10 Things To Know About Sedimentary Rocks

Sedimentary Rocks Inside The White Volta. Photo by Tahiru Rajab. Wikimedia

10 Things To Know About Sedimentary Rocks

Sedimentary rocks are a type of rock formed by layers of sediment (a fancy word for small bits of stuff like sand, dust, or even bits of different rocks) that accumulate over time.

Sedimentation refers to the processes that cause these particles to settle in place. 

Only under low oxygen (anoxic) conditions does iron(II) oxide (FeO) form, giving the rock a grey or greenish colour.

The texture of a sediment is defined by the size, shape, and orientation of its clasts (the original pieces of rock).

The cloth of the rock refers to the 3D orientation of the clasts. The size and shape of clasts can be used to determine the velocity and path of modern-day within the sedimentary environment that moved the clasts from their beginning; large, calcareous mud settles best in slow-moving water, whereas gravel and larger clasts settle best in fast-moving water.

1. Sedimentary rocks are formed by the accumulation and consolidation of sediment

Sedimentary rocks are formed by the deposition and compaction of sediment from a variety of sources.

Sediment can be weathered and eroded from existing rocks, transported by wind, water, or ice, and deposited in a new location.

These free debris become compacted and cemented collectively over time, forming strong sedimentary rock.

These rocks frequently contain clues about past environmental conditions and Earth’s history, making them important for geologic examination and research.

2. Sedimentary rocks are the most common type of rock on Earth’s surface

The sedimentary rock cover of the Earth’s crust’s continents is extensive (73% of the Earth’s current land surface), but sedimentary rock is estimated to be only 8% of the crust’s volume.

Sedimentary rocks are only a thin veneer on top of a crust dominated by igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata, forming a bedding structure. They are frequently found in large structures known as sedimentary basins. On Mars, sedimentary rocks have also been discovered.

3. Sedimentary rocks are often layered

Structurally tilted sedimentary rocks. Photo by James St. John. Wikimedia

Sedimentary rocks are frequently layered, with each layer representing a unique time period and environment.

Positive forms of sedimentary rocks can sometimes be used to determine the relative age of the surrounding rocks using a system known as stratigraphy.

This entails examining a collection of rock layers and the fossils that comprise them to determine the order in which they were deposited.

4. Sedimentary rocks can contain a variety of fossils

Sedimentary rocks, unlike most igneous and metamorphic rocks, form at temperatures and pressures that do not destroy fossil remnants. These fossils are frequently only visible under magnification.

In nature, dead organisms are usually quickly removed by scavengers, bacteria, rotting, and erosion, but in rare cases, these natural processes are unable to occur, resulting in fossilization. When the sedimentation rate is high (so that a carcass is quickly buried), in anoxic environments (where little bacterial activity occurs), or when the organism had a particularly hard skeleton, the likelihood of fossilization increases. Larger, well-preserved fossils are uncommon.

Fossils, as part of a sedimentary rock, go through the same diagenetic processes as the host rock. For example, a calcite shell can dissolve, and a silica cement can then fill the cavity. Similarly, precipitating minerals can fill cavities that were previously occupied by blood vessels, vascular tissue, or other soft tissues. This preserves the organism’s form while changing its chemical composition, a process known as permineralization. Permineralization is most commonly associated with amorphous silica (chalcedony, flint, chert), carbonates (especially calcite), and pyrite.

5. Sedimentary rocks are classified into three main categories

Structurally tilted Jurassic sedimentary rocks. Photo by James St. John. Wikimedia

Sedimentary rocks are divided into three basic types based on their origin and composition: clastic, chemical, and natural.

Clastic sedimentary rocks are formed by the accumulation and cementation of rock and mineral fragments, or clasts, transported and deposited by wind, water, or ice.

These rocks are frequently distinguished by their grain size and sorting, which can provide information about the energy and distance of sediment transport.

Chemical sedimentary rocks form as a result of the precipitation of dissolved minerals in water, such as limestone or evaporites.

Organic sedimentary rocks are formed by the accumulation and compaction of organic substances, such as plant or animal remains or chemical secretions from organisms such as shells or coral.

These rocks can be identified by their natural content and frequently have distinct textures and structures.

Understanding the distinctive forms of sedimentary rocks and their properties can assist geologists in interpreting geologic records of a region and better comprehending the strategies that shaped the Earth’s surface.

6. Clastic sedimentary rocks are made up of fragments of other rocks

Clastic sedimentary rocks are made up of cemented-together rock fragments (clasts). Individual grains of quartz, feldspar, clay minerals, or mica are commonly used as clasts. Any mineral, however, may be present. Clasts can also be lithic fragments made up of multiple minerals.

Clastic sedimentary rocks are classified based on the dominant particle size. The Udden-Wentworth grain size scale is commonly used by geologists to classify unconsolidated sediment into three fractions: gravel (>2 mm diameter), sand (1/16 to 2 mm diameter), and mud (1/16 mm diameter). Silt (1/16 to 1/256 mm diameter) and clay (1/256 mm diameter) are two types of mud.

This scheme is used to classify clastic sedimentary rocks; conglomerates and breccias are mostly made of gravel, sandstones are mostly made of sand, and mud rocks are mostly made of mud. This three-part division is mirrored in older literature by the broad categories of rudites, arenites, and lutites.

7. Organic sedimentary rocks are composed of the remains of plants and animals

Sedimentary rocks in Jasper National Park, Alberta, Canada. Photo by Milorad Dimić MD, Serbia. Wikimedia

Organic sedimentary rocks form as a result of the accumulation and lithification of organic debris like leaves, roots, and other plant or animal material.

Coal, for example, is formed as a result of the accumulation and compaction of natural cloth, including plant matter, that has been buried and subjected to high stress and heat over hundreds of thousands of years.

Limestone can also be made of natural fabric, such as shells and skeletons of marine organisms that have been cemented together over time.

Other natural sedimentary rocks include diatomite, which is made up of diatom silica skeletons, and oil shale, which is made up of natural-wealthy sediment from historical lakes and oceans.

8. Sedimentary rocks contain valuable resources

Sedimentary rocks can contain valuable assets such as coal, oil, and natural gas, which are all formed from the remains of organic material trapped and preserved in the rock.

Other valuable resources found in sedimentary rocks include various minerals and metals such as iron ore, copper, and gold.

9. Sedimentary rocks can be used to interpret past environments and climates

Sedimentary rocks can provide important clues about past environments and climates, and they may help us understand Earth’s geologic records.

They also help identify a location’s geologic records.

Geologists can reconstruct the history of sediment deposition and erosion in a location by analyzing the layering and relationships between distinct sedimentary rock units.

This data can be used to interpret beyond tectonic activities, such as mountain level uplift and erosion, and to understand the evolution of landscapes over time.

10. Natural water filters are made from sedimentary rocks

Sandstone and limestone are two sedimentary rocks that can act as natural water filters. Impurities and pollutants can become trapped within the pores of the rock or filtered out by the rock matrix as water flows through it.

Similarly, sandstone is frequently used as a herbal filter in water wells and groundwater systems to help remove suspended solids and other impurities from the water.

Sandstone’s porosity allows water to slowly waft through it, giving the rock time to attract and filter impurities.

Overall, sedimentary rocks can play an important role in preserving the best and most accessible water sources on our planet.