Ocean Basins and Continental Margins (Chapter 4)

How to study the ocean floor

1) Rock dredging: drag metal basket up slope to collect sample. 2) Submersibles - remotely operated vehicle (ROV) 3) Rocks sampled using drill ships. Takes a lot of time. Uses satellite and GPS.

Continental Shelf

A gently sloping, shallow area of the ocean floor that extends outward from the edge of a continent. They are much more like the continent than like the deep-ocean floor, and they may have hills, depressions, sedimentary rocks, and mineral and oil deposits similar to those on the dry land nearby. The shelf break marks the abrupt transition from continental shelf to continental slope. The depth of water at the shelf break is surprisingly constant Tapered, thinned, 'flooded' extension of the continent Highly variable in width Gently sloping submerged surface

Continental Slope

A steep incline of the ocean floor leading down from the edge of the continental shelf. The transition between the gently descending continental shelf and the deep-ocean floor. Continental slopes are formed of sediments that reach the built-out edge of the shelf and are transported over the side. At active margins a slope may also include marine sediments scraped off a descending plate during subduction In general, continental slopes at active margins are steeper than those at passive margins. The true edge of the continent. Seaward edge of continental shelf Relatively steep slope Relatively narrow *Transition between shelf and ocean floor.*

Abyssal Hills and Plains

Abyssal plains and hills account for nearly all the area of deep-ocean floor that is not part of the oceanic ridge system. They are Earth's most common "landform."

Abyssal Plains

Abyssal plains are flat, featureless expanses of sediment- covered ocean floor found on the periphery of all oceans. They are most common in the Atlantic, less so in the Indian Ocean, and relatively rare in the active Pacific. They lie between the continental margins and the oceanic ridges. Flatness is caused by the smoothing effect of the layers of sediment. Most of the sediment that forms the abyssal plains appears to be of terrestrial or shallow-water origin, not derived from biological activity in the ocean above. Some of it may have been transported to the plains by winds or turbidity currents. **limited tectonic or volcanic activity**

Trench

An arc-shaped depression in the deep-ocean floor. These creases in the seafloor occur where a converging oceanic plate is subducted. The water temperature at the bottom of a trench is slightly cooler. Trenches are the deepest places in Earth's crust. (The deepest one is the Mariana Trench). Trenches are among the most active geological features on Earth. Great earthquakes and tsunamis often originate in them. No continental rise occurs along coasts with trenches because the sediment that would form the rise ends up at the bottom of the trench.

Sheeted Dikes

Basalt in vertical layers - series of parallel walls. (Newer in the center). Volcanic rocks that form in a series of parallel walls and are found beneath pillow basalt.

Black Smokers

Black smokers have larger, darker "chimneys," and those chimneys omit water/smoke that is black. They are found primarily in the bathyal zone and the abyssal zone. They emit minerals with high concentrations of sulfide, including iron, copper, zinc and arsenic. They occur in areas with temperatures between 350 and 400 degrees celsius.

Passive Continental Margin

Continental margins facing the edges of diverging plates are called passive margins because relatively little earthquake or volcanic activity is now associated with them. Because they surround the Atlantic, passive margins are sometimes referred to as Atlantic-type margins.

Parts of a Continental Margin

Continental margins have three main divisions: a shallow, nearly flat continental SHELF close to shore; a more steeply sloped continental SLOPE seaward; and an apron of sediment—the continental RISE—that blends the continental margins into the deep- ocean basins

Active Continental Margin

Continental margins near the edges of converging plates (or near places where plates are slipping past one another) are called active margins because of their earthquake and volcanic activity. Because of their prevalence in the Pacific, active margins are sometimes referred to as Pacific-type margins. Active margins coincide with plate boundaries but passive margins do not. Passive margins are also found outside the Atlantic, but active margins are confined mostly to the Pacific

Hot spots and Mantle Plumes

Create linear volcanic chains with that progreesively age in one direction. Mantle plumes and hot spots remain stationary under moving plates. Plate movement above mantle plumes and hot spots.

Ocean Ridge

Created at a divergent boundary where the ocean plates spread apart. Magma rises out to form underwater mountains. An oceanic ridge is a mountainous chain of young basaltic rock at the active spreading center of an ocean (often devoid of sediment) In places they project above the surface to form islands such as Iceland, the Azores, and Easter Island. The rift zones associated with oceanic ridges are sources of new ocean floor where lithospheric plates diverge. The oceanic ridges are widest where they are most active. The youngest rock is located at the active ridge center, and rock becomes older with distance from the center

Island Arc

Curving chain of volanic islands and seamounts. Found parallel to trenches.

Turbidity Currents

Dense current that carries large amounts of sediment down the continental crust Turbidity currents occur when turbulence mixes sediments into water above a sloping bottom.

Depth of Seafloor vs Age

Depth to seafloor varies as a function of square-root of time. Cools with increasing age. Increases density as well as thickness away from ridge. Seafloor deepens away from mid-ocean ridges.

Summary 1

Divergent plate boundaries - allowing 'passive' rise of solid mantle rock. Mantle melts in parts to produce basalt magma. Basalt magma rises, crystallizes and forms new crust. Most abundant magma production on earth happens on spreading centers (MOR) The upper layer of the oceanic crust is made of pillow basalt (cooling in water) and it is covered with the sediment layer. The layer below is basaltic sheeted dikes The bottom layer is gabbro (slowly cooled basaltic melt) The structure of the ocean crust is defined by a) the distribution of rock types and b) seismic velocities Mid-ocean ridges sit high because they are hot Decrease in seafloor elevation with age/time

Guyots

Flat-topped seamounts that once were tall enough to approach or penetrate the sea surface. Generally they are confined to the west-central Pacific. The flat top suggests that they were eroded by wave action when they were near sea level. Their plateau-like tops eventually sank too deep for wave erosion to continue wearing them down. Like the more abundant seamounts, most guyots were formed at spreading centers and transported outward and downward as the seafloor moved away from a spreading center and cooled.

Fracture Zones

Fracture zones are seismically inactive areas that show evidence of past transform fault activity - move in the same direction (as opposed to opposite as lithospheric plates do on a transform fault)

Guyot vs Seamount

Guyots have flat tops because they were once tall enough to be eroded by waves at the ocean's surface. Seamounts have a similar origin but retain their more pointed volcano shape because they never reached the surface Both associated with mid-ocean ridges or hotspots Size: Abyssal hill - seamount - guyot

Mantle Plume

Heated lower mantle rock that rises toward the lithosphere because it is less dense than surrounding mantle rock. The largest known plume (the superplume) is currently lifting all of Africa.

Hydrothermal vent

Hydrothermal vents are located on active ocean ridges, they are similar to underwater geysers and are usually found near volcanically active plates where two tectonic plates diverge (move away from one another) - including spreading centers. The diverging plates create an opening in the ocean floor where hot gases and minerals escape from Earth's interior and combine with freezing seawater. As the minerals cool they form unique structures that are similar in appearance to chimneys. In addition to spreading centers, they are also found at hotspots and ocean basins. There are two types of hydrothermal vents and both are host to a unique variety of organisms that depend on the vents chemical process in order to survive, including tube worms. They help to maintain/regulate the water chemistry in the ocean as well as the overall circulation. There are two types of hydrothermal vents: black smokers and white smokers.

Gabbro

Igneous rock found in abundance - a dense, dark, greenish color rock that is related to basalt and usually found in the deeper parts of oceanic crust (as opposed to basalt at the surface).

Trenches and Island Arcs

Island arcs, curving chains of volcanic islands and seamounts, are almost always found parallel to the concave edges of trenches. Trenches and island arcs are formed by tectonic and volcanic activity associated with subduction - found at spreading centers.

Pillow Lava

Lava that cools underwater, taking on a distinctive pillow-like shape as it hardens. Characteristic of volcanism at mid-ocean ridges. Basalt that erupted or flowed underwater. Found in the top layer of oceanic crust;

Summary 2

Magma erupts to produce volcanic rocks and landforms that are fractured, so seawater circulates through them Seawater heats up at base' of system and exits at vents There are black smokers and white smokers Hydrothermal plumes influence ocean chemistry and benthic ecology Hydrothermal vents can support life without input from photosynthesis

Ocean is Divided into Two Parts

Near shore the features of the ocean floor are similar to those of the adjacent continents because they share the same granitic basement. The transition to basalt marks the true edge of the continent and divides ocean floors into two major provinces. The submerged outer edge of a continent is called the continental margin. The deep-sea floor beyond the continental margin is properly called the ocean basin More than half of Earth's solid surface is at least 3,000 meters below sea level. The average depth of the ocean is much greater than the average elevation of the continents.

Major Features of Ocean Basins

Oceanic ridges Transform faults and fracture zones Abyssal hills Abyssal plains Seamounts, ocean islands and large plateaus Trenches and island arcs

Hotspots

Places where molten material from the mantle reach the lithosphere. Not always located at plate boundaries. As lithospheric plates slide over these fixed locations the plates are weakened from below by rising heat and magma. A volcano can form over the hot spot, but because the plate is moving, the volcano is carried away from its source of magma after a few million years and becomes inactive. It is replaced at the hot spot by a new volcano a short distance away. A chain of volcanoes and volcanic islands result. (Ex: hawaii)

Terranes

Plateaus, isolated segments of seafloor, ocean ridges, ancient island arcs, and parts of continental crust that are squeezed and sheared onto the face of a continent. The thickness and low density of terranes prevent their subduction. Terranes can also contain fragments of dense oceanic crust. Roughly .0001% of oceanic lithosphere is not subducted, but rather abducted (scraped off) into the edges of the continents.

Abyssal Hills

Small, sediment-covered extinct volcanoes or intrusions of once-molten rock. Associated with seafloor spread- ing; they form when newly formed crust moves away from the center of a ridge, stretches, and cracks. Some blocks of the crust drop to form valleys, and others remain higher as hills. Lava erupting from the ridge flows along the fractures, coating the hills. This helps explain why abyssal hills occur in lines parallel to the flanks of the nearby oceanic ridge and why they occur most abundantly in places where the rate of seafloor spreading is fastest. **Formed primarily by faulting**

Ocean Basin

The deepest part of the ocean floor; made up of rolling hills and flat plains. The submerged outer edge of a continent is called the continental margin. The deep seafloor beyond the continental margin is properly called the ocean basin.

Continental Rise

The gently sloping section of the continental margin located between the continental slope and the abyssal plain Along passive margins, the oceanic crust at the base of the continental slope is covered by an apron of accumulated sediment called the continental rise. Most of the sediments that form the continental rise are transported to the area by turbidity currents - deep-ocean currents are an important factor in shaping continental rises. Thick accumulation of sediments at base of slope. Laterally extensive (hundreds of km)

Width of Continental Shelf Varies Depending on its Location

The width of a shelf is usually determined by its proximity to a plate boundary - the shelf at the passive margin (east of South America) is broad, but the shelf at the active margin (west of South America) is very narrow. Shelf width depends not only on tectonics but also on marine processes: fast-moving ocean currents can sometimes prevent sediments from accumulating. The shelves of the active Pacific margins are generally not as broad and flat as the Atlantic shelves. Active-margin shelves have more varied topography than passive-margin shelves; the character of continental shelves at an active margin may be determined more by faulting, volcanism, and tectonic deformation than by sedimentation.

Sediment on the ocean floor

The young spreading ridges of the ocean floor are almost free of sediment, and the oldest edges of the basins support layers of sediment 15-20x as thin as the age of the ocean itself would suggest. This is because the sediments are subducted at a plate's leading edge. Is thickest toward the subduction zone and thin or absent on the ridge

Transform Fault

Transform faults are fractures along which lithospheric plates slide horizontally - earthquakes are common along transform faults. Transform faults are the active part of fracture zones

Seamounts

Volcanic projections that do not rise above the surface of the sea. May be found alone or in groups of 10 to 100. Though many form at hotspots, most are thought to be submerged inactive volcanoes that formed at spreading centers. A conical underwater mountain formed by a volcano and rising 1000 meters or more from the sea floor.

White smokers

White smokers have smaller "chimneys," and their "smoke" is either completely clear or a faint, milky color. They emit minerals that are lighter in color, including barium, calcium and silicon. They occur at temperatures that are less than 300 degrees celsius, significantly lower than black smokers, most likely because they are located farther from their heat source.