particularly composition, tell us about the original
magma and how it evolved. Rocks that contain mainly dark,
ferromagnesian minerals such as
pyroxenes and olivine are mafic.
Rocks that contain mainly light, silica-rich minerals such as quartz
and feldspars are felsic.
Classification System is based on what minerals are found in
Texture of a rock
tells us about the environment in which the rock was
formed and the cooling rate.
indicates a very quick cooling rate. Silica-rich, felsic rocks can form
glassy texture easier than mafic rocks, however there are mafic rocks
that do it too. Rocks with glassy texture include obsidian, pumice,
tachylite and scoria.
show different types of grain size. Those are classified as
coarse-grained, medium size, and fiine-grained. The larger the mineral
grains, the slower was cooling of magma at the time when the minerals
formed. This is a large group of rocks that contains granite, rhyolite,
basalt, gabbro, and many, many others - the vast majority of all
Fragmental textures are
formed in the air from volcanic debris and ash. These include
hyaloclasite, volcanic breccia and pyroclastic rocks such as tuff.