What are the subgroups of rocks?

Answer 1

All rocks are classified into three groups: Igneous, Metamorphic, and Sedimentary. Igneous rocks form from the cooling of magma or lava. Igneous rocks divide into intrusive igneous rocks, and extrusive igneous rocks. Intrusive igneous rocks form from magma and cool slowly, thus having large crystals and a chopped up texture. Extrusive igneous rocks form from lava and cool fast, thus having small or no crystals and a fine grained or glassy texture. An example of an intrusive igneous rock would be granite, porphyry, or pegmatite. An example of an extrusive igneous rock would be basalt or obsidian. Metamorphic rocks are rocks changed by heat and pressure over time. For instance, shale, with heat and pressure, becomes slate. Limestone, with heat and pressure, becomes marble. Granite, with heat and pressure, becomes gneiss (pronounced "nice"). Sandstone, with heat and pressure, becomes quartzite. Metamorphic rocks divide into two groups: Foliate and non-foliated. Foliated is a term used to describe metamorphic rocks that have grains arranged in parallel layers or bands. An example of that would be gneiss (again, pronounced "nice"). Non-foliated is a term used to describe metamorphic rocks that have grains arranged randomly. For example, marble is non-foliated because it has no pattern to it's grains. Sedimentary rocks are types of rocks that form when particles from other rocks or the remains of plants and animals are pressed and cemented together. When sedimentary rocks are formed, they undergo a series of processes.

Erosion - the destructive process in which water or wind loosens and carries away fragments of rock.

Deposition - the process by which sediment settles out of the water or wind that is carrying it.

Compaction(creates layers of compacted rock) - the process by which sediments are pressed together under their own weight.

Cementation - the process by which dissolved minerals crystallize and glue particles of sediment together into one mass.

Sedimentary rocks divide into two main subgroups. Clastic and non-clastic. Clastic sedimentary rock is sedimentary rock that forms when rock fragments are squeezed together under high pressure. These clastic rocks are grouped by the size of their fragments(or grains). Non-clastic is either organic or chemical. Organic rock is sedimentary rock that forms from the remains of organisms deposited in thick layers. Chemical rock is sedimentary rock that forms when minerals crystallize from a solution. The solution in chemical rock is calcium carbonate. Examples of organic rocks are coal, and limestone. Coal is formed in swamps from plants. Limestone is formed from living things in the ocean. An example of a chemical rock would be halite(rock salt). Another example would be some types of limestone. Examples of clastic rocks would be sandstone, shale, and conglomerate.

Just as an afterthought, I'll add for fun how coral reefs are formed. They are made from the skeletons of small coral animals. This makes them organic sedimentary rocks. Coral animals absorb calcium from the ocean and combine it with oxygen and carbon to form calcite, which is why their skeletons are so special. Coral reefs grow slowly over time, and are normally in warm water and close to shore. They are a precious resource(as A LOT of ocean organisms depend on them) and they take years and years to build. They help scientists understand how the earth used to be, which is very valuable to our history.

I'll also include a little part about the rock cycle.

Igneous rocks can be formed when plates collide and rocks are pushed so deep that they melt and form magma, or they can be formed when a volcano forms.

Sedimentary rocks can be formed when plates collide and uplift. Over time, erosion leads to sediment, and sedimentary rock.

Plates can also push rocks down where heat and pressure create metamorphic rocks. When metamorphic rocks are changed so many times, they are pushed into the mantle(part of earth's inside), and become molten magma, which eventually leads to an intrusive or extrusive igneous rock.

This is why you can trust me: I get really good grades in science.