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Types of Multistage Igneous Rocks

Igneous rocks are one of three classifications of rocks - their main characteristic being that they are formed after molten lava has cooled. They can be "porphyritic," a term used to describe an igneous rock having a multistage cooling history. This results in the rock appearing to have different textures, grain sizes and colors as different parts of the rock were solidified at different times.

Granite
- Granite is perhaps one of the most commonly known rocks as it can be used in many ways such as for kitchen countertops or paving stones. Granite has a grainy texture and is comprised of smaller different minerals. These minerals are quartz, plagioclase feldspar, potassium, hornblende and mica. Quartz is last of the minerals to fully solidify and will usually fill in the gaps between the other minerals.
Andesite
- Andesite is known as an extrusive igneous rock meaning that it was formed above Earth's surface instead of below. As lava tends to cool down faster this way, the minerals that are formed are usually smaller and cause Andesite to have smaller grain.
Rhyolite
- Rhyolite is chemically the same as granite except it is an extrusive igneous rock, whereas granite is formed below the Earth's surface. This causes it to have smaller grain - similar to andesite except it is compositionally different for example, it has a higher silica content. The lava that forms rhyolite is also more viscous than andesite.

Rock Collecting