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History of Earth – Seafloor Structure and Features of Ocean

Grade 8
Jun 3, 2023

Seafloor Structure and Features of Ocean

In this article, we’ll learn about the seafloor structure and features of the ocean. Let’s begin

Introduction to Earth’s Crust

There are two types of Earth’s crust: oceanic crust and continental crust.

The oceanic crust, which extends 5-10 kilometers (3-6 kilometers) beneath the ocean floor, is mostly made up of various types of basalts.

The oceanic crust is constantly being formed at mid-ocean ridges, where tectonic plates are separating apart. Young oceanic crust forms magma that wells up from these rifts as the Earth’s surface cools.

The continental crust is thicker and denser than the oceanic crust. The continental crust is the outermost layer of Earth’s lithosphere that forms the planet’s continents and continental shelves and is created near plate boundaries between continental and oceanic tectonic plates.


Types of Crust

Plate Tectonics and the Ocean Floor

Bathymetry, or the shape of the ocean floor, is largely the result of plate tectonics. The Earth’s outer rocky layer is made up of about a dozen large sections known as tectonic plates, which are arranged like a spherical jigsaw puzzle floating on top of the Earth’s hot flowing mantle.

Convection currents in the molten mantle cause the plates to move a few centimeters around the Earth each year. Many ocean floor features are formed as a result of interactions between these plates. The ocean floor (sea floor) spreads due to the movement of plates.

The shifting plates can collide (converge), diverge (diverge), or slide past (transform). When plates converge, one plate may move beneath the other, causing earthquakes, volcanoes, and deep ocean trenches.

Molten magma flows upward between plates where they diverge, forming mid-ocean ridges, underwater volcanoes, hydrothermal vents, and new ocean floor crust. Faults that connect two areas where plates are converging or diverging are known as transform boundaries. These continental boundaries typically form zig-zag patterns.


Plate boundary

Seafloor Spreading

The process by which tectonic plates—large slabs of Earth’s lithosphere—split apart from each other is known as seafloor spreading.

Features of the Ocean

Based on its physical conditions, the ocean can be divided into several categories. These are the categories derived from the land:

Intertidal or littoral – Areas located between high and low tide. Crabs, barnacles, muscles, and other plants and animals rely on this ecosystem.

Continental shelf – The area between low tide and the shelf’s edge.

Continental slope – A steep slope of the ocean bottom to the ocean floor.

Abyssal plane: The ‘bottom’ of the ocean, where it is relatively flat, is known as the abyssal plane.

Hadal zones: These are regions with deep trenches in the abyssal plane.

The science of topography is the study of landforms. When we talk about the topography of the ocean floor, we’re referring to the various shapes that the ocean floor can take.

Continental Margin

The seafloor’s topography is bumpy and irregular, with many high mountains and deep depressions. The Marianas Trench, which is in the Pacific Ocean, is roughly 11 km deep.

Continental margin

The continental margin is the area where continents’ edges meet the ocean. It is made up of sediment-covered continental crust that eventually meets oceanic crust. Continental margins are the ocean’s shallowest areas.

Landforms in the Ocean (Submarine Relief)

Under the ocean, there are mountains, basins, plateaus, ridges, canyons, and trenches.

Submarine Relief refers to the relief features found on the ocean floor.

The ocean basins are classified into four major groups. They are as follows:

  1. Continental shelf
  2. Continental slope
  3. Abyssal plains
  4. The ocean deeps/submarine trenches

Features of the Ocean floor

Features of the Ocean floor

Continental Shelf

The continental shelf is the shallowest part of a continental margin that extends seaward from the shore.

Continental shelves range in width from 30 to 60 kilometers.

The continental shelf on the Pacific coast of the United States is only a few kilometers wide, whereas the continental shelf on the Atlantic coast is hundreds of kilometers wide.

The average depth of water above continental shelves is approximately 130 meters. Remember that during the last ice age, the sea level was approximately 130 m lower than it is now; thus, most of the world’s continental shelves must have been above sea level at the time. As a result, today’s coastlines are very different from what they were during the last ice age.

At the time, the Bering land bridge connected Siberia to North America, Great Britain to Europe, and a large landmass existed where the Bahamas are now only scattered islands.

When the Earth’s surface began to warm after the last ice age and the continental ice sheets melted, the sea gradually engulfed the continental shelves. Beaches and other coastal landforms from that era are now submerged and spread far beyond the current shoreline.

Commercially valuable fish now inhabit the continental shelves’ shallow, nutrient-rich waters.

Continental shelves are very important to human beings. They provide fish, minerals such as sand and gravel, and so on.

The shelves provide a significant portion of the world’s petroleum and natural gas.

On continental shelves, coral reefs are also common.

The presence of submarine canyons that extend to the continental slope is one of the most striking features of the continental shelf. These canyons are ‘steep-sided valleys’ cut into the seafloor.

They resemble the gorges found on the continents.

Underwater landslide is one of the causes of the submarine canyon’s formation.

Continental shelf

Continental Slope

The seafloor drops rapidly beyond the continental shelves to depths of several kilometers, with slopes averaging nearly 100 m/km. These sloping areas are known as continental slopes. The continental slope is the true edge of a continent, according to marine geologists, because it generally marks the edge of the continental crust.

This slope is cut by deep submarine canyons in many places. Submarine canyons are similar to land canyons, and some are larger than the Grand Canyon in Arizona.

Submarine canyon and Underwater canyon

Turbidity currents, which are rapidly flowing water currents along the seafloor that carry heavy loads of sediments, cut these submarine canyons, similar to mudflows on land.

Turbidity currents may form as a result of underwater landslides on the continental slope triggered by earthquakes, or they may form as a result of sediment stirred up by large storm waves on the continental shelf.

Turbidity currents

Continental Rise

A continental rise is the gently sloping accumulation of deposits from turbidity currents that form at the base of the continental slope. The rise gradually thins out and eventually merges with seafloor sediments beyond the continental margin.

The continental slope ends in deeper depressions on the seafloor, known as deep-sea trenches, in some places, particularly around the Pacific Ocean. There is no continental rise at the foot of the continental margin in such places.

Continental rise

Deep-Ocean Basins

Beyond the continental margin are ocean basins, which cover approximately 60% of the Earth’s surface and contain some of the most interesting topography on the planet.

The Abyssal Plains

Abyssal plains are the flattest parts of the ocean floor, 5 or 6 km below sea level. Abyssal plains are plains covered by hundreds of meters of fine-grained muddy sediments and sedimentary rocks deposited on the top of basaltic volcanic rocks.

Abyssal plains

The Abyssal Plains have Two Types of Relief Features.

  1. Submarine ridges are ocean mountains found near the middle of the ocean. Submarine ridges are also known as mid-oceanic ridges. Volcanoes and earthquakes are common. The Mid-Atlantic Ridge is the world’s longest submerged mountain ridge.
  2. Seamounts and guyots: Submerged volcanoes with sharp tops are known as seamounts and guyots. Guyots or Table mounts are seamounts with a flattened top.

Seamount and Guyot

Deep Sea Trenches (Trenches in the sea):

The deep-sea trenches, which are elongated, sometimes arc-shaped depressions in the seafloor several kilometers deeper than the adjacent abyssal plains, are the deepest parts of the ocean basins. Many deep-sea trenches are found near volcanic island chains, such as Alaska’s Aleutian Islands, and the majority of them are found near the Pacific Ocean’s margins.

Deep-sea trenches are relatively narrow, measuring about 100 kilometers wide, but they can stretch for thousands of kilometers. The Peru-Chile trench, depicted in the Figure below, is nearly 6000 km long and 40 km wide on average. The Mariana Trench in the Pacific Ocean is the world’s deepest known location.

Deep Ocean basin trenches

Mid-Ocean Ridges

The mid-ocean ridges, which run through all of the ocean basins and have a total length of more than 65,000 km — a distance greater than the circumference of the Earth — are the most visible features of the ocean basins.

The tallest peaks in mid-ocean ridges rise above the ocean as volcanic islands.

Volcanic eruptions and earthquakes are common on mid-ocean ridges. These ridges’ crests frequently have valleys called rifts running through their centers. Rifts can reach depths of up to 2 km.

The mid-oceanic ridge

Ridges in the middle of the ocean do not form continuous lines. They are divided into shorter, stepped sections that run at right angles across each mid-ocean ridge. The areas where these breaks occur are referred to as fracture zones, as illustrated in Figure 16.21. Fracture zones are about 60 km wide and stretch for thousands of kilometers across the seafloor.

Facture zone

Hydrothermal Vents

A hydrothermal vent is a hole in the seafloor that allows magma-heated fluid to erupt. The majority of hydrothermal vents are found at the bottom of rifts in mid-ocean ridges.

When metal oxides and sulfides are present in the heated fluid erupting from these vents, they immediately precipitate out of the fluid and form thick, black smoke-like plumes.

A black smoker is a type of hydrothermal vent that ejects superheated water at temperatures of up to 350°C. The black smokers were discovered in a rift valley of a mid-ocean ridge.

Warm water is emitted by the second type of vent, known as a white smoker. Seawater circulating through hot crustal rocks in the centers of midocean ridges causes smokers. Plate tectonics is the primary cause of mid-ocean ridges and the volcanic activity associated with them.

Hydrothermal vent

Seafloor Spreading

Divergent plate boundaries cause seafloor spreading. The heat from the mantle’s convection currents makes the crust more plastic and less dense as tectonic plates slowly move apart. Less dense material rises, forming mountains or elevated areas of the seafloor.

The mid-ocean ridge is the location of seafloor spreading, which occurs when tectonic plates—large slabs of Earth’s lithosphere—split apart from one another.

Harry H. Hess, an American geophysicist, proposed seafloor spreading in 1960. Hess was able to map the ocean floor and discover the mid-Atlantic ridge by using sonar (mid-ocean ridge). He also discovered that the temperature near the mid-Atlantic ridge was warmer than the surface farther away.

He suspected that the high temperature was caused by magma leaking from the ridge. This hypothesis on the shifting position of the Earth’s surface supports Alfred Wegener’s 1912 Continental Drift Theory.

Evidence of Seafloor Spreading

Several pieces of evidence had been gathered to support Harry Hess’s hypothesis about seafloor spreading. This evidence came from molten material investigations, seafloor drilling, radiometric age dating, fossil ages, and magnetic stripes. However, this evidence was also used to support the Theory of Continental Drift.

The Mid-Atlantic Ridge, for example, separates the North American and Eurasian plates, as well as the South American and African plates.

The East Pacific Rise is a mid-ocean ridge that separates the Pacific plate from the North American plate, the Cocos plate, the Nazca plate, and the Antarctic plate in the eastern Pacific Ocean.

The Southeast Indian Ridge marks the point at which the southern Indo-Australian plate diverges from the Antarctic plate.

Mid-Atlantic ridge

Seafloor spreading is not uniform across all mid-ocean ridges. Tall, narrow underwater cliffs and mountains are located on slowly spreading ridges.

The slopes of rapidly spreading ridges are much gentler. For example, the Mid-Atlantic Ridge is a slow-spreading center. Every year, it spreads 2-5 centimeters (.8-2 inches) and forms an ocean trench the size of the Grand Canyon.

In contrast, the East Pacific Rise is a rapidly spreading center. Every year, it spreads 6-16 centimeters (3-6 inches). There is no ocean trench at the East Pacific Rise because the seafloor is spreading too quickly for one to form!

The newest and thinnest crust on Earth is found near the center of the mid-ocean ridge, which is the actual location of seafloor spreading.

Seafloor Structure


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