ocean floor Flashcards
Bathymetry
- bathos (depth) metro, (measurement)
- measurement of oceanic depth
- originally measured by lowering weighted lines overboard
echo sounder
- instrument for measuring depth, referred to as sonar, sound navigating, and ranging
- transmit sound wave into water in order to produce an echo when it bounces of an object
- velocity is unknown, about 1500 m/sec
- distance equals velocity times time divided by two
high resolution multi beam sonar
- employs an array of sound sources and listening devices
- obtains a profile of a narrow strip of sea floor
ocean floor from space
- satellite uses radar altimeters to measure the anomalies of the ocean surface
- ANOMALY is the difference between the measured and theoretical ocean surface
- variation in sea surface elevation is caused by water gravitational attraction to a different submarine landforms
- sea surface mimics the shape of sea floor
how can geoligists see rock structures beneath the surface
- use a seismic reflection profile - strong low frequency sounds are produced by explosions (depth charges) or air guns
- sound waves penetrate beneath the seafloor and reflect off the contacts between rock layers and fault zones, just like sound reflects off the bottom of the sea.
continental margins (first main province)
passive and active margins
passive continental margins
- not associated with plate boundaries
- little volcanism and few earthquakes
- along coastal areas that surround the Atlantic Ocean
continental SHELF
- flooded extension of the continent
- varies greatly in width
- gently sloping
- contains important mineral deposits
- mantled by extensive glacial deposits
continental SLOPE
- marks the seaward edge of the continual shelf
- steep structure
- boundary between continental crust and oceanic crust
continental RISE
- found in regions where the trenches are absent (east and west coasts of Atlantic Ocean)
- continental slope merges into a more gradual incline - the continental rise
- at the base of the continual slope TURBIDITY CURRENTS deposit sediment that forms DEEP SEA FANS
active continental margins
- convergent plate boundaries
- descends abruptly into a deep oceanic trench
- located around the pacific ocean
deep oceanic trench
- long narrow features
- deepest parts of ocean, can be about 11 km deep ( marina trench)
- sites where lithospheric plates plunge into the mantle
- associated with volcanic activity
deep ocean basins
second main province
features of deep marine basin
- abyssal plains
- seamounts
- guyots - flat topped sea mounts
- oceanic plateaus
abyssal plains
- most level on earth
- sites of thick accumulation of sediments
- found in all oceans
seamounts
- isolated volcanic peaks
- form near oceanic ridges
- may emerge as an island
- may sink and form flat topped sea mounts called guyots
oceanic plateaus
- vast outpouring of basaltic lavas through fissure eruptions on the ocean floor create extensive volcanic structures called oceanic plateaus
- found in every ocean
oceanic ridge ( third main province of ocean floor )
- largest topographic feature on earths surface
- over 70,000 km ( 43 000 miles in length )
- covers 20% of earths surface
- winds through all major regions
anatomy of oceanic ridge
- term ridge is misleading, widths of ridge is 1000 to 4000 km, so to gives the appearance its broad, elongated swells
- zone of extensive faulting and earthquakes
- area of high heat flow
- numerous volcanic structures
oceanic ridge and sea floor spreading
- concept formulated by harry hess in the 1960s
- sea floor spreading occurs along relativity narrow zones, called rift valleys, located at the crest of ocean ridges
- some ridges exhibit deep down-faulted rift valleys
- as plates move apart from ridge crust, magma wells up into the newly created fractures and generates new silvers of oceanic lithosphere
- new lithosphere moves away from the ridge crust in a conveyor belt type fashion
- zones of active rifting are 20 to 30 km wide
why are oceanic plates elevated?
- new oceanic lithosphere is hot, less dense and occupies more volume than cooler rocks
- as the basaltic crust travels away from the ridge crest it is cooled by seawater, thermally contracts, becoming more dense and occupy less volume.
spreading rates and ridge topography
- ridge systems exhibit topographic differences that are controlled by spreading rates
- at slow spreading rates ( 1 to 5 cm per year), a prominent Rift Valley develops along the ridge crest that is usually 30 to 50 km across and 1500 to 3000 meters deep
- SLOW spreading rate forms deep rift valley
- INTERMEDIATE - (5 to 9) rift valleys that develop are shallow ( less than 200 meters deep
- at spreading rates greater than 9 cm per year NO MEDIAN Rift Valley develops and these areas area usually narrow and extensively faulted
structure of oceanic crust
ophiolite complex
- layer 1 ( top) unconsolidated sediments
- layer 2 consisting of basaltic pillow lavas
- layer 3 numerous interconnected dikes called sheeted dike complex
- layer 4 ( bottom) gabbros
formation of oceanic crust
- process starts with magma that originates from partially melted mantle rock peridotite.
- formation of layer 4
- mafic magma mixes up with oceanic water and forms a bottom layer - gabbro
- formation of layer 3
- molten rock rises, injects into fractures and creates the sheeted dike complex
- formation of layer 2
- the submarine lava flows squeezes from fractured floor of Rift Valley of mid ocean ridge. It chills quickly and congeal, producing large tube shaped protuberances known as pillow basalts
- formation of layer 1
marine sediments ( mainly clay and silt) accumulate above pillow basaltic lavas
supercontinent cycle also known as Wilson cycle ( J. Tuzo Wilson- Canadian geologist
- all processes that result in opening and closing of oceanic basins
- rifting and dispersal of one supercontinent ( birth of an ocean ) followed by a long period as fragments are reassembled under a different configuration ( demise of an ocean)
birth of new ocean basin
- begins with the formation of continental rift
- rift splits landmasses into two or more smaller segments
- landmasses became separated by a narrow sea
- as tensional stress continue to pull land segments apart, narrow sea grow wider into an ocean
examples of continental rifting
East African and Red sea rift
- Red Sea is an example of a Rift Valley that has lengthened and deepened in a narrow linear sea
- if spreading continues the Red Sea will grow wider and develop an oceanic ridge similar to the Atlantic Ocean
Basin and Range province, California
- mantle plume under the Yellowstone, caused basin and range province to be broken by a series of normal faults and stretched twice its original width
mechanisms for continental rifting
- mantle plums and hot spot intraplate volcanism
- role of tensional stress
A. mantle upwelling creates tensional forces cause the upper crust to be broken along normal faults
B. large slabs of rock, generating a rift zone
C. further spreading generates a narrow sea
D. eventually, an expansive ocean basin and ridge system are created
The demise of an ocean ( closure)
- plate movements have been reconstructed for the past 200 million years using magnetic stripes on the ocean floor
- research indicates that parts, or even entire oceanic basins, have been destroyed along subduction zones
- main reason for oceanic plate to be destroyed is a difference in rates of its generation and subduction
- when the rate of subduction ( on one end) is greater than the rate of formation ( on the other end ) the plate will disappear in subduction zone
why oceanic lithosphere subducts
-overall density is greater than the density of underlying mantle and overriding continental crust
ex the faralon plate once occupied much of the eastern pacific Basin
- beginning 180 million years ago the farallon plate was subducting beneath the americas faster than it was being generated
- the plate got continually smaller and now only fragments of the original plate remain as the Juan de Fuca, cocos and nazca plates
