Geology Ch.9 The Seafloor
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Oceanic crust is thinner and denser than continental crust and it is made up of basalt and gabbro. Continental crust has a composition close to that of granite.
What percentage of Earth is covered by oceans?
an instrument that scientists use to detect the depth at a certain part of the ocean, it detects sound waves that travel from the ship to the seafloor and back
Depth is calculated by knowing the velocity of sound in water and the time required for the waves to reach the seafloor and return to the ship.
similar to echo sounding, but is even more useful. Strong waves from an energy source reflect from the seafloor, and some of the waves penetrate the seafloor layers and reflect from various horizons back to the surface.
a famous surface vessel, which has carried scientists to the seafloor in many areas to make observations and collect samples.
Preserved slivers of oceanic crust, along with part of the underlying upper mantle.
What does an ideal ophiolite consist of?
rocks of the upper oceanic crust, especially pillow lava and sheet lava flows underlain by a sheeted dike complex consisting of vertical basaltic dikes. A complete ophiolite consists of deep-sea sedimentary rocks underlain by rocks of the oceanic crust and upper mantle.
made up of a gently sloping continental shelf, a more steeply inclined continental slope, and in some cases, a deeper, gently sloping continental rise.
a gently sloping section of seafloor lying between the shore and the more steeply dipping continental slope.
the outer margin of the continental shelf
Steeply inclined part of the continental margin
gently sloping rise that merges with the continental slope. This rise is absent around the margins of the Pacific, where continental slopes descend directly into an oceanic trench.
underwater flows of sediment-water mixtures with densities greater than that of sediment-free water. As a turbidity current flows onto the relatively flat seafloor, it slows and begins depositing sediment, the largest particles first, followed by the smaller particles.
yielded by the deposition of turbidity currents. These fans constitute a large part of the continental rise.
steep-sided canyons, present on continental shelves, best developed on continental slopes.
Turbidity currents are are thought to be the primary agent responsible for their erosion.
Active Continental Margin:
develops at the leading edge of a continental plate where oceanic lithosphere is subducted.
Passive Continental Margins:
within a plate rather than at a plate boundary, and they lack the volcanic and seismic activity found at active continental margins. Earthquakes do occasionally happen at these margins.
Sediment forms a series of overlapping submarine fans and develops a continental rise.
What is the difference between passive and active continental margins?
At passive margins, the sediment forms a series of overlapping submarine fans that develop a continental rise. At active margins, sediment is dumped in a trench and does not form a rise.
- flat surfaces covering vast areas of the seafloor. In some areas they are interupted by peaks rising more than 1km, but they are the flattest most featurless areas of the Earth.
- They are found near continental rises.
Long, steep-sided depressions on the seafloor near convergent plate boundaries, constitute no more than 2% of the seafloor. These are important because it is here that oceanic lithosphere is consumed by subduction. Crust at oceanic trenches is cooler and slightly denser.
Oceanic Ridge system:
submarine mountaneous topography. Runs through the arctic ocean through the middle of the Atlantic and curves around South Africa. Composed mostly of gabbro and basalts.
Submarine Hydrothermal Vents:
at or near spreading ridges where cold water seeps through oceanic crust, is heated by the hot rocks at depth, and then rises and discharges into the seawater as plumes of hot water with temperatures as high as 400 degrees C.
many of the plumes of hot water are black because dissolved minerals give them the appearance of black smoke.
Info on hydrothermal vents:
- Near the vents live communities of organisms, such as bacteria, crabs, mussels, starfish, and tubeworms.
- The heated water from the vent reacts with oceanic crust, transforming it into a metal-rich solution that discharges into seawater and cools, precipitating iron, copper, zinc sulfides and other minerals.
- these large fractures are hundreds of km long.
- Shallow focus EQ's take place along these fractures.;
- The reason oceanic ridges have so many fractures is that plate divergence takes place irregularly on a sphere, resulting in stresses that cause fracturing.
rise more than 1 km above the seafloor, and if they are flat topped they are called guyots.
volcanoes that originally extended above sea level. However, as the plate upon which they were located continued to move, they were carried away from a spreading ridge, and as the oceanic crust cooled, it descended to greater depths.
smaller than seamounts, average 250 m high.
- long, narrow ridges and borad plateu-like features rising as much as 2 to 3 km above the seafloor.
- They lack seismic activity (hence Aseismic).
- Referred to as microcontinents, because they were probably small fragments separated from continents during rifting.
- Most form a linear succession of hot spot volcanoes.
- May develop at or near an oceanic ridge, but each volcano that is formed is carried laterally with the plate that it originated on.
Sediments on the sea floor:
- mostly fine grained.
- silt and clay-sized particles.
- Derived from:
- 1. windblown dust and volcanic ash from continents and volcanic islands.
- 2. shells of microscopic organisms
- 3. chemical reactions in seawater
- Most sediment is pelagic, meaning that is settled from suspension far from land.
brown or red and is composed of clay-sized particles from the continents or oceanic islands.
made up of mostly tiny shells of marine organisms.
consists primarily of calcium carbonate skeletons of marin organisms such as foraminifera
composed of silica skeletons of single-celled organisms such as radiolarians (animals) and diatoms (plants).
- moundlike, wave-resistent structure composed of the skeletons of marine organisms.
- Restricted to shallow, tropical seas where the water is clear and its temp. does not fall below 20 degrees C.
- Depths are rarely more than 50 m, depending on sunlight penetration because many corals rely on symbiotic algae that must have sunlight for energy.
- 1.fringing reefs - solidly attached to the margins of an island or continent. Rough, tablelike surface, as much as 1 km wide, and on the seaward side, slop steeply down to the seafloor.
- 2. Barrier Reefs - similar to fringing reefs, except that a lagoon separates them from the mainland.
- 3. Atols - circular or oval reefs surrounding a lagoon. They form around volcanic islands below sea level as the plate they lie on is carried progressively farther from an oceanic ridge. Common in the western Pacific Ocean basin. Started out as fringing reefs, but grew into atolls as the plate they were on moved further into the ocean.
Resources of Oceans:
- Seawater contains many elements in solution, some of which are extracted for industrial and domestic uses. Produces salt by evaporation.
- Large portion of the worlds magnesium comes from seawater.
Exclusive Economic Zone (EEZ):
claimed by the U.S. as a resource area. The U.S. claimed rights to all resources within an area used as an energy resource.
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