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The rock cycle is the continuous process of the changing states of earth minerals. It describes the formation, breakdown, and reformation of a rock. To understand rock cycle, we must learn about types of rocks first.

This course will cover types of rocks and the rock cycle.

Types of Rocks

A rock is made of collection of one or more minerals. Some rocks are made from interlocking mineral crystals that fit tightly together, whereas others are made up from broken fragments, or grains, of older rocks and minerals which have been cemented together.

The mineral grains in a rock are so tiny that they can only be seen with a microscope. There are three types of rocks: igneous, sedimentary and metamorphic.

All of them are a part of the rock cycle (Figure 1).

Figure 1. Types of rocks in rock cycle
Figure 1. Types of rocks in rock cycle

Igneous rocks form when magma or lava cools. Their properties vary greatly based on how long the substance took to cool.

Sedimentary rock forms when sediments like clay build up and cement together. They are layered and often grainy.

Metamorphic rock forms when igneous or sedimentary rock gets pulled toward the mantle and is subjected to heat and pressure. They are crystalline.

Igneous Rocks

Igneous rocks (derived from the word Ignis means fire) are formed when molten hot material cools and solidifies (Figure 2).

Figure 2. Tungurahua Volcano eruption, Ecuador
Figure 2. Tungurahua Volcano eruption, Ecuador

Most of the crust is composed of igneous rock. Igneous rock can be intrusive or extrusive. When they are formed inside of the earth, they are called intrusive, or plutonic igneous rocks.

On the other hand, if  they are formed outside on Earth’s crust, they are called extrusive, or volcanic igneous rocks (Figure 3).

Figure 3. Intrusive and extrusive igneous rock
Figure 3. Intrusive and extrusive igneous rock

Intrusive Igneous Rock

This rock forms when magma does not break the surface. As the surrounding earth insulates it, the magma takes a long time to cool and solidify, allowing for large crystals to grow within the rock.

They have a coarse texture with large mineral grains. Granite is an example of this type of rock.

Extrusive Igneous Rock

This rock forms when magma erupts on the surface. These rocks cool quickly on or very near the surface of the earth.

An example is found where magma encounters cold ocean water at mid-ocean ridges and cools very quickly. Crystals do not form in this type of rock before it solidifies.

Basalt is an example of this type of rock (Figure 4).

The most common types of igneous rocks are granite, andesite, basalt, dacite, dolerite (also called diabase), gabbro, diorite, peridotite, nepheline, obsidian, scoria, tuff.

Figure 4. Studlagil basalt canyon, Iceland
Figure 4. Studlagil basalt canyon, Iceland

Sedimentary Rock

Sedimentary rock is formed as sediments from broken rock that are cemented together to form a new rock.

Sediment is a naturally occurring material, broken down by weathering and erosion and is subsequently, transported.

These rocks form through the deposition of material at the Earth’s surface and within bodies of water. They are laid down in layers.

These rocks might contain fossils from animals and plants that become trapped in the sediment before it becomes a rock.

Sedimentary rocks, such as sandstone, form from clasts, or pieces of other rock (Figure 5 ). Organic sedimentary rocks, such as coal, form from hard, biological materials like plants, shells, and bones.

Some common sedimentary rocks are argillite, breccia, chalk, chert, claystone, coal, conglomerate, dolomite, limestone, gypsum, greywacke, mudstone, sandstone, shale, siltstone, turbidite.

Figure 5. The antelope canyon
Figure 5. The antelope canyon

Metamorphic Rock

Metamorphic rock is formed when sedimentary, igneous, or other metamorphic rocks are subjected to heat and pressure from burial or contact with intrusive or extrusive igneous rocks (Figure 6).

Figure 6. Metamorphism (regional, dynamic, hydrothermal and contact forces)
Figure 6. Metamorphism (regional, dynamic, hydrothermal and contact forces)

Metamorphic rocks (Figure 7) can have crystals and minerals from the initial rocks as well as new minerals resulting from the metamorphosis process.

The most common metamorphic rocks are amphibolite, schist, eclogite, gneiss, hornfels, marble, migmatite, phyllite, quartzite, serpentinite, slate.

Figure 7. Marble stones
Figure 7. Marble stones

The Rock Cycle

The rock cycle is a process in which rocks are continuously transformed between the three rock types igneous, sedimentary and metamorphic (Figure 8).

Figure 8. The rock cycle
Figure 8. The rock cycle

As weathering, or gradual wearing away, of igneous rock occurs, the rock is broken down into smaller sediments. The sediment is transported by water and wind and get deposited in specific locations, such as a river delta.

A large buildup of a specific type of sediment can get buried by other layers. Over time, the layers can become cemented together to form a sedimentary rock, such as, sandstone or shale that forms from clay.

Metamorphic rocks are formed when igneous or sedimentary rocks are dragged down by subduction, when one tectonic plate along a convergent boundary moves under another.

These rocks are not pulled down into the mantle far enough to melt. The intense heat and the pressure due to the Earth’s mass and gravitational pull causes the rock to crystalize and change from one kind to another.

The rock can eventually be forced back to the surface (Figure 9).

Figure 9. Process involved in the rock cycle
Figure 9. Process involved in the rock cycle