origin and evolution of the ocean floor (13) Flashcards
HMS Challenger
December 1872 to May 1876
First comprehensive study of the global ocean
What did the HMS Challenger study?
127,500 kilometer voyage that traveled to every ocean except the Artic
(made the first recording of the deepest-known point on the ocean floor in
Bathymetry
The measurement of ocean depths and the carting of the topography of the ocean floor
Sonar
An instrument that uses acoustic signals (sound energy) to measure water depths
What sonar and acronym for?
Sound Navigation and Ranging
Echo sounder
An instrument used to determine the depth of water by measuring the time interval between emission of a sound signal and the return of its echo from the bottom
Sidescan sonar
(after WW2) US navy developed this device to scan for explosive devices
List three major provinces of the ocean floor
Continental margins
Deep-ocean basins
Oceanic (mid-ocean) ridges
Continental margin
the portion of the seafloor that is adjacent to the continents
What may be included in the continental margin?
Continental shelf
Continental slope
Continental rise
Passive continental margin
A margin that consists of a continental shelf, continental slope, and continental rise. They are not associated with plate boundaries and therefore experience little volcanism and few earthquakes
How do passive continental blocks develop?
Continental blocks rift apart and are separated by continued seafloor spreading
Continental shelf
The gently sloping submerged portion of the continental margin, extending form the shoreline to the continental slope
What does the continental shelf consist of?
Continental crust with sedimentary rocks and sediments eroded form the adjacent landmass
What is the average inclination of a continental shelf?
1/10th of a degree
Why do continental shelfs have political and economic significance?
They contain extensive reservoirs of oil and natural gas, and they support important fishing grounds
Continental slope
The steep gradient that leads to the deep-ocean floor and marks the seaward edge of the continental shelf
What is the average inclination of a continental slope?
About 5 to 25 degrees
Continental rise
The gently sloping surface at the base fo the continental slope
What does the continental rise consist of?
A thick accumulation of sediment that has moved down the continental slope and onto the deep-ocean floor
How are most of the sediments are delivered to the seafloor?
Turbidity currents
Deep-sea fan
A cone-shaped deposit at the base of the continental slope. The sediment is transported to the fan by turbidity currents that follow submarine canyons
Submarine canyon
A seaward extension of a valley that was cut on the continental shelf during a time when sea level was lower, or a canyon carved into the outer continental shelf, slope, and rise by turbidity currents
Turbidity current
A downslope movement of defense, sediment-laden water created when sand and mud on the continental shelf and slope are dislodged and thrown into suspension
Active continental margin
A margin that is usually narrow and consists of highly deformed sediments. Such margins occur where oceanic lithosphere is being subjected beneath the margin of a continent
Accretionary wedge
A large wedge-shaped mass of sediment that accumulates in subduction zones. Here sediment is scraped from the subjecting oceanic plate and accreted to the overriding crustal block.
Subduction erosion
A process in subduction zones in which sediment and rock are scraped off the bottom of the overriding plate transported into the mantle
Deep-ocean basin
The portion of seafloor that lies between the continental margin and the oceanic ridge system. The region comprises almost 30 percent of Earth’s surface
Abyssal plains
Remarkably flat areas
Seamounts/Guyouts
Tall volcanic peaks
Oceanic plateaus
Extensive areas of lava flows piled one atop the other
Deep-ocean trench
A narrow, elongated depression of the seafloor
What are the only two trenches in the Atlantic?
The Puerto Rico trench
The South Sandwich trench
Volcanic island arc
A chain of volcanic islands generally located a few hundred kilometers from a trench where there is active subduction of one oceanic plate beneath another
Continental volcanic arc
Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent
What are examples of continental volcanic arcs?
Andes
Cascades
Seismic reflection profiler
An instrument for viewing the rock structure beneath a blanket of sediment that uses strong, low-frequency sound waves that penetrate the sediments and reflect off the contacts between rock layers and fault zones
What are the sediments of abyssal plains?
Fine sediments from land transported far out to sea by winds, ocean currents, and turbidity currents
Mineral matter that has precipitated out of seawater
Shells and skeletons of microscopic marine organisms.
How tall does a seamount rise?
at least 1000 meters above the deep-ocean floor
What are examples of volcanic islands?
Easter Island, Tahiti, Bora Bora, the Galapagos Islands, and the Canary Islands
What are examples of islands that formed over volcanic hot spots?
Hawaiian Island-Emperor chain
Guyot
Formed as a moving plate slowly carries inactive volcanic islands away from the elevated oceanic ridge or hot spot over which they formed, they gradually sink and disappear below the water surface
Oceanic plateaus
An extensive region on the ocean floor that is composed of thick accumulations of pillow basalts and other magic rocks that, in some cases, exceed 30 kilometers in thickness
What percent of the Earth’s total surface is comprised by deep-ocean basins?
30%
Atoll
A coral island that consists of a nearly continuous ring of coral reef surrounding a central lagoon
Coral
Tiny animals that generally cluster in large numbers and form colonies when linked
Reef
Large calcium carbonate structures
What do reefs require?
Temperature of about 24 degrees Celsius
Clear, sunlit water
Depth no more than 45 meters
HMS Beagle
Charles Darwin voyage (1831-1836)
Darwin’s observation of coral reef progression
development from (1) a fringing reef along the margins of a volcano to (2) a barrier reef with a volcano in the middle to (3) an atoll, consisting of a continuous or broken ring of coral reef surrounding a central lagoon
The plate tectonics theory provides the most current scientific explanation of how _______
Volcanic islands become extinct and sink to great depths over long periods of time
Mid-ocean ridge
A continuous mountainous ridge on the floor of all the major ocean basins and varying in width from 500 t0 5000 kilometers. The rifts at the crests of these ridges represent divergent plate boundaries
How are oceanic ridges characterized?
Extensive normal and strike-slip faulting
Earthquakes
High heat flow
Volcanism
Rift valley
A long, narrow trough bounded by normal faults. It represents a region where divergence is taking place.
Why is the oceanic ridge elevated?
Newly created oceanic lithosphere is hot and therefore less dense than cooler rocks of the deep-ocean basin (lithosphere gradually cools and contracts)
What percent of magma is produced along the oceanic ridge system in associated with seafloor spreading?
60%
Layer 1 of the oceanic crust
A sequence of deep-sea sediments or sedimentary rocks compose the upper layer. These sedimentary deposits are very thin near the axis of oceanic ridges but may be several kilometers thick next to continents.
Layer 2 of the oceanic crust
Below the sediments layer lies a rock unit composed mainly of basaltic lavas that contain abundant pillowlike structures called pillow lavas
Layer 3 of the oceanic crust
The middle, rocky layer is made up of numerous interconnected dikes that have a nearly vertical orientation, called the sheeted dike complex. These dikes are former pathways where magma rose to feed pillow basalts on the ocean floor.
Layer 4 of the oceanic crust
The lowest unit is mainly gabbro, the coarse-grained equivalent of basalt, which crystallized at depth without erupting.
Ophiolite complex
The sequence of rocks that make up the oceanic crust. The three-layer sequence includes an upper layer of pillow basalts, a middle layer or sheeted dikes, and a lower layer of gabbro
Sheeted dike complex
A large group of nearly parallel dikes
Pillow lava
Basaltic lava that solidifies in an underwater environment and develops a structure that resembles a pile of pillows
Hydrothermal and metamorphism causes _________
Dark silicates (olivine to pyroxene) to form new metamorphic minerals such as chlorite and serpentine
Hot water dissolves ___
ions of silica, iron, copper, and occasionally silver or gold
Block smoker
A hydrothermal vent on the ocean floor that emits a black cloud of hot, metal-rich water
Continental rift
A linear zone along which continental lithosphere stretches and pulls apart. Its creation may mark the beginning of a new ocean basin.
Examples of narrow continental rifts
East African Rift
Rio Grande Rift
Baikal Rift
Rhine Valley
Example of wide continental rifts
Basin and Range region in the western United States
Supercontinent cycle
The idea that the rifting and dispersal of one supercontinent is followed by a long period during which the fragments gradually reassemble into a new supercontinent
Mantle plume
Consists of hotter-than-normal mantle rock that has a large mushroom-shaped head hundreds of kilometers in diameter attached to a long, narrow, trailing tail
Triple junction
A point where three lithospheric plates meet
Spontaneous subduction
A process that occurs at Mariana-type subduction zones in which old, dense lithosphere sinks into the mantle at a steep angle by its own weight creating a deep trench
What drives subduction?
The lithospheric mantle, which makes up about 80% of the descending oceanic slab
Forced subduction
A process that occurs at Peru-Chile-type subduction zones in which lithosphere is too buoyant to subduct spontaneously but is forced beneath the overriding plate
What are the three remaining fragments of the once extensive Farallon plate?
Juan de Fuca
Cocos
Nazca
