with coarse grains (called phenocrysts) imbedded in the matrix. This is called a "Porphyritic" rock.
Sedimentary Rocks
There are two types of sedimentary rocks, chemical and clastic.
- Chemical sedimentary rocks form by precipitation of minerals from an aqueous solution or by some other chemical or biochemical reaction.
- Clastic sedimentary rocks form when existing rocks are weathered into small fragments called sediment, eroded and transported to another site, and deposited, compacted and lithified.
Metamorphic Rocks
Metamorphic rocks are the result of the application of heat, pressure and directed stress, or some combination of these effects applied to pre-existing rock of any type. The process by which metamorphic rocks are produced is called metamorphism.
There are three types of metamorphism Regional, Dynamic and Contact.
Regional metamorphism occurs when heat, pressure and directed stress are applied to pre-existing rock. Regional metamorphism involves the recrystallization and reorientation of the minerals in the existing rock by a process called solid state recrystallization.
Contact metamorphism occurs when molten magma is intruded into existing country rock and heats the surrounding country rock, causing recrystallization of some of the mineral grains in the existing rock.
Dynamic metamorphism occurs when rock is fragmented and foliated in a fault and then recrystallized in the solid state by the pressure and heat of deformation. Dynamic metamorphism also accounts for only a tiny percentage of the metamorphic rocks of the world.
The Rock Cycle
Rocks are in a constant process of transition from their current states and rock type classifications to the next phase of their existence as another rock type. The rock cycle is essentially a road map that allows us to follow the sequence of processes by which rocks from each of the three major rock types are formed from members of the other rock types.
To test your understanding of the rock cycle, click the link below for an exercise.
Here are a few examples of the rock cycle. In each example one rock or material is shown, and you must pick the correct processes in the correct order that would result in the formation of the rock or material.
