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Rocks are classified into three major groups based on how they formed
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Igneous Rocks
• Can be divided into two categories based on where they form

A. Intrusive Rocks
– igneous rocks that form underground from magma
– Cool slowly allowing crystals to grow large giving it a coarse-grain texture

B. Extrusive Rocks – igneous rocks that form at the surface from lava
– Cool quickly resulting in the crystals to be small giving it a fine grain texture

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II. Sedimentary Rock – rock that forms over time as sediment is squeezed and cemented together
– Sediment
– small, solid pieces of material that comes from rocks or living organisms
• Sediment is deposited in layers by running water or wind
• Sediment is cemented together by pressure from overlying layers
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Three Main Groups of Sedimentary Rocks:
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1. Clastic
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2. Chemical
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3. Organic

1.
Clastic Rock
– sedimentary rocks that form from the broken fragments of other rocks
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Clastic Rocks are classified based on the size of their fragments
Example: Conglomerate
– rock fragments that consist of gravel and pebbles

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Some clastic rocks are made of smaller particles ( sandstone)

2. Chemical Rocks
– rocks that form when minerals precipitate out of solution
– Example: when ocean water evaporates
(halite)

3.
Organic Rocks
– rocks that form as a result of organic processes
Shells of marine animals like corals, clams, mussels sink to the ocean floor when they die.
– Example: Limestone and calcite (chalk)

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III. Metamorphic Rocks
– rock that have been changed by temperature, pressure, or chemical reactions.
– Most metamorphic rocks form under high temperatures and pressures deep underground.
(slate and schist– shown below)

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Foliation: a layering of minerals in a rock due to intense pressure squeezing the crystals together.

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Metamorphic Rocks like gneiss show banding of light and dark minerals.

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Examples:
– Slate: parent is shale
– Gneiss: parent is granite/schist
– Quartzite: parent is sandstone
– Marble: parent is limestone

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Rock Cycle – series of processes in which rocks continuously change from one type to another.