Geography/Y3/Weathering

Rocks
Rocks are usually a composite of naturally occuring minerals.

Differenciation Between Rocks

 * Formation
 * Mineral Composition
 * Hardness (Friability)
 * Texture
 * Grain Size

Minerals
Minerals are homogeneous, inorganic soilds that are naturally occuring. They usually have a specific chemical composition and crystal structure. It may be a compound or an element. Eg. Diamond

Differenciation Between Minerals

 * Colour
 * Luster
 * Transparency
 * Technical Crystal Habits
 * Descriptive Crystal Habits
 * Twinning
 * Cleavage
 * Fracture
 * Hardness (Moh Scale)
 * Specific Gravity
 * Streak
 * Associated Minerals
 * Notable Localities

Types of Rocks
Rocks are divided into three types: Igneous, Sedimentary and Metamorphic.

Igneous Rocks
Igneous rocks are formed by cooling and solidification of molten rock. If they are cooled in water, they would cool fast and will have a very small crystal size as the crystals do not have time to clump. However, if they are cooled internally or on the Earth surface, it will cool slowly and would result in a rock with a very big crystals. Igneous rocks do not have layers, have no fossils and are generally hard. Igneous rocks can be further divided into Intrusive and Extrusive. Intrusive Igneous rocks are those cooled in the Earth. Some examples include Granite and Grabbo. Extrusive Igneous rocks are are those cooled on the surface of the Earth. Some examples include Basalt, Obsidian and Pumice.

Sedimentary Rocks
Sedimentary rocks are weathered and eroded igneous, sedimentary and metamorphic rocks that are cemented together. They could contain organic materials such as animals and plant remains. Plant and animal fossils are preserved well. They may have a layered appearance and could be stratified if layered. They may have a crystalline appearance and rock particles. Sedimentary rocks can be further divided into three types: Chemical, Biological and Clastic.

Chemical Sedimentary rocks are formed when there is more evaporation than precipitation over a body of water. If the water is not pure, after evaporating, it will leave behind a precipitate. As crystals of minerals form up, they cement, compact and accumulate. Limestone, for example, is made up of calcium carbonate crystals.

Biological Sedimentary rocks are formed when dead sea animals, corals and shells are compressed and cemented together over time. Also, plants and animals near a swamp can decompose, piling layer on layer. An example is coal.

Clastic Sedimentary rocks are clumped pieces of weathered rocks, such as pebbles, sand, gravel, silt and clay. The layers of pieces are piled one on top the other exerting pressure, which compresses the pieces and cements the individual pieces together.

Metamorphic Rocks
Metamorphic rocks are sedimentary or igneous that have undergone immense pressure and heat. This changes the chemical composition and physical structure of the rock. This also destroys any fossils or original layers. These rocks are usually found in areas whee magma is rising, pressure zones (due to Earth movement) and are squeezed and heated by mountain building forces. Metamorphic rocks can be split into two categories: Foliated and Non-Foliated.

Foliated Metamorphic rocks are banded. Some examples are Slate, which have thin sheets that split easily, Schist, which is slate with more pressure, producing wavy bands, coarse grain and big crystals, and Gneiss, which is granite after high pressure and heat.

Non-Foliated Metamorphic rocks do not have layers. Examples are marble and Quartzite.



Denudation
Denudation refers to all processes that cause a reduction of rearrangement of landforms. Some of these processes include weathering, mass movement, erosion, transportation and deposition. They are usually done by agents such as moving water, air, ice and the pull of gravity.

Impacts of Denudation
Soil formation is long and slow, one inch of soil takes 500-1000 years to form. Although soil is constantly being formed, it is also being constantly being eroded.

Mechanical Weathering
Physical weathering refers to the breakdown of rocks to smaller fragments of the same rock insitu (in the same place, no movement).