Composition and Layers of Oceanic Crust

In Earth, the crust is the topmost layer above the mantle.Previously, we discussed the two types of crust: continental crust and oceanic crust.Continental crust ranges in thickness between 25 and 70 km, making up roughly 70% of Earth's total crust volume, although it only covers about 40% of the planet's surface.There is a much thinner oceanic crust, ranging from 5 to 10 km in thickness.

Continental crust is composed primarily of felsic rock and has an average density of 2.7 g/cm3.Light elements like silicon, aluminum, oxygen, sodium, and potassium are present in felsic rock.Because of these lighter elements, continental crust is less dense than oceanic crust, which has an average density of 2.9 g/cm3.

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Layers of more dense rock form the crust of the ocean.Geologists did not manage to drill through the oceanic crust to the mantle in 2014.Currently, the deepest it has been possible to drill is approximately two kilometers.Inferred knowledge and observation have provided scientists with most of what they know about the oceanic crust today.In the oceanic crust, ophiolites, for example, are parts that have been uplifted and exposed above sea level, often above continental crust (Fig. 7.55).Scientists can determine characteristics of the oceanic crust, including the layered structure, by observing ophiolites and data from existing drills and seismic data.

Life Cycle of the Oceanic Crust

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During the rock cycle, every rock on the planet is continually recycled.As millions of years of geological time unfolded, three types of rocks transitioned between each other (Fig. 7.56).During the formation of new crust, molten magmas are cooled and crystallized to form igneous rock at volcanoes and mid-ocean ridges.There are three types of igneous rocks: basalt, granite, and andesite (Fig. 7.57 A).Sediments may form as a result of the weathering and erosion of igneous rocks.Sedimentary rocks are formed due to the deposition and hardening of sedimentary rocks (Fig. 7.57 B).There is a third type of rock that can be formed by both igneous and sedimentary rock types.A metamorphic rock is formed when igneous or sedimentary rocks are exposed to high heat and pressure.Among the metamorphic rocks are marble, slate, schist, and gneiss (Fig. 7.57 C).As described in Figure 7.56, metamorphic rocks can also become sedimentary rocks upon weathering, erosion, and sediment deposition.

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The image was credited to Dr. Mark A. Wilson, College of Wooster, at Wikimedia Commons

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This image is from Wikimedia Commons, via Roll-Stone

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Likewise, all three types of rocks found in the earth's crust -- igneous, sedimentary, and metamorphic -- can be recycled back to their original molten states.Subduction zones are regions where oceanic crust is pushed back into the mantle.At mid-ocean ridges and volcanic hotspots, new oceanic crust is formed as old oceanic crust is subducted and melted into magma.According to this recycling process, 60 percent of Earth's surface is recycled every 200 million years, meaning that the oldest recorded oceanic crust rock is roughly the same age.Oceanic crust ages vary depending on location due to this recycling process.New crust is forming along mid-ocean ridges, which are younger than zones farther away (Figure 7.58).However, continental crust is rarely recycled.Oldest rocks on Earth are located on continental crust in northern Canada and western Australia and date from 3.8 to 4.4 billion years ago.


Deep Sea Sediment

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Sediments are natural materials that have been broken down into smaller pieces over time.Deep sea sediments are a feature of the oceanic crust that researchers have been able to explore in depth (Fig. 7.59) due to the examination of sediment cores.

Lithogonal sediments and biogenic sediments are the two most common types of sediment on the ocean floor.

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During the erosion and weathering of the continental crust, small rocks and minerals are formed that are characteristic of the lithium-rich deposits.Sediments derived from limestone can be carried to the ocean by runoff, rivers, and wind.After large rain events, you can often observe large plumes of lithogenic sediments near shorelines (Fig. 7.60).

Lithogonous sediments remain in suspension and cause water turbidity since they are always being moved by currents and shoreline waves.Upon reaching the coastline and relatively calmer waters, they settle out.Smaller particles settle out farther away from the shore than larger particles like rocks and sand.Due to the slow sinking of small particles, lithogenous sediments can be transported for a great distance by ocean currents.No rewrites were found

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These sediments, also known as "oozes," are primarily made up of phytoplankton and zooplankton.As plants and animals die, their remains sink slowly to the seafloor.Organisms such as bacteria consume large amounts of organic matter - the carbon-based parts of the organisms - which helps to return carbon to the ecosystem.What remains is composed of harder structures such as shells and skeletons.Carotenes are those whose skeleton was composed of calcium carbonate, and silicates are those whose skeleton was composed of silicates.In a sinking medium, small particles tend to aggregate into visible clumps.Researchers first observed this phenomenon in manned submersibles and described the phenomenon as marine snow (Fig. 7.61).

Beaches and Sand as well as Module 2 Unit 7: Seafloor Chemistry Topic 7.1 Types of Sediments provide more details about sediments.

Ocean water contains calcareous and siliceous compounds with unique properties.While both dissolve as they sink, their rate of dissolution varies depending on the temperature.Biogenous remains make up about one percent of sediments.The solid form of calcium carbonate is much more common in warm water that is rich in CO2 and under high pressure. In cold water containing CO2, however, calcium carbonate dissolves rapidly.The calcium compensation depth (CCD) is the depth at which calcium completely dissolves.Consequently, calcareous sediments are rarely found in deep sea sediments below the CCD.This can vary in depth.The Pacific basin has depths ranging from approximately 4.2 to 4.5 kilometers.In areas where calcareous sediments can settle, such as mid-ocean ridges, volcanoes, and seamounts, some seafloor features can rise above the CCD.Calcareous compounds, on the other hand, dissolve slowly in warm water, so siliceous compounds are important both in deep sea sediments and in shallower areas where cool water is upwelling.


Seafloor Volcanoes and Hydrothermal Vents

The hydrothermal vents in mid-ocean ridges are located in spreading zones.Hydrothermal vents on the ocean floor are similar to geysers and hot springs on continents where water runs under the surface and rises to areas that are very hot.As a result of the boiling water, steam rises to the surface.The spreading seafloor creates fissures and cracks where seawater percolates.Water is heated by geothermal sources to temperatures of up to 400 °C during its descent.Copper, zinc, iron, and sulfur are dissolved during this process.Due to the high hydrostatic pressure, the water doesn't boil despite being very hot.Super heated water rises out of the vents because it is buoyant, and it meets relatively cold and oxygen-rich ocean water, which precipitates many of the dissolved minerals.As a result of their dark billowing appearance, black smokers are called black precipitates when they contain the majority of sulfide precipitates. (Fig. 7.63 A).Smokers who are white emit minerals of lighter hues (Fig. 7.63 B).Some of these particles form chimney structures around the vents (Fig. 7.64).There were 55 meter tall chimneys found in the Atlantic basin of the ocean in 2000.Such vents are found in spreading regions on the seafloor.

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NOAA provided these images

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Photo by National Oceanic and Atmospheric Administration (NOAA)

When scientists first examined photos of hydrothermal vents, one of the most surprising discoveries they made was the strong benthic community surrounding them.There are many types of organisms that have adapted to live in these extreme environments.(Figure 7.65). Examples of these are crabs, mollusks, and worms.In these communities, the foundation of the food web is composed of microorganisms that break down compounds such as hydrogen sulfide and methane and convert them into energy and food.Almost everywhere else on Earth, the sun is the source of energy.It is possible for some vent tube worms to adapt so they are completely dependent on symbiotic microbes that transform hydrogen sulfide and methane into food (Fig. 7.65).The worm provides a suitable environment and a steady supply of nutrients for the microorganisms, and the microbes provide the worm with food.

First hydrothermal vents were discovered near the Galapagos rift, a 2 km deep rift in the eastern Pacific ocean basin.While surveying deep water, scientists noticed unusual hotspots.Hydrothermal vents have been viewed firsthand during subsequent dives with submersibles.

You can learn more about deep sea ecosystems in Module 4 Unit 4: Aquatic Ecosystems, topic 4.4 Offshore Marine Ecosystems.