– a rock is a collection of one or more minerals.
Ms. Hartnett's Earth Science 1

1. Most rocks have a number of minerals in common. In other words, only a small number of minerals are very common.
2. Most rocks are heterogeneous. This means that they are made of two or more different minerals and are not the same throughout.
3. Some (very few) rocks are homogeneous or monominerallic.
This means that they are made of only one type of mineral and they are the same throughout.
4. Rocks are divided into 3 major classes. They are sedimentary, igneous, and metamorphic.
5. Rocks are classified by how they formed or origin .
6. Structure, texture, and composition (what a rock is made of) help us to figure out the environment that the rock formed in (origin).
Ms. Hartnett's Earth Science 2

Ms. Hartnett's Earth Science 3

Sedimentary Rocks – rocks that formed from the accumulation of deposited rock particles, fragments, and organic material underwater. ( Remember, they are deposited in horizontal layers)?
There are 3 basic categories of sedimentary rocks:
1. Clastic
– the rock fragments are lithified (glued) together to form a rock. These rocks are made of different pieces stuck together. Conglomerate and Breccia have the largest particles.
2. Crystalline/Chemical - rocks that formed from ions (smallest particles) that were dissolved in the sea water. These rocks usually form as the ocean water evaporates away and leaves the dissolved minerals behind. These are also monominerallic.
3. Bioclastic/Organic – rocks formed from biologic products.
(Things that died and solidified together over time. Coal and limestone are great examples.
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Ms. Hartnett's Earth Science 5

rocks formed from the cooling and solidification of molten materials (magma). Whenever lava cools, an igneous rock forms.
There are two sub-classes of igneous rocks:
1. Intrusive/Plutonic – igneous rocks that cooled and solidified beneath the Earth’s surface (The magma cools underground).
These rocks take a very long time to cool because they are not on the surface. Because they take a longer time to cool, they end up having large crystals/grains.
2. Extrusive/Volcanic – igneous rocks that cooled and solidified on the Earth’s surface. Lava on the surface cools to form igneous rocks. These rocks cool very quickly and have very small crystals/grains or none at all (glass).
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Ms. Hartnett's Earth Science 7

– rocks that have been changed by the action of heat and pressure from occurrences inside the Earth.
Metamorphic rocks were once sedimentary, igneous, or a different type of metamorphic rock. Because of the heat and pressure of magma coming in contact with rock, the former rock changes into a new one.
The rock does not melt while it changes into a metamorphic, but it almost does. It just deforms and distorts. What kind of rock would form if rock melted and then re-cooled? ______________
There are two groups of metamorphic rocks:
1. Foliated
– alignment of minerals in paper thin layers or bands.
2. Non-foliated – no alignment of minerals.
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Ms. Hartnett's Earth Science 9