Chapter 10 - Ocean Floor

Created by

Kirsty Camba

Cards (49)

Picture of Ocean floor
December 1872 to May 1876
Challenger made the 1st comprehensive study of the global ocean
During the 127,500 km voyage, the ship traveled to every ocean except for Arctic
the HMS Challenger measured the depth of the deepest spot on the seafloor in 1875.
this sport, Challenger Deep is 10,995 meters deep.
The first bathymetric map was created by Charles Wyville Thomson
Picture of Ocean floor
Modern Bathymetric Techniques
Bathymetric - measurement of ocean depths and charting of the shape or topography of ocean floor.
today, sound energy is used to measure water depths.
Sonar - an acronym for sound navigation and ranging
Echo sounders - first devices that used sound to measure water depths.
Picture of Ocean floor
Modern Bathymetric Techniques
Echo sounders were developed in 20th century
work by transmitting a soundwave into the water to produce an echo when it bounces of any object.
a sensitive receiver intercepts the reflected echo & a clock recors the travel time to fractions of a second.
depth = 1/2 (1500 m/sec x echo travel time)
1500 m /sec= speed of sound waves in water
Picture of Ocean floor
Bathymetry determined from continous monitoring of these echoes is plotted to obtain a profile of ocean floor.
Picture of Ocean floor
2. Mapping ocean floor from space.
Water's surface is not perfectly flat because:
Massive structures like seamounts and ridges exert stronger than average gravitational attraction so they produce elevated surfaces on the ocean surface.
Satellites w/ radar altimeters are able to measure subtle differences in sea level 6 by bouncing microwaves off the sea surface.
Provinces of the ocean floor
3 major areas:
continental margins
deep - ocean basins
oceanic (mid-ocean) ridges.

Continental Margins
outer margins of the continents where continental crust transitions to oceanic crust.
2 types: passive and active
Nearly entire Atlantic Ocean and a large portion of Indian Ocean are surrounded by passive continental margins.
By contrast, most of Pacific Ocean is bordered by active continental margins.
Provinces of the ocean floor (Continental Margins) 
Passive continental Margins.
tectonically active regions located some distance from plate boundaries.
not associated w/ strong earthquakes / volcanic activity.
develop when continental blocks rift apart and are separated by continued seafloor spreading.
as a result, continental blocks are firmly attached to the adjacent oceanic crust.
most passive margins are relatively wide and are sites where large quantities of sediments are deposited.
features w/ passive continental margins: continental shelf, slope and rise.
Continental Margins
Passive Continental Margins
Continental shelf
gently sloping, submerged surface that extends from the shoreline toward the deep ocean basin.
consist mainly of continental crust, capped w/ sedimentary rocks and sediments that ended from adjacent landmasses.
extends seaward more than 150 km (930 miles) along some continental margins.
average inclination is about one-tenth of 1 degree, a slope so slight that it would appear as a horizontal surface.
featureless; however, some areas are mantled by extensive glacial deposits and quite mugged.
Continental Margins
Passive Continental Margins
Continental Shelf
in addition, some continental shelves are dissected by large running valleys from the coastline into deeper waters.
Many of these shelf valleys are seaward extensions of river valleys on the adjacent land mass.
They were eroded during the last Ice Age (Quaternary) when enormous quantities of water were stored in ice sheeks on the continents rather than in the world's oceans, lowering sea level by atleast 100 meters (330 ft)
This gave proof that portions of continental shelf were once above sea level.
Continental Margins
Passive Continental Margins
Continental Slope.
seaward edge of continental shelf.
a relatively steep structure that forms the boundary between continental crust & oceanic crust.
inclination averages about 5 degrees and sometimes exceed 25 degrees in other places.
Continental Margins
Passive Continental Margins
Continental Rise
consists of a thick accumulation of sediment that has moved down at the continental slope & unto deep-ocean floor.
most of the sediments are delivered to the seafloor by turbidity currents, mixture sediment and water that periodically flow down submarine canyons.
when these muddy sluries emerge from the mouth of a canyon, they deposit sediments that form deep-sea fans.
as fans from adjacent submanne canyons grow, they merge to produce a sediment at the bottom of continental slope, forming continental rise.
Continental Margins
2. Active Continental Margins
located along convergent plate boundaries where oceanic lithosphere is being subjucted beneath the leading edge of a continent.
deep ocean trenches are the major topographic expression at convergent plate boundaries
Accretionary wedge: chaotic accumulation of deformed sediment and scraps oceanic crust.
prolonged plate subductions can produce massive accumulations of sediments along active continental margins.
Continental Margins
2. Active Continental Margins
Subduction Erosion
opposite process
rather than sediment accumulating along the front of the overriding plate, rediment & rock are scraped off the bit of the oven ding plate and transported down into the mantle.
effective where cold, dense oceanic lithosphere subducts at a steep angle (e.g. Mariana trench)
sharp bending of the subducting plate causes faulting in ocean crust.
Deep Ocean Basins
between the continental margins and oceanic ridge
almost 30% of earth's surface
contain deep-ocean trenches, abyssal plains, seamounts and guyots, oceanic plateaus.
Features of Deep Ocean Basins?
Deep Ocean basins
Deep- ocean trenches
long namow creases in the seafloor that are the deepest parts of the ocean floor.
most are located along the margins of pacific ocean where many exceed 10km (6 miles) in depth.
e.g. Challenger Deep - deepest known part of the world ocean.
only 2 trenches are located at Atlantic
Puerto Rico trench
South Sandwich trench
Features of Deep Ocean Basins? 
Deep Ocean basins
Deep ocean trenches
trenches are sites of plate convergence where slabs of oceanic lithosphere subduct and plunge into the mantle.
aside from earthquakes as results of plate subduction , it also triggers volcanic activity.
as a result, a trench tends to run parallel to an arc shaped row of active volcanoes called volcanic island arc wherein the overriding plate is oceanic.
continental volcanic - oceanic plate subducts under continental plate.
Features of Deep Ocean Basins? 
Deep Ocean basins
2. Abyssal Plains
flat features of the deep ocean floor
most level places on Earth
e.g. cast of Argentina has less than 3 meters (10 ft) of relief over a distance exceeding 100 km (800 miles)
monotomous topography is occassionally interrupted by the protruding summit of a partially buried volcanic peak (seamounts)
with reismic reflection, it was determined that the featureless topography of abyssal plain is due to thick accumulations of sediments that have buried on otherwise rugged ocean floor.
Features of Deep Ocean Basins?
Deep Ocean basins
2. Abyssal Plains
the nature of sediments indicate that these plains consist of 4 materials;
fine sediments transported far out to sea by turbidity currents.
mineral matter that has precipitated out of seawater
shells and skeletons of microscopic morine organisms.
wind-blown clay particles derived from the land.
Abyssal gains are all found in oceans.
Atlantic Ocean has the most extensive abyssal plains bcs it has few frenches to act traps for sediments carried down to the continental slope.
Features of Deep Ocean Basins?
Deep Ocean basins
3. Volcanic Structures
Seamounts & Volcanic Islands
Seamounts
may rise hundreds of meters above the sumunding topography
estimated that more than a million seamounts exist.
some grow large enough to become oceanic islands, but do not have sufficiently long eruptive history to build a structure above sea level.
most common in the pacific.
e.g. Emperor seamount chain w/c stretches from the Hawaiian Islands to the Meutian trench from over volcanic hotspots. Others are born near oceanic ridges.
Features of Deep Ocean Basins?
Deep Ocean basins
3. Volcanic Structures
Seamounts & Volcanic Islands
Volcanic Island
if a volcano grows large enough before plate motion carries it away for its magma source, this structure emerges.
eg. Easter Islands, Tahiti, Bora bora, Galapagos Islands and Canary.
Features of Deep Ocean Basins? 
Deep Ocean basins
3. Volcanic Structures
Guyots
Inactive volcanic islande are gradually but inevitably lowered to near sea level by forces of weathering and erosion.
As a moving plate slowly carries volcanic islands away from the elevated oceanic ridge or hotspot over w/c they formed, they gradually sink and disappear below water surface.
Submerged, flat segmounts formed in this manner are called guyots.
Features of Deep Ocean Basins? 
Deep Ocean basins
3. Volcanic Structures
Oceanic Plateaus
ocean floor contains several massive oceanic plateaus.
form when the bulbous head of a rising mantle plume mets, producing vast outs of fluid basaltic tavas.
some oceanic plateaus appear to have formed quickly in geological terms.|
e.g. Java Plateau - less than 3 miliion yrs ago
e.g. Kerguelen Plateau - 4 million yrs ago
Features of Deep Ocean Basins?
Oceanic Plateaus
Coral Apills - Darwin's Hypothesis.
ring shaped structures that extend from slightly above sea level to depths of several thousand meters.
Corals
tiny animals that generally cluster in large number and form colonter when linked.
most create a hard external skeleton made of CaCo3.
some build large ca coz structures called reefs.
Features of Deep Ocean Basins?
Oceanic Plateaus
Reef-building corals grow best in waters w/ an average annual temp. of about 24°C.
They cannot survive prolonged exposure to temperatures below 18°C or above 30°C
also requires clear, sunlit water.
depth of most active reef is limited to no more than 45 meters.
Features of Deep Ocean Basins?
Oceanic Plateaus
Charles Darwin formulated a hypothesis on the origin of ringed-shaped atolls.
Darwin noticed a progression in coral reef development form
a fringing reef along the margins of a volcano.
a barrier reef w/ a volcano in the middle
to an atoll consisting of a continuous or broken ring of coral reef.
Features of Deep Ocean Basins?
Oceanic Plateaus
The essence of many volcanic islands gradually sink.
He also hypothesized that corals responded to the gradual changes in water depth caused by the subsiding volcano by building the reef complex upward.
Eventually, volcanoes would submerge beneath the sea and its remnant would be Covered by an atoll.
Features of Deep Ocean Basins?
Oceanic Plateaus
Plate tectonics theory provide the most current explanation of how volcanic islands become extinct and sink.
Some volcanic islands form over relatively stationary mantle plumes, causing the lithosphere to be warmed and buoyantly lifted.
Over time, these volcanic islands gradually sink as the moving plates carry them away from the region of hot-spot volcanism because the lithosphere cools, becomes denser and sinks.
Anatomy of Oceanic Ridge?
along well-developed divergent plate boundaries, the seafloor is elevated, broad linear swell called oceanic ridge/ mid-ocean ridge or rise.
Oceanic Ridge
winds thru all major oceans.
longest topographic feature exceeding 70, 000 km in length.
crest of the ridge typically stands 2 to 3 km above the adjacent deep-oces basins and marks the plate boundary, where new oceanic crust is created.
Anatomy of Oceanic Ridge?
Oceanic Ridge
large sections of the oceanic ridge system have been named based on their locations within various ocean basins.
a ridge may run through the middle of an ocean basin where it is called mid-ocean ridge.
e.g. Mid-Atlantic Ridge and Mid-Indian Ridge
ridge system is broken into segments that range from a few tens of Kilometers to hundreds of kilometers in length.
each ridge segment is offset from the adjacent segment by a transform fault.
Anatomy of Oceanic Ridge? 
Oceanic Ridge
oceanic ridges are as high as some mountains but the similarities end there.
Whereas most mountain ranges on land form when the compressional for associated w/ continental collisions fold and metamorphose thick sequence of sedimentary rocks.
Oceanic ridges form where upwelling from mantle generates new oceanic crust.
Oceanic ridges consist of layers and piles of newly formed basaltic rocks. are buoyantly uplifted by hot mantle rocks.
Anatomy of Oceanic Ridge?
Rift valleys
along the axis of some segments of oceanic ridges are there deep. faulted structures.
30 to 60 km (20 to 30 miles) wide and have walls that tower 50 to 2500 meters above valley floor.
Mid-Atlantic Ridge
almost comparable to Arizona's Grand Canyon.
Oceanic Ridges & Seafloor Spreading? 
Oceanic ridges form due to seafloor spreading
as plates of oceanic lithosphere move apart, the warm mantle beneath rises & undergoes decompression melting.
the resulting magma is basaltic.
Some magma erupts along the ridge axis, but much of it crystallizes at depth.
the freshly cooled basalt or gabbro makes up the new oceanic crust.
Oceanic ridges are elevated features because they are warm and therefore less denser, older oceanic lithosphere that makes up the deep-ocean basins.
Oceanic Ridges & Seafloor Spreading?
Heat loss causes the oceanic crust to subside and be buried.
After so million yrs, crust that was once an oceanic ridge can become an abyssal plain.
The rate at w/e seafloor spreading occur determines the shape of an oceanic ridge.
Ridges w/ slow spreading rates (1 to 5 com/yr) have prominent rift valleys and rugged topography.
Those w/ fast spreading rates (greater than 9 cm/yr) lack rift valleys and show a smoother more subdued a topography.
Oceanic Ridges & Seafloor Spreading?
Nature of oceanic crust
4 distinct layer
Layer 1: sequence of deep-sea sediments or sedimentary rocks, sediments are very thin near the axes of oceanic ridges but may be several kilometers thick next to continents.
Layer 2: consist of rock units comprised mainly of basaltic lavas that contain abundant pillow like structures called pillow lavas.
Oceanic Ridges & Seafloor Spreading?
Nature of Oceanic Crust
Layer 3: middle, rocky layer made up of numerous interconnected dikes that have a nearly vertical orientation called sheeted dike complex.
these dikes are former pathways where magma rose to feed pillow basalts on the ocean flour.
Layer 4: mainly gabbro which crystallized deeper in the crust w/out erupting.
When fragments of oceanic crust and underlying mantle are discovered on land, they are called ophiolite complex.
Oceanic Ridges & Seafloor Spreading?
How does Oceanic Crust form?
the molten rock that forms oceanic crust originates of partial melting of the ultramafic mantle rock.
as 2 divergent plates move apart, fractures open perpendicular and lava moves up thru these cracks toward the seafloor.
once the lava has cooled and sealed these fractures shut, they become preserved as dikes of the sheeted dike complex.
Magma that cools at depth crystallizes to gabbro.
Any lava that makes it to the seafloor is erupted as pillow lavas, w/c are gradually buried by deep sea sedimentation.
Oceanic Ridges & Seafloor Spreading?
Interactions between seawater and oceanic crust
along mid ocean ridges, seawater flows thru fissures in the oceanic crust and is heated by the hot surrounding rock.
the warmer the seawater, the more chemically active it becomes.
the hot water causes hydrothermal metamorphism and dissolves metal ions
these hot solutions may spew out of the crust as black smokers.
Continental Rifting: New Ocean Basin?
Evolution of an ocean Basin
opening of a new ocean basins begin w/ the formation of a continental rift, an elongated depression along w/c the entire lithosphere is stretched and thinned.
where the lithosphere is thick, cool and strong, rifts tend to be narrow.
e.g. East African Rift, Rio Grande Rift and Rhine Valley
by contrast, where the crust is thin hot and weak, rift can be more than 1000 kilometers wide
e.g. Basin and Range region in the US
Continental Rifting: New Ocean Basin?
Evolution of an Ocean Basin
East African Rift
a continental rift that extends thru eastern Africa for approx. 3000 km consists of several interconnected rift valleys that split into eastern and western sections around Lake Victoria.
whether this rift will eventually develop into a spreading center is uncertain.