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What Are the 3 Types of Rocks?

Geology | Rock and Minerals

Types of Rocks

Earth is covered with these 3 types of rocks are:

  1. Sedimentary rocks
  2. Metamorphic rocks
  3. Igneous rocks

Here’s how they are different from each other. And how they form. Find out more below.

Types of Rocks

1. Sedimentary Rocks

Sediment Size

The key ingredient to sedimentary rocks is sediments. Sediments are just rocks that have been broken down by weathering. Sedimentary rocks form from two key processes:

  • First, compaction squeezes material together.
  • Second, cementing glues the squeezed material together.

If you start with a big boulder and speed up time 10,000 years ahead, it would erode away into tiny, little sediments. These weathered sediments are the ingredients that cement into sedimentary rocks like sandstone, limestone, and shale.

If you look at the Earth’s crust, only 5% of it is composed of sedimentary rocks. But 75% of land surface contains sedimentary rocks. So, it’s rare to find sedimentary rocks at the ocean bottom.

Clastic Sedimentary Rocks

If you take the term “Clast”, all it means is just sediments. So clastic rocks form from compacted sediments. Also, clastic rocks are inorganic. In other words, they don’t contain any living material.

There are different types of clastic rocks. For example, we can branch off into conglomerate, breccia, sandstone, siltstone and shalestone rocks.

  • “Conglomerate” have a variety of size sediments such as boulders, sand and clay. One key characteristic is they are rounded and smoothed.
  • “Breccia” also has different size sediments all compacted and cemented together. Instead of being rounded, sediments are jagged.
  • “Sandstone” are clastic sedimentary rocks that have compacted sand grains.
  • “Siltstone” is compacted silt.
  • “Shalestone” is compacted clay particles.

Organic (Crystalline) Sedimentary Rocks

Organic and crystalline rocks don’t necessarily form from compacted sediments. Instead, it involves some type of chemical process.

  • “Evaporates” are crystalline rocks that form from evaporation. For example, this process occurs in oceans, lakes, or other standing bodies of water. It has salt dissolved in it. When seawater evaporates, it stays in the form of rock salt crystals. For example, gypsum, anhydrite, and halite are evaporites.
  • “Precipitates” are crystalline but do not involve evaporation. It involves precipitation and crystallizes at the bottom of a body of water.
  • “Biological matter rocks” or “bioclastic rocks” formed from compacted biological materials. For example, coal forms from compacted plant remain. There are also rocks that form from shells which are compacted sea creatures.

2. Metamorphic Rocks

Metamorphic Rock

The process of formation of metamorphic rocks starts with existing rocks. Then, they undergo some sort of change due to immense heat or pressure.

For example, a rock made of sand is “metamorphosed” into another type of rock when it comes in contact with intense heat. They can’t melt because then it would be heading for the igneous state.

When it’s metamorphosed, it’s soft and pliable like cookie dough. This intense pressure for metamorphic rocks comes from inside Earth. Earth’s major mountain chains contain metamorphic rocks because it’s at plate tectonics boundaries where this intense pressure exists.

Contact Metamorphic Rocks

Contact metamorphism involves existing rocks coming into contact with intense heat. This heat generally comes from lava or magma.

When layers of rocks come in close contact with magma, they can undergo metamorphosis into another type of rock.

And this usually happens because magma plumes move to the upper part of the crust.

So, contact metamorphism involves the surrounding rock being burned from intense heat. Heat ranges from around 300° to over 800°C.

Regional Metamorphic Rocks

Instead of from heat, the key catalyst for regional metamorphism is mostly from pressure. For example, when there are two convergent plates pushing together, there will be immense pressure at the fault in between.

It’s at this fault in the middle where rocks will undergo regional metamorphism. For example, gneiss is a metamorphic that forms due to intense pressure. Gneiss is known for having bands where all the layers are squeezed.

Then, if you are even more pressure to gneiss, of would melt into igneous rocks. Gneiss is usually the extent of metamorphism that we see.

3. Igneous Rocks

Magma Lava Difference

Igneous rocks form after cooling and solidifying from magma or lava. So when molten rock cools, they turn solid and become igneous rocks. First, you have to understand the difference between lava and magma. And it’s all about location.

For a volcano when it’s deep inside the Earth, it’s magma. Once it erupts, it’s lava. Then, it cools and solidifies. But it matters where it cools and solidifies.

Because of the immense pressure that igneous rocks undergo, it erases any fossil evidence. Likewise, this is true for the immense heat from metamorphic rocks. So if you see a rock with a fossil, you can automatically assume that it is a sedimentary rock.

There are two types of igneous rocks. Based on texture and composition, we can classify them into two categories:

  • Intrusive (or plutonic) rocks form inside the Earth from magma.
  • Extrusive rocks cool and solidify on the surface.

Intrusive (Plutonic) Igneous Rocks

For intrusive (or plutonic) rocks, magma is hot inside Earth so it cools slowly. When it cools, minerals form slowly.

So crystals grow very large for intrusive igneous rocks.

For example, gabbro, granite, and pegmatite are intrusive rocks.

Extrusive Igneous Rocks

For extrusive rocks, because they cool quickly, the crystals are smaller.

Some types of extrusive igneous rocks can even have pockets of air bubbles inside them.

For example, extrusive rocks include basalt, andesite, and obsidian.

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