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A rock is a naturally occurring mass of inorganic or organic material that forms a significant part of the earth's crust.
A rock can also be defined as a solid substance that occurs naturally because of the effects of three basic geological processes: magma solidification; sedimentation of weathered rock debris; and metamorphism.
From the physical properties of rock, earth scientists can determine:
 The general type of environment in which the rock formed (volcanic, tropical marine, arid terrestrial, etc.)
 The general water depth (if marine or fresh water)
 The relative age of the rock (using fossils) or possibly its absolute age (if radioactive minerals are present)
 Its original position and orientation on the surface of earth
TYPES OF ROCK
Igneous rocks - produced by solidification of molten magma from the mantle.
Sedimentary rocks - formed by burial, compression, and chemical modification of deposited weathered rock debris or sediments at the earth's surface.
Metamorphic rocks - created when existing rock is chemically or physically modified by intense heat or pressure.
ROCK TYPE AND CLASSIFICATION
Igneous Rock - Rocks formed by crystallization from a melt (magma)
1.
Extrusive (volcanic) - produced when magma flows on the earth's surface
2.
Intrusive (plutonic) - produced when magma solidifies at depth beneath the earth
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Rate of cooling & temperature changes determine the texture & degree of crystallization while the composition of the magma will determine the composition of the igneous rock
Sedimentary Rocks -Layered or stratified rocks formed at or near the earth's surface in response to the processes of weathering, erosion, transportation and deposition.
Sedimentary rocks are generally stratified, fine-grained or composed of fragments of older rocks
Formation of sedimentary Rocks:
Sediment = loose particulate material (clay, sand, gravel, etc.)
Sediment becomes sedimentary rock through lithification, which involves: Compaction, Cementation, Recrystallization

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Characteristics of Sedimentary Rocks
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Stratified because the sediments are laid down in horizontal layers called strata
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May also contain fossil i.e: remains, prints or other indications of plants & animals found buried in rocks
Metamorphic Rocks
"Metamorphism" comes from the Greek: Meta = change, Morph = form, so metamorphism means to change form.
 refers to the changes in mineral composition and texture that result from subjecting a rock to pressures and temperatures
Characteristics of Metamorphic Rocks:
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Harder and more compact than the rock they are made from.
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Minerals are arranged in bands or recrystallised to form new or larger minerals
Do Rocks Always Stay the Same?
• James Hutton: a farmer and Scottish geologist developed the concept of the rock cycle.
• He proposed that rock changes from one form to another over time, the matter making up rocks is neither created nor destroyed but simply changed
How does it work?
• Most surface rocks started out as igneous rocks- rocks produced by crystallization from a liquid.
• When igneous rocks are exposed at the surface they are subject to weathering
• Erosion moves particles into rivers and oceans where they are deposited to become sedimentary rocks.
• Sedimentary rocks can be buried or pushed to deeper levels in the Earth, where changes in pressure and temperature cause them to become metamorphic rocks.
• At high temperatures metamorphic rocks may melt to become magma.
• Magma rises to the surface, crystallize to become igneous rocks and the processes starts over.
So why does this matter?
• Knowledge of rocks fundamental to study of earth sciences
• Forms the basis for the study of plate tectonics