How do we classify rocks?

Article by Rezolutions Design

Any good rockhound is bound to have a rock or two that they're particularly proud of. It might be a stunning specimen of amethyst, a beautiful piece of petrified wood, or a curious-looking rock that they just can't quite identify. Rocks can be really cool - they come in all sorts of shapes, sizes, and colors and can be found just about anywhere.

But how do we classify rocks? What makes one rock different from another?

Well, to classify a rock, you must think like a geologist! Geologists look at many different features to determine a rock's type. The most important feature is the rock's composition, or what the rock is made of. Rocks are classified into three groups based on their composition: igneous, sedimentary, and metamorphic. So, how do geologists know which rocks are igneous, sedimentary, or metamorphic?

Let's take a closer look.

What Is A Rock?

A rock is an aggregation of different mineral materials bound together by physical or chemical forces. This means that a rock must possess structural integrity to be considered a rock and not just a loose pile of minerals. For instance, sand aggregates would not be considered rocks because they lack this structural integrity. However, if the sand is cemented or otherwise bound together, then it can be classified as a sandstone.

The force that binds the different minerals together can be either chemical or physical. Chemical binding agents include hydration (water molecules chemically bonding with other minerals), precipitation (minerals coming out of solution and binding together), and adsorption (molecules binding to the surface of other minerals). An example of a rock that is held together by a chemical binding agent is limestone, which is precipitated by the presence of calcium carbonate in water.

On the other hand, physical binding agents do not involve any chemical reactions. Instead, they are typically the result of interlocking mineral crystals or the pressure of overlying rocks.

These strong forces are formed when minerals are welded together by the heat from molten lava or when rocks are crunched and mashed together by the forces of plate tectonics. In this case, no new minerals are formed, but the existing minerals are simply rearranged and intergrown with one another.

When classifying rocks, you must determine how the rock was formed, their chemical and mineral composition, and what other rocks they are similar to.

This will give you clues as to what kind of minerals are present and how they are arranged. The three main rock types - igneous, sedimentary, and metamorphic - are all classified based on their geological formation processes.

Geological Classification Of Rocks

Rocks can be classified based on their formation process. The three main categories are igneous rocks, sedimentary rocks, and metamorphic rocks.

1.   Igneous Rocks

Igneous rocks are rocks that form when hot, molten rock (magma) cools and hardens. This can happen either below the Earth's surface (intrusive/plutonic igneous rocks), on the surface (extrusive igneous rocks), or in between (subvolcanic igneous rocks).

Intrusive or plutonic igneous rocks form when magma slowly cools below the Earth's surface. Because the magma cools slowly, the minerals have time to grow large and well-formed crystals. Ordinary plutonic rocks are mostly perceived as homogeneous in mineralogy and texture.

Examples of intrusive igneous rocks are granite and diorite.

Extrusive igneous rocks, on the other hand, form when magma erupts and quickly cools as it pours on the Earth's surface. Because the magma cools quickly, it doesn't have time to form large crystals. Extrusive igneous rocks have small, microscopic crystals and a glassy texture. They can have vesicles, which are small holes formed when gas bubbles escape from the cooling magma.

Examples of extrusive igneous rocks include basalt and obsidian.

When rocks are formed in between the Earth's surface and its interior, they are called subvolcanic or hypabyssal rocks. These rocks are similar to intrusive rocks but have a finer grain structure since the magma cools more quickly.

Examples of subvolcanic or hypabyssal rocks include andesite and dacite.

2.   Sedimentary Rocks

Sedimentary rocks are rocks that form from the accumulation of sediments. These sediments can be transported by gravity, water, wind, or ice, and they can be:

Biologic/Organic: Made from the remains of plants or animals

Clastic: Made from the weathering and breakdown of other rocks

Chemical: Formed when minerals precipitate out of the water and are eventually redeposited after the water evaporates or when the water becomes over-saturated.

Examples of sedimentary rocks include conglomerates, shale, sandstone, and limestone.

3.   Metamorphic Rocks

Metamorphic rocks are rocks that have been changed by heat and pressure. The word "metamorphism" comes from the Greek words "meta," meaning change, and "morph," meaning form.

Metamorphic rocks can be formed from any type of rock: igneous, sedimentary, or even metamorphic. The rocks are changed by heat and pressure deep underground, where the temperatures and pressures are much higher than at the Earth's surface.

For example, a metamorphic rock called gneiss can be formed from the metamorphism of granite. The heat and pressure cause the large minerals in the granite to break down and re-form into smaller minerals. The new minerals align themselves in bands, giving the rock a striped appearance.

Other examples of metamorphic rocks include slate, marble, and quartzite.

Classification By Chemical Composition Of Rocks

Rocks are classified into three types based on their chemical composition:

Argillaceous rocks

If clay content is predominant in a rock, then it is classified as an argillaceous rock. The main component of clay is aluminum silicate. These rocks are soft in nature, can crush easily, and have a soapy feel.

Examples of argillaceous rocks are shale and mudstone.

Calcareous rocks

These are rocks where the main chemical component is calcium carbonate. This can come from the remains of marine organisms, like coral and mollusks, that have been deposited on the sea floor and then lithified (turned to stone). They are generally hard, but their durability can be affected by surrounding environmental conditions, like rainwater or acid.

Examples of calcareous rocks are limestone and marble.

Siliceous rocks

These are rocks where the main chemical component is silica. They mainly form from weathering and the breakdown of other rocks, with the resulting sediments being lithified. The presence of silica makes these rocks very hard and resistant to weathering.

Examples of siliceous rocks are quartzite and chert.

Classification By Physical Properties Of Rocks

Rocks can be classified into three types based on their physical properties:

Stratified rocks

These are rocks that have been layered by sedimentation. The individual layers are called strata, and they can be of different sizes and compositions and can be seen when the rock is cut or broken open. Examples of stratified rocks are sandstone and shale.

Unstratified rocks

These are crystalline rocks that have no layering. They are made up of interlocking crystals that have grown in all directions. The most common type of unstratified rock is granite.

Foliated rocks

Foliated rocks are metamorphic rocks that have been deformed by heat and pressure. The individual grains in the rock have aligned themselves into foliation planes. This gives the rock a layered or banded appearance. Examples of foliated rocks are gneiss and slate.

Other physical properties that can be used to classify rocks include color, grain size, and hardness.

Rock Identification Methodology

Rock identification is a systematic process that begins with a careful description of the rock. This includes recording its color, texture, and any other physical characteristics.

Geologists use petrographic descriptions to identify rocks. This involves looking at the rock under a microscope and describing its grain size, shape, and any other features. The composition of the rock (e.g., the minerals it contains) is also important for identification.

Once a rock has been described, geologists compare it to other rocks they have already classified. This process of comparing and contrasting rocks is called the comparative method. Using the comparative method, geologists can often identify rocks without requiring complex laboratory analyses.

However, there are some cases where the comparative method is insufficient to make a positive identification. In these cases, geologists may need to use more sophisticated techniques, such as X-ray diffraction or electron microprobe analyses.

Despite these more sophisticated techniques, the comparative method is still the most common way to identify rocks. It is a fast and inexpensive way to identify rocks and doesn't require any special equipment.

Use Declan's Classification Chart To Know Your Gem!

Rock identification can be done in several ways, but using a rock classification chart is the most common.

A rock classification chart is a tool that geologists use to identify rocks using a series of diagrams that show the different characteristics of rocks. These characteristics include the rock's texture, grain size, and composition.

At Declan's Mining Co., we have an easy-to-use rock identification chart to help you identify common rocks. This chart is an excellent resource for beginners and can be used as a reference tool by more experienced rock hounds.

The chart is organized by rock type and includes information on the most common rocks in each category. It also includes a brief description of each rock and its most distinguishing characteristics. If you are a rock enthusiast or would love your kid to learn more about rocks, this is the perfect tool for you!