Rocks are classified into three main groups. Igneous rocks are rocks that crystallize as molten rock (magma or lava), cools, and hardens. Sedimentary rocks are formed from cemented rock fragments, chemical precipitates, and organic remains. Metamorphic rocks are formed when other rocks are altered by heat and pressure.
The rock cycle describes how rocks can be transformed from one type into another. For example, a sedimentary rock such as limestone can be transformed into the metamorphic rock marble if it is buried deep in Earth’s crust and subjected to high temperature and pressure. If an igneous rock such as granite is uplifted and exposed on Earth’s surface, the action of plants, rain, ice, and snow will break it up into sediments. The sediments will be smoothed and rounded as they are carried to the ocean, and eventually they may be deposited on a beach. Over time, the beach sand may be buried under other layers of sediment and cemented into sandstone.
Convection currents in Earth’s mantle provide the energy to keep the rock cycle moving. These convection currents cause the motion of tectonic plates. New crust is created in mid-ocean ridges and old crust is destroyed in subduction zones. Mountains are created along convergent and collisional plate boundaries. In these regions, rocks created deep below Earth’s surface are uplifted and exposed to the weathering forces of rain, sunlight, wind, ice, and plants. Over millions of years, these forces act to break down rocks into smaller particles,forming sediments and soil. The sediments are transported by rivers and streams to the ocean, where they are deposited in layers on the continental shelf. Over time, these layers are buried and harden into sedimentary rock.
Igneous rocks are the ones that were superheated and originally liquid. They often start their lives below the crust and then get pumped out. There are two basic types of igneous rocks. There are the rocks that make it to the surface(extrusive) and the ones that are stuck in the crust just below the surface (intrusive). These igneous types have all hardened after being molten rock. Some examples of igneous rock are granite, all volcanic rock, basalt, and obsidian.
This rock type is created by heat and/or pressure. Even though heat is involved, they didn’t start off as molten rock. You often find metamorphic rock near volcanoes and sources of super hot rock. The heat from the magma changes all of the rock around it. Heat/pressure has changed these rocks from one type into a new type. The result is a metamorphic rock. Some examples are marble, jade, slate, and gneiss. Because pressure and heat are involved, these rock types are usually found deep beneath the surface. They are also found near fault lines where plates push against each other and create enormous pressures. Over time, because of the movement of the crust, these metamorphic rocks are pushed to the surface where you can find them every day.
The last of the big three rock types is probably the rarest… unless you live near the coast. Sedimentary rock types are created when sediment compresses. Here’s the setup… A river flows through a canyon and picks up a bunch of silt. That sediment and silt runs downstream and deposits where the river ends. It could be in a flood plain or a valley, but we’re using a coastline as an example. When that material gets to the beach, it sits there. Now if you watch this happen over millions of years, more and more sediment builds up and compacts. That compacted sediment eventually becomes a type of rock. Examples of sedimentary rock include sandstone, amber, anthracite, and limestone.