Just as biologists need to distinguish between different kinds of plants and animals, geologists depend on being able to classify different types of rocks. It turns out that the best way to classify rocks is based on the process in which they were formed. There are three main processes that result in the three overarching categories of rocks. These categories, which we will explore in detail, are igneous, sedimentary, and metamorphic rocks.
Minerals are the building blocks of rocks, but they are not rocks in and of themselves. Rocks are aggregates of different kinds of minerals. Granite is a classic example of this. The distinct granules of color in granite are different kinds of minerals. Together, those mineral crystals make the resulting granite rock.
Each mineral has a unique chemical composition that influences the characteristics of the resulting mineral. For example, quartz is silica dioxide. Quartz is always hard and either transparent or translucent. Other minerals have their own characteristics.
Some examples of other common minerals include feldspar, mica, and carbonate.
The super-hot center of our Earth is liquid magma, or molten rock. When this magma cools as a result of getting closer to the Earth’s surface, it forms igneous rocks. Just as water turns to ice in the freezer, liquid magma turns into solid rocks when it comes into contact with cooler temperatures. The inside of the Earth is about 2500 degrees Fahrenheit, so anywhere near the Earth’s crust is very cold in comparison.
There are two main categories of igneous rocks; intrusive and extrusive.
It is a bit easier to imagine the formation of extrusive igneous rocks. These rocks form when lava (which is magma when it reaches the surface of the Earth) cools into rocks. Volcanoes form extrusive igneous rocks when they erupt.
The lava cools from a liquid to a solid incredibly quickly during this process. This quick cooling results in smaller mineral grains within the rock. In general, if an igneous rock has mineral crystals that are smaller than 1mm, it is an extrusive igneous rock.
This rapid cooling can result in unusual rocks. Volcanic glass has no mineral crystals because the lava cooled so fast none were able to form. Pumice is a rock that floats because air was trapped in the lava and wasn’t able to escape before the lava cooled. These air pockets are bubbles in the rock that allow it to float. Other examples of extrusive igneous rocks include pumice, basalt, andesite, rhyolite, and obsidian.
Intrusive igneous rocks, on the other hand, form from the cooling of magma below the Earth’s surface. This crystallization happens in hotter environments. Since intrusive rocks form more slowly, their mineral crystals are larger than 1mm. Examples of intrusive igneous rocks include diorite, gabbro, and granite.
While lava and magma are the basis of igneous rocks, sediments are the building blocks of sedimentary rocks. Sediments are small bits of rocks caused by the erosion, or breaking down, of other rocks. Water and wind cause this erosion, which is sometimes called weathering.
Sediments can be incredibly small or as large as boulders. The size of the sediment granules determines the resulting sedimentary rock. Sediment types in order from smallest to largest are clay, silt, sand, pebble, cobble, and boulder. These ‘boulders’ are anything over 25cm, or about one foot. Clay, on the other hand, is any sediment smaller than 1/256th of a millimeter.
After eroding from their parent rocks, these sediments are either pressed together or glued together to form sedimentary rocks. This process is lithification.
Clastic sedimentary rocks form from compacted (pressed together) or cemented (glued together) sediments. Sediments compact when layers and layers of sediment get deposited over the same area. The weight of the upper layers presses down on the weight of the lower layers, which compacts the lower layers into rocks.
Cementation happens when water flows through compacted sedimentary rocks. Water is almost never pure because it usually carries minerals as it flows. As this minerals-laden water flows through compacted sediment, some of the minerals in the water settle into the pores in the rock. This process is called cementation. This is also how fossils form, which is why sedimentary rocks are the only rocks with fossils in them.
If a clastic sedimentary rock is formed from clay-sized particles, it is called shale. Sand-sized particles make sandstone, silt-sized result in siltstone, and larger particles make conglomerate rocks.
These are the only rocks made from life itself! Organic sedimentary rocks form in the same way as clastic rocks, but the parent ‘rock’ isn’t a rock at all. Instead, the parent material is a type of life. Examples include crushed up seashells and layers upon layers of dying plants or algae. Seashells result in limestone and the dying plants create coal.
Precipitate and evaporate types of sedimentary rocks are a little more difficult to understand. Evaporate rocks occur when mineral-laden water consistently evaporates in the same place over and over again. When the water evaporates it leaves behind any minerals it was carrying. Over a long period of time, these minerals build up to create layers of sediment, which then compress into rocks.
An example of evaporate rocks is halite. As salty water evaporates, it leaves behind salt (NaCl), which over geologic time becomes halite.
Precipitate rocks occur when minerals fall out of a body of water. Water can only carry a certain amount of minerals. If the water has any more minerals than it can carry, these minerals fall to the bottom of the body of water. Again, over time this precipitation of minerals creates layers that compact each other. Gypsum forms through precipitation.
The final category of rocks is metamorphic rocks. Like sedimentary rocks, metamorphic rocks form from parent rocks. When rocks are heated or squished hard enough through pressure, they change into metamorphic rocks. The two methods of metamorphic rock formation are contact and regional metamorphism.
Have you ever gotten burned by touching something really hot? Did your skin blister and look a little bit different where it was burned? This is basically how contact metamorphism works. When lava or magma comes in contact with existing solid rock, the liquid magma superheats the rock that it is touching. This extreme oven created by the magma bakes the existing rock into something different. That existing rock metamorphizes into a contact metamorphic rock. Contact metamorphism requires lots of heat.
Regional metamorphism requires pressure instead of heat. There is an incredible amount of pressure where two tectonic plates converge. This pressure squeezes the existing rocks so hard that they metamorphize into regional metamorphic rocks.
Sometimes metamorphic rocks form by a combination of both regional and contact processes.
Geologists put metamorphic rocks on a scale from low grade to high grade. The lower the grade of rocks means the metamorphosis happened with less temperature and pressure. The higher-grade rocks experience higher temperatures or pressure. The types of rocks that form (in order from low to high grade) are called slate, phyllite, schist, and gneiss. Typically, with any additional pressure or heat, gneiss will melt and turn back into magma.
The rocks on Earth are constantly changing, but the amount of minerals and magma on Earth is not. This is because rocks go through the rock cycle. This cycle isn’t the same for every rock, but it always starts with rocks being born as igneous rocks. They are then weathered into sedimentary rocks or smushed into metamorphic rocks. At some point, they will melt back into the Earth’s mantle and become igneous rocks again one day.
Quartzite is a great example of the rock cycle. When quartz erodes from an igneous rock it sometimes makes a sedimentary rock of pure quartz sediment. This pure quartz sandstone can then undergo metamorphic processes, which turn it into quartzite. Therefore, any quartzite has, at one point, been magma, igneous, sedimentary, and metamorphic.
Geologists have named hundreds of types of rocks. Knowing the basic methods of formation for igneous, metamorphic, and sedimentary rocks is only the surface of an entire field of study.
The types of rocks in an area can give us clues into what Earth has looked like throughout time. For example, metamorphic rocks can act as the writing in a book about our Earth’s tectonic past. By knowing the depths, temperatures, and ages of different types of metamorphic rocks form at, scientists can piece together the development of Earth’s plate tectonics. Geologists can learn all sorts of surprising insights by examining these rocks!