Question

What is the origin of the sediments that make up continental shelves? Why are the shelves on the eastern coast of North America so much wider than those along the west coast?

Short answer

The sediments that make up continental shelves originate from the erosion of nearby landmasses, which are then transported to the ocean by rivers, glaciers, and wind. The eastern coast of North America has wider continental shelves due to its passive margin, characterized by low tectonic activity and extensive sediment accumulation, along with higher rates of sediment deposition from major rivers. In contrast, the west coast of North America has narrower continental shelves due to its active margin with high tectonic activity and fewer and smaller rivers transporting sediments to the ocean.

Step-by-step solution

Origin of sediments making up the continental shelves

Continental shelves consist of the submerged part of a continent extending from the shoreline to the steep slope which descends to the ocean floor. They are formed by the accumulation of sediments eroded from the nearby landmasses and delivered to the ocean by rivers, glaciers, and wind. These sediments include materials such as sand, silt, clay, and organic matter. Over time, the deposition of sediments creates layers, which can compact and eventually turn into sedimentary rocks, such as sandstone, shale, and limestone.

Why are continental shelves on the eastern coast of North America wider than those on the west coast?

The width of the continental shelves is determined by several factors, including the rate of sediment deposition, tectonic processes, and sea-level changes. The continental shelves along the eastern coast of North America are wider mainly for the following reasons: 1. Passive Margin: The eastern coast of North America is part of a passive margin, which is formed when the continental crust is located away from active plate boundaries. Passive margins are characterized by low tectonic activity, extensive sediment accumulation, and gentle slope. These factors contribute to the formation of wide continental shelves. 2. Higher Rates of Sediment Deposition: The geology and climate of the eastern coast of North America support a higher rate of erosion and sediment transportation than the west coast. Major rivers such as the Mississippi, Hudson, and St. Lawrence transport large amounts of sediments from the continental interior to the coast, contributing to the growth of the continental shelves. In contrast, the west coast of North America is characterized by an active margin, where the tectonic activity is high due to the interaction between the Pacific plate and the North American plate. This tectonic activity causes the formation of steep slopes, narrow continental shelves, and deep trenches along the west coast, as well as more frequent earthquakes and volcanic eruptions. Additionally, the west coast has fewer and smaller rivers compared to the east coast, which means less sediment is transported to the ocean, resulting in narrower continental shelves.

Key concepts

Sediment Deposition

Sediment deposition is a natural process where materials like sand, silt, clay, and organic particles accumulate in a new location. These materials originate from the erosion of landmasses. Various forces such as rivers, glaciers, and wind carry these sediments until they settle, typically at a lower elevation. On continental shelves, the deposited sediments come from nearby land. This deposition occurs over long periods. As layers build up, they often compact and harden into sedimentary rocks, creating formations like sandstone, shale, and limestone.
Sediments play a crucial role in forming the continental shelves, which are gently sloping areas at the edges of continents. The continuous supply of sediments helps expand these shelves over time as deposits stack up layer by layer.

Passive Margin

A passive margin forms where the edge of a continental plate moves away from tectonic activity and is found along the eastern coast of North America. These regions are typically far from active plate boundaries. As a result, passive margins experience very low tectonic activity. This means less geological upheaval such as earthquakes and volcanoes.
The characteristics of passive margins promote wider continental shelves. These margins showcase gentle slopes and lots of sediments are deposited here due to the lack of tectonic disruption. The eastern North American margin is a classic example. Here, vast amounts of sediments accumulate and create broad, expansive continental shelves. Major rivers like the Mississippi and Hudson transport sediments from the continent into the ocean, facilitating this process.
The eastern coast of North America, with its passive margin, provides a contrast to the narrower, more geologically active margins found on the west coast.

Tectonic Activity

Tectonic activity refers to the movement and interaction of Earth's plates. It plays a vital role in shaping continents, ocean basins, and geological features. This activity includes the creation of mountains, earthquakes, and volcanoes, and profoundly influences continental shelf formation.
On the western coast of North America, active tectonic margins are present. Here, the interaction between the Pacific Plate and North American Plate results in considerable geological activity. This activity leads to the formation of steep ocean floors and narrow continental shelves. As tectonic plates converge, they often create deep-sea trenches and trigger frequent earthquakes.
In contrast to passive margins, active tectonic regions like the western U.S. have less sediment deposition and more dramatic underwater topography. Fewer large rivers add to this difference, resulting in less sediment reaching their continental shelves, creating steep and narrow ocean margins.