How to Guide: Classification of Rocks by PRO Geologist

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Classifying rocks is an important part of keeping track of all the information we’ve learned about rocks over the course of history. It helps us do important things like quickly finding rocks based on their traits, guessing what rocks will be like, learning about geological processes in certain places, and much more.

There are three main types of rocks based on how they formed: volcanic rocks, metamorphic rocks, and sedimentary rocks. These groups are further divided into subgroups based on things like structure, material or chemical makeup, and how or where they formed.

This piece will explain the various ways rocks are grouped, along with the types of rocks that are found in each group and the criteria used to decide which rocks to put together. We will also talk about the most common ways geologists divide rocks into groups and show some examples of each type.

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Why Do Geologists Classify Rocks Into Groups?

Scientists from numerous branches of study have faced the difficulty that when data collection expands in size, it becomes vital to standardize the information by categorizing it into groups and subgroups based on common qualities.

Here’s a basic but comprehensible illustration of why information is structured into a system: consider trying to discover certain information in a big book.

However, there are no page numbers in this book, and it is not separated into paragraphs. In this instance, your duty will be incredibly difficult. However, if the pages are numbered and the book is separated into paragraphs, you may readily locate the information you want.

The same idea applies to a vast database of rock-related information. Throughout human history, a wealth of information about rocks has been acquired.

This information may be organized and classified to make it much easier to explore, search for, and extract useful facts about rocks.

The first descriptions of rocks date back to ancient civilizations that were interested in understanding the Earth’s geological properties. One famous example is Theophrastus, a Greek philosopher and Aristotelian pupil who published a book titled “On Stones” circa 300 BCE.

This work is regarded as one of the first known systematic descriptions of rocks and minerals. Theophrastus categorized rocks according to their physical qualities and made observations on their formation and occurrence.

As we can see, the urge to organize information about rocks originated thousands of years ago. Today, there are several categorization systems for rocks based on diverse features that serve certain purposes, such as:

Identification of rock types: When geologists encounter a rock, they must correctly identify it. To do this, the geologist examines the rock’s various physical and chemical characteristics. By utilizing a classification system for rocks, the geologist can determine the name of the rock and gather other information about its type.
Prediction of rock properties: by identifying the rock and determining its rock type, one can infer the properties associated with that particular rock type. This is particularly important in engineering geology, as different rock types have varying load-bearing capacities, permeability, and other relevant properties.
One common language for geologists: Classification provides a standardized system for geologists to communicate and share information about rocks. By using common classification schemes, geologists can convey information about rocks more effectively, enabling clearer communication within the scientific community and facilitating the exchange of knowledge.
Predicting resources: Different rock types have varying economic significance and may contain valuable resources such as minerals, ores, or fossil fuels. Geologists can identify potential resource-bearing formations and predict their distribution by classifying rocks. This information is crucial for exploration and mining industries to efficiently locate and extract valuable resources.
Geological mapping: Classification helps create geological maps that visually represent rock types and their spatial distribution. These maps help geologists understand the geology of an area, including its structure, stratigraphy, and potential geologic hazards. Geological maps are essential for land use planning, infrastructure development, and environmental management.
Understanding Earth’s history: Rocks record Earth’s geological history. By classifying rocks into groups, geologists can interpret the processes that formed them, such as volcanic activity, sedimentation, or metamorphism. Studying rock groups allows geologists to reconstruct past environments, understand tectonic processes, and unravel our planet’s complex history.

In general, geologists classify rocks as the basis for understanding the history of the Earth, communicating scientific information, and making informed decisions in various fields related to earth sciences, including geology.

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How Do Geologists Classify Rocks?

Geologists classify rocks based on their origin (genesis), mineral composition, structure and texture, and other parameters that must be considered when addressing specific tasks. Geologists use various rock classification systems in their work depending on the type of work being performed.

The science that studies rocks’ origin, composition, structure, classification, and transformation processes is called Petrology. Please do not confuse it with petrography. Petrography is a narrower discipline within petrology that focuses on the detailed microscopic examination of rocks.

Let’s start with the most common classification of rocks: classification based on the formation type (genesis). According to this classification, all rocks can be divided into 3 types:

Igneous rocks: formed from the cooling and crystallization of magma.
Sedimentary rocks are formed when weathered fragments of other rocks are buried, compressed, and cemented together or minerals precipitate directly from the solution.
Metamorphic rocks: result when existing rocks are changed by heat, pressure, or reactive fluids, such as hot, mineral-laden water.

These types are further divided into subtypes.

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Igneous Rocks

The preferred method for classifying any rock type – igneous, sedimentary, or metamorphic – is based on texture and composition (chemical and mineral composition).

Textural Criteria

Textural criteria are commonly considered first, as textures provide the best evidence for rock origin and permit classification into the broadest genetic categories. Texture refers to the size and arrangement of mineral grains within the rock.

The first step in igneous rock description is for geologists to determine whether the rock falls into one of the following 5 categories by texture:

Phaneritic texture: The crystals that compose the rock are readily visible to the naked eye.
Aphanitic texture: The crystals, if any, are too small to be seen readily by the naked eye.
Fragmental texture: The rock is composed of pieces of disaggregated igneous material, deposited and later amalgamated.
Glassy: Rapid cooling prevents mineral grains from forming, resulting in a glass-like texture. Example: Obsidian.
Vesicular: Contains cavities (vesicles) formed by trapped gases. Example: Pumice.

All igneous rocks are divided into two groups according to the place of their formation relative to the Earth’s surface:

Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth’s crust. If a rock exhibits a phaneritic texture, it crystallizes slowly beneath the surface of the Earth, which is called plutonic. Examples include granite and diorite.

Volcanic or extrusive rocks form when lava cools and solidifies on the Earth’s surface. If it is aphanitic, it crystallizes rapidly at the Earth’s surface and is called volcanic. Examples include basalt and obsidian.

Fragmental rocks are collectively called pyroclastic. Pyroclastic rocks are composed of fragments ejected during explosive volcanic eruptions.

If it is glassy, vesicular, or fragmental, you cannot determine mineralogy, and hence, the name is simply obsidian for a glass, tuff for a fragmental, or pumice/scoria for a vesicular rock.

Mineral and Chemical Composition-Based Classification

Igneous rocks are further classified based on their mineral composition, which is determined by the proportions of black and light minerals present. The main compositional categories are:

Felsic (Acid igneous rocks): Rich in light-colored minerals, mainly quartz and feldspar. These rocks have more than 66% SiO2 content, e.g., granite rhyolite.
Mafic (Basic igneous rocks): Rich in dark-colored minerals like pyroxene and olivine. The SiO2 content in these rocks varies between 45 to 52%, e.g., gabbro and basalt.
Intermediate: Contains minerals between felsic and mafic compositions. These rocks have 52 to 66% of SiO2, e.g., syenite or diorite, trachyte, andesite.
Ultramafic (Ultrabasic igneous rocks): Composed predominantly of dark-colored minerals, including olivine and pyroxene. In these rocks, SiO2 content is less than 45%. They have high Mg content, e.g. dunite, peridotite, and pyroxenite.

As you may have noticed, this system is also based on SiO2 components. The grouping of rocks on the basis of their silica contents is a chemical parameter.

This classification system is very convenient when conducting fieldwork. During fieldwork, it can be quite difficult and sometimes impossible to determine the mineral composition of the rock, so geologists are guided by the ratio of light and dark minerals.

Composition type	Rocks name
INTRUSIVE
Felsic		Granite
Intermediate		Diorite, Syenite, Monzonite
Mafic		Gabro
Ultramafic		Peridotite, Dunite, Pyroxenite, Hornblendite
EXTRUSIVE
Felsic		Rhyolite, Obsidian
Intermediate		Trachyte, Andesite, Dacite
Mafic		Basalt
Ultramafic		–

The classification of igneous rocks

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Metamorphic Rocks

Metamorphic rocks start as one type of rock and — with pressure, heat, and time — gradually change into a new type of rock.

Geologists classify metamorphic rocks based on their texture, mineral composition, and the processes that led to their formation.

Texture-Based Classification

Foliated:

These rocks have a layered or banded appearance due to the alignment of minerals or the presence of visible parallel planes of preferred orientation.

Examples include slate, schist, phyllite, and gneiss. These rocks formed in an environment with either directed pressure or shear stress.

Non-foliated:

These rocks lack a layered or banded appearance and typically have a more uniform grain structure.

Examples include marble, hornfels, and quartzite. These rocks formed in an environment without direct pressure or relatively near the surface with very little pressure at all.

Metamorphic Grade-Based Classification

Metamorphic rocks can also be classified based on their metamorphic grade, which reflects the intensity of the metamorphic process.

The grade is typically determined by the temperature and pressure conditions experienced by the rock during metamorphism. Common grade-based terms include:

Low-grade: Rocks that have undergone mild metamorphic conditions, resulting in minimal mineral changes. Examples include slate and low-grade schist.
Medium-grade: Rocks that have experienced intermediate levels of temperature and pressure, leading to more pronounced mineral changes. Examples include medium-grade schist and amphibolite.
High-grade: Rocks that have undergone intense metamorphism at high temperatures and pressures, resulting in significant mineral changes. Examples include gneiss and granulite.

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Mineral Composition-Based Classification

Metamorphic rocks can also be classified based on their mineral composition, which is determined by the types of parent rocks and the specific metamorphic processes involved.

Here are some common types:

Calcite-rich: Metamorphic rocks dominated by the mineral calcite. Examples include marble and calc-schist.
Quartz-rich: Metamorphic rocks rich in quartz. Examples include quartzite.
Mica-rich: Metamorphic rocks with a significant presence of mica minerals. Examples include phyllite and mica schist.
Garnet-rich: Metamorphic rocks containing abundant garnet crystals. Examples include garnet schist and gneiss.
Amphibole-rich: Metamorphic rocks rich in amphibole minerals. Examples include amphibolite and hornblende schist.

Metamorphism intensity/type	Rocks name
FOLIATED
Low Grade (150-450ᴼC)	Slate, Chlorite schist
Low Grade (150-450ᴼC)	Phyllite
Medium Grade (450-550ᴼC)	Schist, Amphibole gneiss
High Grade (more than 550ᴼC)	Gneiss
NONFOLIATED
Regional type of metamorphism	Antracite coal
Contact type of metamorphism	Hornfels
Regional or Contact type of metamorphism	Quartzite, Marble, Metaconglomerate

Table of the classification of metamorphic rocks

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Sedimentary Rocks

These rocks are formed at low temperatures and pressures on Earth’s surface when water, wind, or ice leave them.

Sedimentary rocks are grouped based on their texture, makeup, and the processes that made them, among other things. It can be hard to put sedimentary rocks into groups, but here are the key groups and some popular types:

Clastic (Detrital) Sedimentary Rocks are composed of fragments or grains derived from the weathering and erosion of pre-existing rocks.

They are classified based on the grain size of the particles and the dominant mineral composition:

Conglomerate: Coarse-grained rock composed of rounded gravel-sized particles.
Sandstone: Medium-grained rock composed of sand-sized particles.
Siltstone: Fine-grained rock composed of silt-sized particles.
Mudstone (including Shale): Very fine-grained rock composed of clay-sized particles.

Chemical Sedimentary Rocks form from the precipitation or crystallization of minerals from water solutions.

They are classified based on the dominant mineral present:

Limestone: Rock composed primarily of the mineral calcite (CaCO3).
Dolostone: Rock composed primarily of the mineral dolomite (CaMg(CO3)2).
Evaporites: Rocks formed by the precipitation of minerals from evaporating water bodies, such as rock salt (halite) and rock gypsum.

Organic Sedimentary Rocks are derived from the accumulation and lithification of organic matter, such as the remains of plants or animals.

Examples are peat and coal (formed from the compaction and alteration of plant remains under specific conditions).

Biogenic (Biochemical) Sedimentary Rocks are formed from the accumulation of organic material produced by living organisms, such as shells, coral reefs, and microscopic marine organisms:

Chalk is composed mainly of the microscopic shells of marine plankton.
Coquina rock is made up of loosely cemented shell fragment
Coral reef limestone formed from the accumulation of coral skeletons.

Classification table of sedimentary rocks

Type by genesis	Rocks name
Clastic	Conglomerate, Breccia, Sandstone, Silestone, Claystone, Shalestone
Biogenic and Organic	Peat, Coal, Oil Shale, Limestone, Coral Reef Limestone
Chemical	Chert, Dolostone, Halite, Gypsum, Ironstone, Phosphorite
Pyroclastic	Tuff, Ash, Agglomerate

Classification table of sedimentary rocks

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Conclusion

Geologists classify all rocks into 3 main types based on their origin:

Igneous rocks
Sedimentary rocks
Metamorphic rocks

Each group is further split into smaller groups based on structure, material or chemical make-up, and how or where the rock formed.

One example is igneous rocks, which can be broken down into types that form below the Earth’s surface and types that form on top of the Earth’s surface.

They have different textures depending on where they crystallize. They are also put into four groups based on the minerals they contain: felsic, mafic, intermediate, and ultramafic.

There are different types of sedimentary rocks, such as clastic, chemical, biological, biogenic, and others.

Based on their appearance, metamorphic rocks are divided into two groups: foliated and non-foliated. They are further broken down into low-grade, medium-grade, and high-grade metamorphism based on how much they have changed. Of course, they are put into groups based on the minerals that make them up.

There are also other ways to group rocks into different groups. In industrial geology, for instance, rocks are put into many groups based on their strengths, weaknesses, porosity, permeability, and other characteristics.

During geophysical studies, rocks are also put into groups based on how they behave physically. We’ve only talked about the most popular ways to classify rocks in this piece.

Sources

Petrology of sedimentary rocks – S. Boggs, Jr. 2009

Physical Geology – Thompson & Turk

General Geology – Ivanik, Menasowa, Kyiv 2020

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