Question

Explain why relatively coarse terrigenous sediments (e.g., sand) tend to accumulate close to the continents, while terrigenous clay is dispersed all across the ocean floor

Short answer

Relatively coarse terrigenous sediments, such as sand, accumulate close to continents because they have high settling velocities and settle quickly once they enter the ocean, resulting in them being deposited near their sources. On the other hand, terrigenous clay sediments have lower settling velocities and can be transported much further by ocean currents before settling. This, combined with clay particles' tendency to flocculate, allows them to be dispersed over vast areas of the ocean floor, even far away from the continental sources.

Step-by-step solution

1. Understanding terrigenous sediments

Terrigenous sediments are sediments derived from land and transported by rivers, glaciers, or other processes, into the ocean. Coarse sediments include sand, while fine sediments refer to clay. Due to differences in size and density, these sediments behave differently when transported and deposited in oceanic environments.

2. Sediment transportation and deposition

Sediments are transported from land to the ocean primarily through rivers, glaciers, and wind. During transportation, the sediments settle at different rates depending on their size and density. Larger, denser particles, such as sand, have a higher settling velocity and thus settle out more quickly than lighter, smaller particles like clay. This settling process is known as sedimentation.

3. Deposition near the continents

As coarse sediments like sand have a higher settling velocity, they tend to settle closer to the coast where the river or other sediment transport mechanisms discharge into the ocean. River currents and coastal processes like longshore drift or tidal currents also contribute to keeping the sand close to the continents, resulting in accumulations of sand along beaches, bars, and other nearshore areas.

4. Dispersion of clay across the ocean floor

In contrast to sand, clay particles have a lower settling velocity due to their smaller size and lower density. This means that they can be transported much further by ocean currents before they eventually settle on the ocean floor. Additionally, clay particles can flocculate (stick together) when they encounter chemical changes in the water, further reducing their settling velocity. These factors enable clay to become dispersed across vast areas of the ocean floor, even far from the continental sources.

5. Summary

Relatively coarse terrigenous sediments like sand tend to accumulate close to continents because they have higher settling velocities, allowing them to settle quickly after entering the ocean. As a result, they accumulate near the continent where they were transported by rivers. In contrast, terrigenous clay sediments have lower settling velocities, allowing them to be transported further from their source and dispersed more widely across the ocean floor.

Key concepts

Terrigenous Sediments

Terrigenous sediments originate from land and are carried into the ocean by natural forces. This type of sediment is mainly composed of two kinds of particles: sand, which is coarse, and clay, which is fine.

• Sand particles are larger and heavier, which affects how they travel and settle in the marine environment.
• Clay particles are smaller and lighter, making them behave differently during sedimentation.

Understanding terrigenous sediments is crucial for grasping how sediments are distributed in marine environments. These sediments' size and density play a huge role in determining which particles will settle where, especially in relation to their proximity to land. This natural process explains why certain types of sediments accumulate close to continents while others spread further across the ocean floor.

Sediment Transportation

Sediments from land reach the ocean via various methods such as rivers, wind, and glaciers. Each transportation method has a unique impact on how far and wide the sediments can travel.

• Rivers are significant carriers of sediments, bringing both sand and clay into the ocean.
• Wind especially affects lighter particles that can be carried over longer distances.
• Glaciers can transport large quantities of sediments but are more localized compared to rivers.

During transportation, the sediments' size and density determine how quickly they settle. Larger particles like sand settle faster and closer to the source, such as coastline areas. Smaller particles like clay, however, remain suspended for longer periods, traveling further away from their original deposit areas.

Settling Velocity

The settling velocity of a particle describes how fast it settles to the ocean floor. This factor is key in determining where different types of sediments end up.

• Larger, denser particles like sand have a higher settling velocity. This means they settle more quickly and don't travel far from where they enter the ocean.
• Clay particles, being lighter and smaller, have a lower settling velocity. They tend to drift with ocean currents, settling slowly over vast distances.

This principle helps explain sediment patterns near continental margins, where coarse materials settle quickly, while finer materials disperse in the ocean. Understanding settling velocity helps marine scientists predict sediment distribution in marine environments.

Continental Margins

Continental margins are underwater boundaries that extend from the coastline to the deep ocean.

• They typically include the continental shelf, slope, and rise.
• These areas are richer in nutrients because they receive a high level of sediment deposits from nearby land sources.
• The ocean's dynamic processes heavily influence sediment distribution on continental margins.

Because of their proximity to land, continental margins are the primary locations where terrigenous sediments accumulate, especially those with high settling velocities like sand. Coastal processes like longshore drift and tidal currents can redistribute sediments along continental margins, but generally, coarse sediments remain nearby. Meanwhile, finer sediments like clay may be transported much further before settling, thus getting dispersed across the ocean floor.