- Rocks with a aphanitic texture usually form from lava which crystallizes rapidly on or near the Earth's surface. Because extrusive rocks make contact with the atmosphere they cool quickly and so their crystals have little time to grow.
- Aphanitic textures mean that the rocks crystals are not distinguisable to the naked eye. Examples of this texture are basalt, andesite and rhyolite.
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- This texture occurs during a volcanic eruption and lava is cooled so quickly that crystals do not have time to form. This results in a glass like rock that has little or no crystals such as obsidian and pumice.
- This rock texture is commonly found in volcanic areas that are beside or under the sea, where the magma that escapes is instantly cooled.
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- This texture is basically the opposite of glassy. This texture occurs when magma takes so long to cool that the crystals that form become massive (the sizes can range from a few centimeters to several metres).
- This is typical of pegmatites.
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- This texture occurs when rocks crystallize slowly below the Earth's surface. As a magma cools slowly, the minerals have time to grow to form large crystals.
- This texture is the opposite of aphanitic textures (card 1) because the crystals are visible to the naked eye.
- Example of rocks with these textures are gabbro, diorite and granite.
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- Occurs when rates of cooling change relatively quickly.
- The earlier crystals grow slowly and remain large, whereas sudden cooling causes the rapid crysalitzation of the remaining lava and forms a smaller set of crystals.
- This usually occurs when magma slowly cools under the surface, thus developing large crystals however an eruption occurs and the magma is erupted and cools quickly.
- Basically a rock with two different size crystals in it.
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- These textures occur whe explosive eruptions blast the lava into the air resulting in fragmental, typically glassy materical which fall as volcanic ash, lapilli and volcanic bombs.
- So, the volcanic bombs that are shot into the air during eruptions are actually blocks of newly cooled lava rather than older chunks of rock.
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