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The Oceanic Crust and Seafloor

The content and activities in this topic will work towards building an understanding of the rock cycle and movement of crustal rocks on Earth.

Composition and Layers of Oceanic Crust

The crust is the outermost layer of Earth above the mantle. As discussed earlier, crust can be divided into two types: continental crust and oceanic crust. The continental crust ranges from 25 to 70 km thick and makes up a total of approximately 70 percent of Earth’s total crust volume, though it only covers about 40 percent of the planet’s surface area. The oceanic crust is much thinner, ranging from 5 to 10 km thick.
 

The continental crust has an average density of 2.7 g/cm3 and is composed primarily of felsic rock. Felsic rock is rich in light elements such as silicon, aluminum, oxygen, sodium, and potassium. The presence of these lighter elements is responsible for continental crust being slightly less dense than oceanic crust, which has an average density of 2.9 g/cm3.
 

<p><strong>Fig. 7.55.</strong> An ophiolite rock complex is located on the island of Cyprus. Ophiolites are areas where oceanic crust has been thrust above the continental crust.</p><br />

Oceanic crust is primarily composed of more dense rock, which forms distinct layers. As of 2014, geologists had not been able to successfully drill through the oceanic crust to the mantle. The deepest that scientists have been able to drill is approximately two kilometers. Much of what scientists know about the oceanic crust today has been discovered by observation and inference. Ophiolites, for example, are portions of the oceanic crust that have been uplifted and exposed above sea level, often above continental crust (Fig. 7.55). By observing ophiolites and data from existing drills and seismic information, scientists can infer characteristics of the oceanic crust, in particular layering.


Life Cycle of the Oceanic Crust

<p><strong>Fig. 7.56.</strong> Diagram of the rock cycle</p><br />

All rocks in Earth’s crust are constantly being recycled through the rock cycle. The rock cycle is the transition of rocks among three different rock types over millions of years of geologic time (Fig. 7.56). Igneous rock is formed by the cooling and crystallization of molten magma at volcanoes and mid-ocean ridges, where new crust is generated. Examples of igneous rock are basalt, granite, and andesite (Fig. 7.57 A). Over time, igneous rocks may experience weathering and erosion from exposure to water and the atmosphere to produce sediments. The deposition and hardening of these sediments forms sedimentary rocks (Fig. 7.57 B). Both igneous and sedimentary rock types can transform physically and chemically into a third rock type. Metamorphic rocks are formed when igneous or sedimentary rocks are exposed to conditions of high heat and pressure. Examples of metamorphic rock include marble, slate, schist, and gneiss (Fig. 7.57 C). Metamorphic rocks can also transform to sedimentary rocks through weathering, erosion, and sediment deposition (Fig. 7.56).

<p><strong>Fig. 7.57.</strong> (<strong>A</strong>) Basalt, an example of igneous rock, from Mauna Ulu Lava Field, East Rift Zone, Kilauea Volcano, Hawaii</p><br />
<p><strong>Fig. 7.57.</strong> (<strong>B</strong>) Sandstone, an example of sedimentary rock, Jackson County, Ohio</p><br />
<p><strong>Fig. 7.57.</strong> (<strong>C</strong>) Marble, an example of metamorphic rock, Czech Republic</p><br />


 

<p><strong>Fig. 7.58.</strong> The age of oceanic crust in millions of years. The youngest crust (shown in red) is near mid ocean ridges and spreading zones.</p><br />

All three rock types in the earth’s crust—igneous, sedimentary, and metamorphic—can also be recycled back to their original molten magma form. This process occurs when oceanic crust is pushed back into the mantle at subduction zones. As old oceanic crust is subducted and melted into magma, new oceanic crust in the form of igneous rock is formed at mid-ocean ridges and volcanic hotspots. This recycling accounts for the recycling of 60 percent of Earth’s surface every 200 million years, making the oldest recorded oceanic crust rock roughly the same age. Because of this recycling, the age of the oceanic crust varies depending on location. Areas where new crust is being formed at mid-ocean ridges are much younger than zones further away (Fig. 7.58). By contrast, continental crust is rarely recycled and is typically much older. The oldest recorded rocks on Earth are all located on continental crust in northern Canada and western Australia and date to approximately 3.8 to 4.4 billion years old.

 

Activity

Activity: Crayon Rock Cycle

Simulate the rock cycle using crayons to build an understanding of the processes that occur to create sedimentary, metamorphic, and igneous rock.

 

Deep Sea Sediment

<p><strong>Fig. 7.59.</strong> Deep sea sediment cores can give scientists valuable information about the composition of the seafloor. Notice the various layers of sediment in the figure.</p><br />

Sediments are naturally occurring materials that have been broken down into smaller pieces. One feature of the oceanic crust that scientists have been able to explore in detail is deep sea sediment, often through examination of deep sea sediment cores (Fig. 7.59).

 

The two most common types of sediment on the ocean floor are lithogenous sediments, derived from rocks, and biogenous sediments, which are derived from living organisms.


<p><strong>Fig. 7.60.</strong> Intense rainfall and melting snow can increase sediment runoff into the ocean. This image is from the Mississippi river delta.</p><br />

Lithogenous sediments are small rocks and minerals that are the result of erosion and weathering of the continental crust. Lithogenous sediments can be carried to the ocean by runoff, rivers, and wind. Large plumes of lithogenous sediments can often be observed near shorelines after large rain events (Fig. 7.60).

 

Lithogenous sediments remain in suspension and cause high water turbidity because they are in constant movement due to currents or shoreline surf. When they reach the coastline and relatively calmer water they begin to settle out. Larger particles like rocks and sand settle out very near the shore while smaller particles settle out further away. Since small particles sink slowly, ocean currents can transport lithogenous sediments over a long distance. Small particles (< 4 micrometers) known as abyssal clay make up a large portion of the sediment on the ocean floor. Prior to the theory of plate tectonics, early scientists suggested that since continental erosion was continually taking place, lithogenous sediments should constantly be filling in ocean basins resulting in a very thick layer of sediment. However, early sediment cores revealed a much thinner layer of sediment than expected. This provided further evidence that continental crust was continually being recycled, along with the sediment layer.

 

<p><strong>Fig. 7.61.</strong> Marine snow is made of biogenous particles that clump together and gradually sink to the ocean floor.</p><br />

Biogenous sediments, also sometimes referred to as “oozes,” are composed primarily of the remains of living organisms—phytoplankton and zooplankton. When plants and animals die, their remains slowly sink to the seafloor. Bacteria consume much of the organic matter—the carbon-based parts of the organisms, which helps to cycle carbon back into the biological system. Particles that remain are composed of harder structures like shells and skeletons. They fall into two categories: calcareous if the skeleton was made of calcium carbonate, and siliceous if the skeleton was made from silicates. As small particles sink they tend to aggregate into clumps that are visible to the naked eye. Deep sea researchers first noticed this phenomenon in manned submersibles and coined the term marine snow to describe the particles constantly showering down (Fig. 7.61).

 

For more detail about sediments, see Beaches and Sand, and also Module 2 Unit 7: Seafloor Chemistry Topic 7.1 Types of Sediment.

 

Calcareous and siliceous compounds have unique properties in ocean water. Both substances dissolve as they sink, but at different rates depending on temperature. Only approximately one percent of biogenous remains become sediments. Calcium carbonate dissolves rapidly in cold water that is rich in CO2 and at high pressure, but is relatively common as a solid in warm water. The depth at which calcium completely dissolves is known as the calcium compensation depth (CCD). Consequently, calcareous sediments are not frequently found in deep sea sediments below the CCD. The depth of the CCD varies. In the Pacific ocean basin it ranges from approximately 4.2–4.5 km deep. Some seafloor features such as mid-ocean ridges, volcanoes, and seamounts may rise above the CCD; these are areas where calcareous sediments can be deposited. Siliceous compounds are different than calcareous compounds because they dissolve faster in warm water than cold water, therefore they can be common in both deep sea sediments and in shallower areas where there is a lot of upwelling of cool water.

 

Activity

Activity: Sediment Cores

Simulate taking sediment cores in the ocean to build an understanding of sediment layering and sediment core sampling.

 

Seafloor Volcanoes and Hydrothermal Vents

Mid-ocean ridges and spreading zones are home to hydrothermal vents. Hydrothermal vents in the ocean are analogous to geysers and hot springs on continents where groundwater percolates up to 2 km below the surface to areas that are very hot. The resulting boiling water and steam rush to the surface. At hydrothermal vents, cool seawater percolates down in fissures and cracks created by the spreading seafloor. As water moves down, it is heated from geothermal sources, reaching temperatures as high as 400 °C. Throughout this process, minerals like copper, zinc, iron, and sulfur dissolve in the water. Although the water is very hot, it does not boil due to the high hydrostatic pressure. When the super heated water rises out through the vents because it is buoyant, it meets relatively cold and oxygen rich ocean water and many of the dissolved minerals precipitate out as particles. If the majority of precipitates are sulfides and have a black color, the vents are known as black smokers due to their dark billowing appearance (Fig. 7.63 A). White smokers emit minerals with lighter hues (Fig. 7.63 B). In some cases these particles combine to form chimney structures around the vents (Fig. 7.64). In 2000 scientists discovered a field of chimneys in the Atlantic ocean basin that had reached 55 meters tall. Hydrothermal vents are found in spreading regions on the seafloor.

<p><strong>Fig. 7.63.</strong> (<strong>A</strong>) Black smokers emit dark sulfides, Sully Vent in the Main Endeavour Vent Field, northeast Pacific ocean basin.</p><br />
<p><strong>Fig. 7.63.</strong>&nbsp;(<strong>B</strong>) White smokers emit minerals such as barium, calcium, and silicon, Champagne Vent, Marianas Islands Marine National Monument.</p><br />

 


<p><strong>Fig. 7.64.</strong> Deep sea chimneys measuring 9 m tall from base to tip, East Diamante Volcano, Marianas Islands Marine National Monument. Chimneys form at hydrothermal vents when particles dissolved in the superheated fluid from the vent meets cold ambient water and precipitates out.</p><br />
<p><strong>Fig. 7.65.</strong> Deep sea vents, such as this one in the Galapagos Islands, are home to diverse communities of crabs, mussels, tube worms, microbes, and many other species.</p><br />


 

One of the most surprising discoveries for scientists, who first looked at photographs of hydrothermal vents, was the highly productive benthic community surrounding them. Many types of organisms have adapted to live in these extreme habitats. These include crabs, mollusks, and worms (Fig. 7.65). The base of the food web in these communities are microorganisms or microbes that use compounds, particularly hydrogen sulfide and methane, from the vents and convert them to useable energy and food. In virtually every other ecosystem on Earth, the ultimate source of energy is the sun. Some vent tube worms have adapted so they are entirely dependent on symbiotic microbes that convert hydrogen sulfide and methane into food (Fig. 7.65). The worm provides a suitable environment and steady supply of nutrients to the microorganisms and the microbes supply the worm with food.

 

Scientists discovered the first hydrothermal vents in 1976 at the 2.5 km deep Galapagos rift in the east Pacific ocean basin. These vents were discovered when scientists observed unusual hotspots during a deep water survey. Subsequent dives using submersibles allowed scientists to view hydrothermal vents firsthand.

 

For more information on deep sea ecosystems see Module 4 Unit 4: Aquatic Ecosystems, Topic 4.4 Offshore Marine Ecosystems.

 

 

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.