Rocks are naturally formed, solid bodies made of minerals. But not all rocks are alike. They come in three main types: igneous, sedimentary, and metamorphic. Each of these three rock types forms in its own unique way. But here’s the really cool part: rocks can transform into different types! This happens really slowly over a long time through something called the rock cycle.

The Rock Cycle

The rock cycle is a special journey that all rocks are on. The rock cycle allows each of the three rock types to become other rock types. There is no “start” or “end” to the rock cycle – it is a continuous process that all rocks are a part of.

Let’s start our journey through the rock cycle on the surface of the Earth, where forces like water and shifting temperatures work endlessly to break rocks down. The broken down pieces of rocks come together to create new rocks, known as sedimentary rocks.

Next, imagine our rock’s journey takes it deep underground. Rocks can move underground due to plate tectonic activity. When a rock is buried deep in the Earth, it is under a lot of pressure from above and starts getting much warmer from the heat inside the Earth. The intense pressure and heat cause big changes in a rock’s structure and a metamorphic rock is formed.

However, when heat and pressure become too intense, a rock will undergo an even more dramatic transformation. The rock will melt, becoming molten rock.  Later, when molten rock cools, it solidifies and becomes our third rock type: igneous rock.

Let’s now explore each of these rock types a little further!

Igneous Rocks

There are two main subtypes of igneous rocks: intrusive and extrusive. The main difference between them is where they form. Igneous intrusive rocks form within the Earth while igneous extrusive rocks form on the Earth’s surface.

Igneous Intrusive

More specifically, an igneous intrusive rock starts as magma, which is what we call molten rock located underground. The magma is insulated deep within the Earth, so it cools very slowly. The slow cooling allows plenty of time for large interlocking crystals to form. Granite, diorite, and gabbro are examples of igneous intrusive rocks.

Igneous Extrusive

In contrast, igneous extrusive rocks form when hot lava cools on the Earth’s surface such as when a volcano erupts. Lava cools very quickly on the surface, so there is not enough time for big crystals to form. If you are trying to decide if an igneous rock is intrusive or extrusive, texture is your biggest clue! While igneous intrusive rocks are always made of large interlocking crystals, igneous extrusive rocks come in a variety of other textures: fine-grained, glassy, bubbly and more! Basalt, rhyolite, and pumice are igneous extrusive rocks.

Sedimentary Rocks

Sedimentary rocks form at or near the Earth’s surface.  They form from deposits of sediments, minerals, and/or organic matter.

Sedimentary Clastic

When you picture sedimentary rocks, chances are you picture a sedimentary clastic rock, such as sandstone, conglomerate, and shale. Sedimentary clastic rocks form from tiny solid particles of rock called sediments. Sediments are made through a process called weathering, in which rocks break down over time into smaller pieces. These pieces are moved around by forces like flowing water, wind, and gravity. Eventually, sediments come together and new rocks are formed through compaction and cementation. Compaction happens when sediments are buried and get pressed into a rock by the weight above. Cementation happens when water evaporates, leaving behind minerals that act like cement, holding the sediments together.

Sedimentary Chemical

However, clastic rocks are not the only sedimentary rocks. There are also sedimentary chemical rocks that are formed by mineral precipitates. What does that mean, you ask? Well, imagine stirring sugar into a glass of water until it disappears. Although you cannot see the sugar, it is still there, dissolved in the water. Like sugar, many minerals dissolve in water too! Most liquid water on Earth contains dissolved minerals.

But how do those dissolved minerals become rocks? Picture an ancient ocean, with water rich in dissolved minerals. As time passes, the ocean begins to shrink, eventually drying up completely. When the water evaporates, the minerals from the water recrystallize to form sedimentary chemical rocks. This process, called precipitation, is the opposite of dissolving. Sedimentary chemical rocks are crystalline but often look fine-grained. Some are made of carbonate and react strongly to acid. Rock salt and limestone are two examples of sedimentary chemical rocks.

Metamorphic Rocks

Metamorphic rocks form when other rocks experience lots of heat and pressure underground. During the transformation, the minerals in the parent rock to reorganize into a more compact structure. This can only happen when the amount of heat and pressure are just right. If the heat and pressure grow too high, the rock will melt into magma instead.

There are two types of metamorphic rocks: foliated and nonfoliated. With a little practice, you can learn to tell them apart.

Metamorphic Foliated

Metamorphic foliated rocks have bands or layers. Some, like gneiss, have alternating bands of light and dark crystals. Others, like phyllite and slate, look like they are made of sheets or layers. Foliation forms when a rock is being squeezed hardest in a particular direction rather than from all sides. This may happen when a rock is buried deep underground. The directed pressure forces the minerals to all line up in the same direction, creating the bands or layers.

Metamorphic Nonfoliated

Metamorphic nonfoliated rocks form when the pressure comes from all sides or if the minerals do not line up. Consequently, nonfoliated rocks do not have bands or layers. Instead, nonfoliated rocks are usually a single solid color and made of interlocking mineral crystals. These crystals can be quite large or appear fine-grained. Some nonfoliated rocks, like marble, are prized for their strength and beauty, which makes them popular in construction and fine arts.

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References and Further Reading

1. Earle, S. (2019). Physical Geology. BCcampus Open Education. (Chapters 3-7). Available: https://opentextbc.ca/physicalgeology2ed/
2. Khan Academy (n.d.). Weathering and Erosion (video). Middle School Earth and Space Science. Available: https://www.khanacademy.org/science/middle-school-earth-and-space-science/weathering-and-erosion
3. Johnson, C., Affolter, M.D., Inkenbrandt, P., & Mosher, C. (2017). An Introduction to Geology. Salt Lake Community College. (Chapters 4-6). Available: https://opengeology.org/textbook/
4. Plummer C.C., Carlson D.H., & Hammersley L. (2019). Physical Geology. McGraw-Hill Education. (Chapters 3-7).

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