Rock Types and the Rock Cycle

In preparation for my upcoming Plate Tectonics Week, I'm presenting the rock types and rock cycle- the basic ideas of geology. For each rock type, I'll be presenting a non-comprehensive list of traits used to identify and classify rocks. Even for skilled geologists, identifying rocks can be an incredibly difficult task, and often requires a certain flexibility. It's better to think of a rock like granite or sandstone as a category for rocks, rather than as a specific thing.

The Rock Types

Igneous Rocks

Columnar basalt, a spectacular type of igneous rock. [Image source]

Igneous rocks are volcanically erupted rocks. They are by far the most common rocks on Earth by volume- over 65% of the Earth's crust are igneous. Oceanic tectonic plates are 99% igneous rocks. 

There are a few different ways to classify igneous rocks. First of all is their silica content. High silica content volcanic rocks are known as felsic rocks. Felsic magmas are known for their high viscosity and explosive eruptions- Mt. St. Helens was a felsic volcano. Low silica content volcanic rocks are known as mafic rocks. Mafic rocks have low viscosity and flow freely- the magmas of Hawaii are examples of mafic rocks. Most of the mantle is also composed of mafic and ultramafic minerals. Oceanic plates are composed of basalt, as are the Hawaiian magmas.

Another major method of classifying volcanic rocks is by eruptive settings- namely, if it was extrusive or intrusive. Extrusive lava eruptions are on the surface- they include basalt, andesite, rhyolite, and more. Intrusive lava occurs underground- examples include gabbro, granite, and diorite. They correspond to each other, however- gabbro is chemically identical to basalt, for instance, and granite is chemically identical to rhyolite. The difference is primarily driven by cooling time- as the intrusive rocks take much longer to cool, they form much larger crystals than the extrusive rocks, so there is a clear and visible difference between them.

Sedimentary Rocks 

Claystone deposited in an ancient lake. [Image source]

Sedimentary rocks are formed by the deposition and cementation of sediments. Sedimentary rocks are extremely diverse, and extremely common. Though it only makes up 8% of the Earth's crust, sedimentary rocks cover 73% of the Earth's land surface. There are quite a few ways of classifying sedimentary rocks.

First off, they can be classified by grain size. A sandstone is almost any sedimentary rock with a specific grain size. (Namely, sand sized.) Mudstones and siltstones have smaller grain sizes, and conglomerates and breccias have larger grain sizes. 

Next, they can be classified by composition. There are three primary types of grain that make up sedimentary rocks- quartz, feldspar, and lithics. Lithics are a mash-up category of anything that doesn't fit in the other two. The composition of the cement is also important- quartz, calcite, clays, and more can act as cement for the rock.

In addition, many sedimentary rocks are fossil-bearing. (They're the only fossil-bearing rocks.) Some are even entirely formed by biological means, like many limestones.

Metamorphic Rocks

Gneiss, a type of metamorphic rock. [Image source]

Metamorphic rocks are any rock of the other two types that are changed into new rocks by the addition of high heat and pressure. They make up around 12% of the Earth's land surface. They tend to be fairly hard and strong, and many of the minerals in them have altered their physical and chemical characteristics from the original rock, or protolith.

Metamorphic rocks tend to also have their own unique structures. Foliation is a sequence of layers formed by non-symmetrical stress applied to the rock. Gneissic banding is an extreme version of this where the rock aquires alternating black and white bands. 

Metamorphic rocks are classified in a few ways. First off, they can be classified by their protolith and chemical composition. Different protoliths will tend to produce different metamorphic rocks. Mudstones, for example, will often produce micas and mica-rich rocks after undergoing metamorphosis. Metamorphic rocks are also often classified by the types of metamorphic structures and foliation produced.

Metamorphic rocks are produced in a few ways. Intrusive volcanics often form metamorphic rocks- the country rock the magma contacts is often altered by the heat. This is known as contact metamorphism. Mountain building events often also produce metamorphic rocks due to the high pressure involved. There are plenty of other types as well.

Fossils from metamorphosed sedimentary rocks are usually destroyed in the process.

The Rock Cycle

[Image source]

The rock cycle is the idea that rocks change between the three types across geological time. Metamorphic, sedimentary and igneous rocks erode into sediment, which forms into sedimentary rock. Igneous or sedimentary rock undergoes metamorphosis, turning them into metamorphic rock. Any of the three can undergo melting and eventually become igneous rock. The Earth is continually recycling the rocks it is composed of- it's the main reason why rocks of older ages are much less common than younger rocks.

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Bibliography:

https://en.wikipedia.org/wiki/Rock_cycle

https://en.wikipedia.org/wiki/Metamorphic_rock

https://en.wikipedia.org/wiki/Sedimentary_rock

https://en.wikipedia.org/wiki/Igneous_rock