Module #9 - Rocks and Minerals

Intrusive igneous rocks are formed from magma that cools and solidifies deep beneath the earth’s surface. The insulating effect of the surrounding rock allows the magma to solidify very slowly. Slow cooling means the individual mineral grains have a long time to grow, so they grow to a relatively large size. Intrusive rocks have a characteristically coarse grain size.

Extrusive igneous rocks are formed from magma that cools and solidifies at or near the earth’s surface. Exposure to the relatively cool temperature of the atmosphere or water makes the erupted magma solidify very quickly. Rapid cooling means the individual mineral grains have only a short time to grow, so their final size is very tiny, or fine-grained. Sometimes the magma is quenched so rapidly that individual minerals have no time to grow. This is how volcanic glass forms.

Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering, then transported to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock.

Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their grain size.

Chemical sedimentary rocks are formed by chemical precipitation. This process begins when water travelling through rock dissolves some of the minerals, carrying them away from their source. Eventually these minerals are redeposited when the water evaporates away or when the water becomes over-saturated.

Biologic sedimentary rocks form from once-living organisms. They may form from accumulated carbon-rich plant material or from deposits of animal shells.

Foliation forms when pressure squeezes the flat or elongate minerals within a rock so that they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.

Non-foliated metamorphic rocks do not have a platy or sheet-like structure. There are several ways that non-foliated rocks can be produced. Some rocks, such as limestone, are made of minerals that are not flat or elongate. No matter how much pressure you apply, the grains will not align! Another type of metamorphism, contact metamorphism, occurs when hot igneous rock intrudes into some pre-existing rock. The pre-existing rock is essentially baked by the heat, changing the mineral structure of the rock without the addition of pressure.

All the rocks at the earth's surface are slowly being altered and broken down. The particles produced by these processes are transported by water, wind, or gravity, and accumulate as sediments. After undergoing several physical and chemical changes, these sediments will eventually become sedimentary rocks. If a sedimentary rock is buried at great depth within the earth's crust, it will be transformed into a metamorphic rock. If a metamorphic rock is subjected to still higher temperatures, it may melt, forming a magma that will crystallize as an igneous rock. The cycle will then begin again. The cycle can also be interrupted or short-circuited as shown by this diagram representing the rock cycle.

Video: Rock Cycle Rap (Optional, but entertaining)