Sedimentary vs. Igneous Rocks: Differences & Similarities

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Sedimentary and igneous rocks are among the basic types of rocks. They have different compositions, appearances, and properties as a result of their genesis. Sedimentary and igneous rocks are strikingly different but also have a few similarities. It’s essential to know which rocks you are working with. Having information about differences and similarities between sedimentary and igneous rocks can help you with their identification and understanding.

The main difference between igneous and sedimentary rocks is that sedimentary rocks can contain fossils and can be deposited in layers as strata. Sedimentary rocks are generally less dense and less hard than igneous. The number of mineral species in igneous rocks is larger than in sedimentary.

It may seem hard to distinguish between a sedimentary and igneous rock in the field. But here we will provide you with a shortlist of properties (checked by real geologists), which will assist you a lot. Sedimentary and igneous rocks can have considerably different appearances, properties, compositions, and genesis. Keep reading to become an expert and to distinguish two types of rocks accurately.

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Sedimentary vs. Igneous Rocks

The most significant differences between sedimentary and igneous rocks derive from their genesis. Sedimentary rocks form as a result of disintegrated pre-existing rocks deposition or from precipitation from a solution. Igneous rocks are crystallized directly from magma or lava.

Further differences in texture and structure, chemical composition, or physical properties of rocks are a result of their contrasting way of formation.

Taking into account the differences and similarities of igneous and sedimentary rocks it is easy to imagine the geological history of the region and to predict what kind of minerals, gems, and ores can be found.

What are Sedimentary Rocks?

Before finding similarities and differences, it’s necessary to clearly identify each type of rock.

Sedimentary rocks are rocks formed on the Earth’s surface by the accumulation and lithification of sediment (detrital rock), which may be composed of geological detritus (minerals) or biological detritus (organic matter) or by the precipitation from solution at normal surface temperatures (chemical rock).

According to USGS, sedimentary rocks are formed from pre-existing rocks or minerals (geological detritus) or pieces of once-living organisms (organic matter). In other words, sedimentary rocks are the lithified equivalents of sediments.

There are 3 major types of sediments based on the type of sedimentation process:

Clastic sediments. This type of sediment is made of particles of various sizes, which are carried in suspension by wind, water, or ice. Examples of sedimentary rocks that result from the lithification of clastic sediments are conglomerates and breccias, sandstones, and mudrocks.

Chemical sediments. The process of chemical sediment formation can be explained the next way. When water evaporates or the concentration of the ions in water gets too high, the ions recombine by chemical precipitation to form minerals that can accumulate to become chemical sediments and chemical sedimentary rocks.

Examples of this type of sedimentary rock are evaporites (halite, which is salt that we consume every day; gypsum), travertine, dolostones, and chemical cherts.

Organic sediments and sedimentary rocks are those derived from living organisms. When the organism dies, the remains can accumulate to become sediment or sedimentary rock. Among the types of rock produced by this process are biochemical limestone, biochemical chert, diatomite, and coal.

What are Igneous Rocks?

Igneous rocks, are any various crystalline or glassy rocks formed by the cooling and solidification of molten earth material.

Igneous rocks form when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots and then rises toward the surface. Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies.

When they are formed inside of the earth, they are called intrusive, or plutonic, igneous rocks. If they are formed outside or on top of Earth’s crust, they are called extrusive, or volcanic, igneous rocks.

Granite and diorite are examples of common intrusive rocks. They have a coarse texture with large mineral grains, indicating that they spent thousands or millions of years cooling down inside the earth, a time course that allowed large mineral crystals to grow.

TIP: Do you want to know more about igneous rocks? If so, find out the main characteristics of igneous rocks in the article below:
6 Main Characteristics of Igneous Rocks

Differences Between Sedimentary and Igneous Rocks

Sedimentary and igneous rocks are formed in extremely different environments. Their genesis gives rise to the differences in structures, textures, properties, and mineral composition.

The main difference between sedimentary and igneous rocks is the fossil presence in sedimentary rocks. Also, sedimentary rocks are characterized by layered bedding and composed of a restrained number of mineral species (quartz or calcite mostly) in comparison with igneous.

Formation of Igneous and sedimentary Rocks

The essential difference between sedimentary and igneous rocks is their formation. Igneous rocks are formed as a result of the solidification of magma or lava, while sedimentary rocks are formed by the deposition of pre-existing disintegrated rocks, or by precipitation from solutions.

Igneous rocks are formed as a result of the cooling and solidification of magma or lava. Temperatures of igneous rocks formation ranges from 650-800^oC (for felsic/rhyolitic magma) to 1000-1200^oC (for mafic/basaltic magma).

The maximum temperature for the formation of sedimentary rocks is considered to be 200 ^oC. But in most cases, sedimentary rocks are formed under normal conditions. They are formed by the deposition of already existing material.

Sedimentation is the term for processes that cause mineral or preexisting rock particles (primary igneous or metamorphic) and/or organic particles (detritus) to settle and accumulate or minerals precipitate from a water solution.

TIP: Other ways of rock formation are fossilization and petrification. In this way, rocks are formed from the remains of plants and animals. Find out the difference between these two ways of rock formation in the article below:
The Difference Between Fossilization and Petrification

Presence of Fossils in Sedimentary Rocks

Organic remnants or fossils are usually found in sedimentary rocks only. Sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants. In contrast to sedimentary rocks, it is impossible to find any organic remnants in the primary igneous magmatic rocks.

Fossils are remains of once-living organisms. They are essential indicators of the environment of sediment deposition. Different living organisms inhabit specific environments.

That’s why fossils are important markers of past epochs’ climates. They are also good for approximating the age of sedimentation because when one type of organism is extinct, the other evolves.

Geologists can ascribe fossil-bearing sedimentary rock to a particular period of time. Often some fossils (diatoms, for example) may only be visible when studied under a microscope or with a loupe.

Examples of organic-bearing sedimentary rocks are biochemical chert, biochemical limestone, and diatomite.

Biochemical limestone is the most common organic sedimentary rock. It is composed of the mineral calcite (CaCO₃), which is precipitated by organisms to form a shell or other skeletal structure. Accumulation of these skeletal remains and their further lithification results in limestone.

Stratification of Sedimentary Rocks

The other significant difference between sedimentary and igneous rocks lies in the form of geological bodies’ occurrence. Sedimentary rocks as a result of sedimentation are deposited in layers called strata or beds, while igneous rocks during magma or lava solidification form plutons, dikes, sills, or laccoliths.

Sediments are deposited in low-lying areas and can extend for a few thousand kilometers. Successive depositional events, which can take millions of years, produce layers called beds, strata, or stratification.

A bed is defined as a layer of rock that has a uniform lithology and texture. Beds form by the deposition of layers of sediment on top of each other.

The sequence of beds that characterizes sedimentary rocks is called bedding. That is usually one of the most evident features of sedimentary rocks.

The layering can be produced by differences in the color of the material, in size of grains, mineral content, or chemical composition. All of these differences can be explained by differences in the environment present during sedimentation events.

Igneous rocks, because of magma and lava cooling, form different rock bodies. The structures of igneous rocks are large-scale features, which are dependent on several factors like:

Composition of magma.
The viscosity of magma.
Temperature and pressure at which cooling and consolidation take place.
Presence of gases and other volatiles.

Igneous rocks are mainly crystalline. Magma that cools at a great depth form bodies of rocks called intrusive bodies or plutonic bodies called plutons. In relatively shallow environments (low depths) intrusions are usually tabular bodies like dikes and sills, or domed roof bodies called laccoliths.

TIP: When magma erupts from a volcano, it forms extrusive or volcanic rocks on the surface. If the magma cools underground, it forms intrusive or plutonic rocks. Find out what types of rocks you can find near volcanoes in the article below:
Ten Most Common Types of Rocks You Can Find In Volcanoes

Rock Abundance on the Earth Crust

Sedimentary rocks are far more common than igneous rocks on the Earth’s surface. 80–90 percent of the Earth’s surface is covered by sediment or sedimentary rocks rather than with igneous or metamorphic varieties. However, igneous and metamorphic rocks constitute the bulk of the Earth’s crust.

Sediments and sedimentary rocks are confined to Earth’s crust only. The Earth’s crust is the thin, light outer solid shell of Earth ranging in thickness from 40–100 kilometers in the continental blocks to 4–10 kilometers in the ocean basins.

The area of outcrop and exposure of sediment and sedimentary rock comprises 75 percent of the land surface and well over 90 percent of the ocean basins and continental margins.

In other words, 80–90 percent of the surface area of Earth is mantled with sediment or sedimentary rocks rather than with igneous or metamorphic varieties. The sediment-sedimentary rock shell forms only a thin superficial layer.

While sedimentary rocks cover the Earth’s surface – the outer part of the crust, igneous and metamorphic rocks constitute the bulk of the crust (up 90–95% of the Earth’s crust by volume).

Number of Mineral Species in Sedimentary and Igneous Rocks

Sedimentary rocks are mostly monomineralic. This means, that it is composed of one mineral only. In most cases, it is quartz or calcite. Igneous rocks are composed of a long list of different mineral species (e.g. quartz, pyroxene, olivine, alkali feldspar), which crystallized directly from the melt.

In contrast to igneous and metamorphic rocks, sedimentary rock usually contains very few different major minerals, which are quartz and calcite mostly.

However igneous rocks can be composed of a lot of different minerals, depending on the chemical composition of the magma. Granite is an example of one of the most common igneous rocks, is composed of

quartz,
alkali feldspar,
plagioclase, and
mica.

TIP: Igneous rocks have unique chemical and mineral compositions. Check out the most interesting and unique facts about igneous rocks in the article below:
11 Cool & Interesting Facts About Igneous Rocks (Must Read)

Similarities Between Sedimentary and Igneous Rocks

There are not so many similarities between sedimentary and igneous rocks. It’s easier to make a mistake between igneous and metamorphic rock than igneous and sedimentary.

The Internet is full of rather philosophical similarities between these two types of rocks, than geological ones. Let us omit similarities like: “they are all made of minerals” and “they are a member of three main rock types”.

Sedimentary and igneous rocks can be quite hard and dense. Chemical cherts or flints are quite hard sedimentary rocks. They are the same hard as granite. Both rock types can be porous and vesicular, e. g. porous limestone as a representative of sedimentary rock, and scoria as an example of igneous rock.

Both sedimentary rocks and igneous can be quite dense and hard. No doubt, that igneous rock is hard. It’s enough to remember paving stones.

At the same time, flint or chert – chemical sedimentary rocks composed of microcrystalline quarts are so hard and durable that ancient people used them for arrowheads and knives.

It’s very easy to differentiate between limestone (sedimentary rock) and gabbro (igneous rock), but there are some examples, which can lead to confusion.

For example, volcanic igneous rocks can be either solid bodies (such as lava flows), or vesicular (such as scoria or other forms of volcanic tephra). Tephra, once erupted, is subject to the vagaries of wind and water. As such, tephra falls can sometimes appear to be sedimentary rather than volcanic.

Igneous rocks can sometimes also show layering, but there will be a noticeable difference in the density and weight of these two rocks. Sedimentary rock will be lighter and less dense, than igneous one.

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Conclusion

Sedimentary and igneous rocks are essential parts of our planet’s life and two out of three players (the third is metamorphic rocks) are in the rock cycle. The main points in separating sedimentary rocks from igneous are the next:

The presence of fossils or any kind of organic material will indicate a sedimentary origin of the rock.
Layering structure is also a sign of sedimentary rock (be careful, igneous rocks also show layering, but they will be substantially harder than sedimentary.
Igneous rocks are formed from magma and lava, which solidify. On the contrary, sedimentary rocks are derived from pre-existing rocks.
Sedimentary rocks are mostly monomineralic. This means, that it is composed of one mineral only. In most cases, it is quartz or calcite.
Sedimentary rocks can be porous. But pay attention, some igneous volcanic rocks can be also porous.
Igneous rocks are crystalline in the structure; sedimentary rocks tend to be fragmentary and stratified.

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How to Start Rockhounding: The Ultimate Beginner’s Guide