What Kind of Rocks Are There – A Guide To Identifying Different Types of Rocks

Rocks are all around us, from the tiniest pebble up to huge mountains. But what kind of rocks are they, and how can they be identified? This guide will take an in-depth look at the different types of rocks and how to easily spot the differences between them.Rocks are one of the most interesting and diversified aspects of geology. Rocks are made up of different minerals and come in many shapes and sizes. They can vary from simple sand grains to huge mountains made from hunks of granite or basalt. In between, there are endless permutations of composition, color, texture and form. One of the most important things to remember when trying to identify a rock is that rocks consist primarily of minerals. Minerals themselves come in many different shapes, sizes, colors and hardness. Therefore, it is important to observe a rock’s physical characteristics carefully during identification. The three major types of rocks are classified as either:

• Igneous rocks
• Sedimentary rocks
• Metamorphic rocks

Additionally, igneous rocks may be further classified as intrusive or extrusive, depending on their formation process. Let’s look at some of the most common types of rocks that are found on Earth today, starting with the igneous rocks.

Igneous Rocks – Intrusive And Extrusive

Igneous rocks are formed from magma or molten rock that has cooled and solidified within or on the Earth’s surface. They are divided into two main types: intrusive igneous rocks and extrusive igneous rocks.

Intrusive Igneous Rocks

Intrusive igneous rocks form from magma that crystallizes within the Earth’s crust and cools slowly underground. They tend to be more coarse-grained and contain large crystals because they have had more time to cool and solidify compared to extrusive igneous rocks. Examples of intrusive igneous rock include:

• Granite
• Gabbro
• Plutonic

Granite is the most common intrusive igneous rock and is composed of quartz, feldspar and mica. It is medium to coarse-grained and usually has an even texture. It is usually very hard and durable. Gabbro is similar in composition to granite but tends to be darker in color with a denser, finer texture. Plutonic rock is usually found in less common cooler conditions, such as magma chambers. It forms from magma that has cooled much slower than in other environments, usually resulting in a coarse-grained texture with large crystals.

Extrusive Igneous Rocks

Extrusive igneous rocks form from magma that has been cooled quickly at or near the surface of the Earth. This generally means that they have a smoother texture and much smaller crystals than intrusive igneous rocks because they have not had as much time to cool and solidify. Examples of extrusive igneous rock include:

• Obsidian
• Pumice
• Basalt

Obsidian is an extrusive igneous rock that forms from rapidly cooled lava flows and is composed primarily of silica. It is usually black or dark grey in color, but can also be found in shades of green and red. Pumice is a light, porous igneous rock that is formed from rapidly cooled magma with high concentrations of dissolved gas bubbles. It is usually white, grey or light brown in color. Basalt is a dark-colored extrusive igneous rock that forms as lava flows on the Earth’s surface. It is composed mostly of plagioclase feldspar with small amounts of olivine, pyroxene and other minerals.

Sedimentary Rocks

Sedimentary rocks are made up of sediment (tiny pieces of eroded rock) that has been compacted and cemented together over time by pressure and chemical processes. There are three basic categories of sedimentary rocks: clastic, biochemical and chemical sedimentary rocks.

Clastic Sedimentary Rocks

Clastic sedimentary rocks are made up of broken pieces or fragments of older rocks that have been joined together by heat and pressure. They are further divided into three categories: sandstone, shale and conglomerate. Examples of clastic sedimentary rocks include:

• Sandstone
• Conglomerate
• Shale

Sandstone forms when naturally occurring grain particles called sediment get compressed by time and heat over time until they become solid. It has a distinct grainy texture due to the presence of quartz, feldspar and other minerals in the sand grains but can come in both heavy and light colors depending on the mineral content. Conglomerate forms from rounded fragments or pebbles that have been cemented together by a cementing material such as calcite or silica. Shale is composed of finely layered flakes or sheets that are made up mainly of clay, calcite or quartz particles.

Biochemical Sedimentary Rocks

Biochemical sedimentary rocks are made up of dissolved compounds or organic remains from organisms such as shells or corals that have been compressed by heat and pressure over time. Examples of biochemical sedimentary rock include:

• Limestone
• Coquina

Limestone is composed mainly of calcite (50-90%) and aragonite (50%) which are mineral constituents of carbonate shells secreted by marine organisms such as coral, algae and clams. It is typically light grey to black in color with a distinctive granular texture due to its high calcium content but can also come in shades of yellow, pink or blue depending on its mineral content. Coquina is a sedimentary rock consisting mainly of shell fragments and other organic material cemented together by smaller amounts of calcite or quartz particles. It is usually harder than limestone, but softer than granite, and often has a smooth texture with bright colors ranging from white to yellow or even pinkish hues due to its high iron oxide content.

Chemical Sedimentary Rocks

Chemical sedimentary rocks are made up of minerals that were dissolved by water, with little to no evidence of any organic material or grains present. They are usually formed in shallow marine basins as dissolved minerals precipitate out from evaporating seawater or hot springs. Examples of chemical sedimentary rocks include:

• Rock Salt (Halite)
• Gypsum

Rock salt (halite) is an evaporite sedimentary rock formed by the precipitation of salt minerals from evaporating seawater over long periods of time. It can come in large beds as wide as 10 meters (30 feet), as well as smaller crystalline deposits called “halite concretions”. Gypsum is typically white or gray in color when it forms but can also come in shades of yellow or pink when mixed with other minerals like iron oxide or carbonates. It is composed mainly of hydrated calcium sulfate which creates flat crystals when it crystallizes out from evaporating liquids such as seawater or hot springs.

Metamorphic Rocks

Metamorphic rocks are formed from pre-existing igneous or sedimentary rocks by heat, pressure or both over long periods of time as part of the Earth’s natural cycling through the rock-forming cycle (weathering, erosion and deposition). Some examples of metamorphic rock include:

• Slate
• Marble
• Gneiss

Slate is a type of metamorphic rock formed from pre-existing sedimentary material such as shale by heat and pressure over long periods of time. It tends to be gray or black in color with a distinctive layered or banded structure due to the recrystallization process it undergoes during its formation. Marble is a type of metamorphic rock that forms when limestone is subjected to heat, pressure and chemical processes deep within the Earth’s crust where temperatures can reach up to 900 degrees Celsius (1650 degrees Fahrenheit). Gneiss is another type of metamorphic rock that is formed when sedimentary or igneous material is subjected to extreme pressures within the Earth’s crust resulting in the formation of intensely foliated layering structures much like marble but darker in color. Learning how to identify different types of rocks can help us understand more about our planet’s rich geological history and movement over time as well as providing insight into its local environment today. As geologists continue to explore more deeply into the Earth’s surface, new discoveries about these earthy materials will be made every day as part our growing understanding about how our planet works – for better or for worse! Britannica Encyclopedia – Rock Geology.