Question

Which describes granite and basalt?


A.


Granite is a coarse-grained intrusive igneous rock, while basalt is a fine-grained extrusive igneous rock.


B.


Both granite and basalt are fine-grained intrusive igneous rocks.


C.


Granite is a coarse-grained extrusive igneous rock, while basalt is a fine-grained intrusive igneous rock.


D.


Granite is a fine-grained extrusive igneous rock, while basalt is a coarse-grained intrusive igneous rock.

Answer 1

Basically the only thing they have in common is the fact that they are both igneous rocks.

Basalt:
- Forms on the surface of the earth
- Because it forms on the surface it cools quickly and has a fine texture (mineral grains are too fine to see with the naked eye).
- The source of this rock comes from partially melted material in the mantle.
- It usually leaves the mantle at mid-ocean ridges, where new seafloor is being formed. That's why most of the ocean crust consists of basalt or gabbro (the intrusive version of basalt).
- Because basalt comes from a mantle source, it's very mafic and consists of dark, dense minerals rich in iron and manganese (usually olivine and pyroxene).

Granite:
- Forms underneath the surface of the earth.
- Because it forms under the surface the magma cools slowly, grains have time to grow and therefore it has a coarse grained texture. Grains can be easily seen with the naked eye.
- Granite forms when a part of the continental crust melts to form magma and solidifies again. The heat needed for this to happen can come from different sources, for example magma from the mantle which causes the crust to melt.
- Because of the above granite will be found on the continental crust (mostly at least).
- The crust consists of lighter minerals than deeper parts of the earth, and that is why the minerals you will find in granite will be lighter, less dense and richer in SiO2 than those found in basalt (granite is therefore a much more felsic rock). Minerals you will typically find is quartz, orthoclase and plagioclase.