Sediment

Small bits of rock move around.

Wind, water, and ice carry them. They travel to new places. Then they land and stay there.

Water moves things like sand and silt. These bits can sink to the bottom of a river. If they get buried, they can turn into rock. This is how new rocks are made.

Wind also moves bits of rock. It can make big sand dunes. Ice can move very large rocks too. These rocks land in piles when the ice melts.

Some bits are big like boulders. Others are tiny like clay. The shapes can be round or sharp. Some bits even have tiny scratches on them.

These bits can land in lakes or oceans. They can even come from dead sea life. The world is always moving these tiny pieces.

Sediment is made of loose, solid bits. These bits come from rocks that break down. This happens through weathering and erosion. Wind, water, or ice then carry the bits to new places. When they land, we call this deposition.

Water is a common way to move sediment. Rivers carry sand and silt to the sea. If these bits get buried, they can turn into rock. This is called lithification.

Other things move sediment too. Wind can move dust to make sand dunes. This is called aeolian transport. Glaciers also move bits. They can carry large boulders. These rocks land in piles called moraines when the ice melts.

We can group sediment by its size. Some bits are big like boulders. Others are tiny like clay. We also look at their shape. Some bits are round with smooth edges. Others are angular with sharp corners. You might even see tiny scratches on their surface.

Sediment is made of loose, solid particles that move to new places. It starts when rocks break down through weathering and erosion. Once broken, these bits are carried away by wind, water, or ice. Gravity also helps pull these particles down. When the particles finally stop moving and land, we call this deposition.

Over a long time, buried sediment can turn into rock through a process called lithification. This is how we get rocks like sandstone and siltstone.

Water is the most common way that sediment moves. In rivers, sand and silt can float in the water. This is called being in suspension. When the river reaches the sea, the particles sink to the bottom. This is called sedimentation.

Wind can also move sediment, which is called aeolian transport. This creates things like desert sand dunes and dust layers called loess. Glaciers move sediment too, often carrying huge boulders. When the ice melts, it leaves behind piles called moraines or till.

Scientists group sediment by its size, shape, and what it is made of. They measure size using the Phi scale. This scale goes from huge boulders down to tiny clay. Boulders can be over 10 inches wide. Clay is much smaller than one micrometer. We also look at the shape of the grains. We check the form, which is the overall shape. We also look at roundness, which tells us if the edges are sharp or smooth. Finally, we look at surface texture. This includes tiny scratches or pits on the grain.

Different parts of the ocean collect different types of sediment. Near the shore, you find littoral sands on beaches. Further out on the continental shelf, you find silty clays. In the middle of the ocean, much of the sediment comes from the skeletons of dead organisms. These are called biological sediments. In deep areas, a system called turbidites moves sediment into deep basins. Any deep spot where sediment gathers is called a sediment trap.

Human activities can change how much sediment moves. For example, cutting down forests can lead to more erosion. In Madagascar, this has created deep gullies called lavakas. This extra sediment can turn rivers a dark red-brown color. It can also harm coral reefs by smothering them. In the South Pacific Gyre, scientists found something amazing in the sediment. They found tiny living things 250 feet below the seafloor. These organisms might be the longest-living life forms ever found.

Sediment is a solid material composed of loose particles. These particles are moved from one location to another through various natural processes. This movement begins with weathering and erosion, which break down larger materials into smaller pieces. Once broken, the particles are transported by wind, water, ice, or gravity. When these particles eventually settle in a new location, the process is called deposition.

Over vast periods of time, buried sediment can undergo lithification. This process turns loose particles into sedimentary rocks, such as sandstone or siltstone.

Transport mechanisms depend on the strength of the flow and the particle's properties. Water is the most frequent transporter, known as fluvial processes. In rivers, particles like sand and silt can travel in suspension. When the water slows down, such as at a sea bed, the particles settle through sedimentation.

Wind, or aeolian processes, can transport fine sediment to create sand dunes or loess. Glaciers also move material, carrying everything from fine silt to massive boulders. When glaciers melt, they leave behind deposits known as till or moraines.

Geologists classify sediment using three main criteria: grain size, shape, and composition. Size is measured using the Phi scale, which is a log base 2 scale. This scale categorizes particles from massive boulders to microscopic colloids. For example, a boulder can be larger than 256 mm. Conversely, clay particles are smaller than 3.9 micrometers. Scientists also examine the composition, which includes the parent rock lithology and chemical makeup.

Particle shape is defined by three specific parameters. The first is form, also called sphericity. This describes the overall shape, such as being spherical, platy, or rodlike. The second is roundness, which measures how sharp the corners and edges are. Descriptions range from very angular to very rounded.

The third parameter is surface texture. This refers to small-scale features like pits, ridges, or scratches. Scanning electron microscopes are often used to see these tiny details. For instance, frosted textures are common in aeolian sediments moved by wind.

Marine environments contain distinct depositional zones. Near the coast, littoral sands form beaches and coastal bars. The continental shelf contains silty clays with increasing biological content. Further out, the shelf margin is mostly made of calcareous faunal skeletons. In the deep ocean, the turbidite system moves sediment into abyssal basins. Any deep depression that collects these materials is called a sediment trap.

This process often leads to a seaward fining of grain size.

Human activity significantly impacts sediment movement and quality. In Madagascar, the removal of vegetation has created deep gullies called lavakas. These gullies can account for 84% of all sediment carried by rivers in certain areas. This causes rivers to turn a dark red-brown color. In the EU and UK, the cost of removing sediment from water erosion exceeds 2.3 billion euro annually. Additionally, watershed development near coral reefs can cause stress. Excess sediment can smother corals or cause algal blooms that limit space for new polyps.

Sediment also holds secrets about the history of life and the planet. Sedimentary rocks often contain fossils of the organisms that lived when the sediment was deposited. Furthermore, lake bed sediments can help scientists determine past climatic conditions. Even deep beneath the seafloor, life persists in unexpected ways. In the South Pacific Gyre, biologists found aerobic microorganisms 250 feet below the seafloor. These organisms were found in sediments up to 101.5 million years old. They may represent some of the longest-living life forms ever discovered.