Sedimentary rock

Rocks can form from tiny bits. These bits are called sediment.

Wind, water, or ice move these bits. They land in one place. Then they settle in layers.

Over time, more layers pile on top. The weight presses the bits down. This makes them stick together.

Some rocks come from dead sea life. Shells pile up on the floor. This can make limestone.

These rocks can hold coal and water. They even exist on Mars!

They tell us about Earth's past.

Sedimentary rocks form from tiny bits called sediment. These bits can be pieces of old rocks. They can also be parts of living things.

Wind, water, or ice move these bits to new places. When the moving stops, the bits settle in layers. We call these layers strata.

Often, these layers form in large areas called sedimentary basins.

There are different ways these rocks make. Clastic rocks form from rock fragments. These fragments are called clasts. They can be big like gravel or tiny like clay. Sandstone is a common clastic rock. It is made mostly of sand.

Some rocks come from living things. This is called biochemical formation. For example, shells from sea life can pile up. This makes limestone.

Coal also forms from old plants.

Other rocks form from chemicals in water. These are called chemical sedimentary rocks. They form when minerals come out of a liquid. As new layers pile up, the weight presses the bottom layers. This process is called diagenesis. It turns loose bits into hard rock.

Sedimentary rocks are special types of rock that cover much of our world. They form when tiny particles called sediment settle in one place. This sediment can be pieces of old rocks or bits of living things.

These rocks cover about 73% of the Earth's current land surface. However, they only make up about 8% of the total volume of the crust. They act like a thin layer over much deeper rocks. Scientists who study these rocks use a field called sedimentology.

There are several ways these rocks form step by step. First, weathering and erosion break down existing rocks into small bits. These bits are moved by water, wind, ice, or gravity. This movement is called denudation.

Next, the particles settle in layers called strata. These layers often collect in large areas called sedimentary basins. Finally, a process called diagenesis turns the loose bits into hard rock. As new layers pile up, the weight creates pressure. This pressure and chemical changes turn sediment into solid stone.

Geologists group these rocks into four main categories based on how they form. Clastic rocks are made from rock fragments called clasts. These fragments can be large gravel or tiny mud.

Biochemical rocks come from living things. For example, coral shells can build up to form limestone. Chemical rocks form when minerals come out of a water solution. Lastly, other rocks form from special events like volcanoes or impacts.

Clastic rocks are often named by the size of their pieces. Geologists use the Udden-Wentworth scale to measure them. Gravel makes rocks called conglomerates or breccias. Sand makes rocks called sandstones.

Mud makes mudrocks, which include siltstones and claystones. Sandstones can be described by their minerals, like quartz or feldspar. They can also be "clean" or "muddy" based on the material between grains. This helps scientists know exactly how the rock was made.

Studying these rocks helps us understand our own world and history. They can tell us about ancient climates and where life used to live.

Sedimentary rocks are also very useful for people today. We find important resources like coal and fossil fuels inside them. They also hold our drinking water and metal ores. Engineers study them to build safe roads, houses, and tunnels. Even on Mars, scientists have found sedimentary rocks.

Sedimentary rocks are essential components of the Earth's crust. They form through the cementation of sediments, which are particles of mineral matter or organic material. These particles accumulate at the Earth's surface through a process called sedimentation. While sedimentary rocks cover 73% of the Earth's current land surface, they only represent about 8% of the total volume of the crust. They act as a thin veneer over a much larger crust made of igneous and metamorphic rocks.

Scientists who study these rocks and their origins practice a discipline called sedimentology.

The formation of these rocks begins with weathering and erosion. These processes break down existing rocks into geological detritus. Agents of denudation, such as water, wind, ice, or mass movement, transport this material to a place of deposition. Sedimentation occurs when these particles settle in place, often forming layers called strata. These layers create a structure known as bedding, and they frequently accumulate in large sedimentary basins.

After deposition, the sediments undergo diagenesis to become solid rock. Diagenesis includes the chemical, physical, and biological changes that occur after a sediment is deposited. As new layers accumulate, the weight of the overlying material increases pressure. This process involves compaction and lithification, where loose particles are turned into stone. One specific mechanism is pressure solution. In this process, material dissolves where grains are under high stress and recrystallizes in open spaces. This acts as a cement that makes the rock harder and more compact.

Geologists classify sedimentary rocks into four main groups based on their origin. Clastic sedimentary rocks are composed of rock fragments, or clasts, that are cemented together. Biochemical sedimentary rocks form when organisms use dissolved materials to build their tissues. Chemical sedimentary rocks occur when minerals precipitate from a water solution that has become supersaturated. Finally, a fourth category includes rocks formed by impacts, volcanism, or other minor processes.

Clastic rocks are further categorized by the size of their particles using the Udden-Wentworth scale. Gravel-sized particles form conglomerates, which have rounded grains, or breccias, which have angular grains. Sand-sized particles form sandstones. Mud-sized particles, which include silt and clay, form mudrocks. Mudrocks can be specifically identified as siltstones, mudstones, or claystones.

Sandstones require a more detailed classification known as the Dott scheme. This scheme looks at the abundance of framework grains like quartz, feldspar, or lithic fragments. For example, a quartz sandstone must contain more than 90% quartz grains. Geologists also look at the muddy matrix between the grains. Sandstones with very little matrix are called arenites, while those with more than 10% matrix are called wackes.

Biochemical and chemical processes create distinct rock types. Biochemical rocks include limestone made from the skeletons of corals or mollusks. They also include coal, which forms from compressed vegetation, and chert, made from microscopic siliceous skeletons. Chemical sedimentary rocks include evaporites like halite, which is rock salt, and gypsum. These form when minerals precipitate from water.

Understanding sedimentary rock sequences is vital for many fields. In civil engineering, studying these rocks helps in constructing roads, tunnels, and canals. They are also major sources of natural resources, including coal, fossil fuels, drinking water, and ores. Furthermore, these rocks serve as a primary record of Earth's history. They provide data on paleoclimatology, palaeogeography, and the history of life.

Even beyond Earth, sedimentary rocks have been discovered on Mars.