Rocks Flashcards
Igneous Rocks
Formed from molten magma or lava. About 85% of the crust of the earth is igneous.
Two igneous rocks:
1) GRANITE
2) BASALT
Granite
A plutonic igneous rock, i.e. it has solidified slowly at depth in the earth. Large grained.
Basalt
A volcanic igneous rock. Lava. It’s cooled quickly. Fine grained.
Sedimentary Rocks
Formed from sediments in the calm parts of rivers, lakes, seas, under high pressure and temperature over long periods of time. They underlie 80% of the earth’s visible crust.
Sedimentary Rocks may contain…
oil, gas, coal.
Layered or stratified. Also may be cemented by minerals carried by water.
Examples of Sedimentary Rocks:
Sandstones, shale, limestones. May contain fossils.
Metamorphic Rocks
Igneous and sedimentary rocks may metamorphose, i.e. change their chemical nature due to reactions with chemically active fluids and/or heat and pressure.
Examples of Metamorphic Rocks:
Slate, marble, schist, gneiss.
Weathering
It is the process whereby rock breaks down into soil, usually over a long period of time.
Types of weathering:
Physical, mechanical, chemical and biological.
Example of physical weathering:
Excessive heat, such as bush fires, can cause rocks to fragment due to stress variations across the parent material.
The Freeze-Thaw cycle
along with excessive heat, this cycle is very damaging to stone. Water seeps into cracks in the rock. Water expands on freezing and will cause the cracks to become larger. The ice melts and the enlarged cracks then fill up with water. The process is repeated until eventually the rock disintegrates.
Mechanical Weathering is caused by the action of…
water, rain, glaciers and wind.
Chemical Weathering
Oxygen and carbon dioxide from the atmosphere, and organic acids from soil, dissolve in water and react with rock to form at various rates new compounds such as clay colloids, silica, carbonates and iron oxides.
