OCEANS, SEAFLOOR, AND CONTINENTAL MARGINS · •Features Found in the Deep‐Ocean Basins •...

TCNJ PHY120 2013 GCHERMAN

OCEANS, SEAFLOOR, AND CONTINENTAL MARGINS

Sources:www.google.comen.wikipedia.orgThompson Higher Education 2007; Monroe, Wicander, and Hazlett, Physical Geology


• Introduction

• Methods Used to Study the Seafloor

• Oceanic Crust ‐ Its Structure and Composition

• The Continental Margins

• Features Found in the Deep‐Ocean Basins

• Sediments on the Deep Seafloor

• Reefs—Rocks Made by Organisms

• Resources from the Oceans

OCEANS, SEAFLOOR, AND CONTINENTAL MARGINS


OCEANS cover more of Earth’s surface than land

•71% of Earth’s area consists of oceans and marginal seas like Caribbean and Mediterranean Seas

•Most of the ocean waters are in the are in the Southern Hemisphere, about 81 % of the surface water.


EARTH’S SURFACE COVER

Continental margins lying below sea level

separate the continents from the

ocean basins.


OCEAN BASINS

• Of the 3 major oceans (Pacific, Atlantic, Indian), the Pacific is the largest, contains over half of the water by volume, and has the greatest average depth — 3940m

• The average depth of all ocean basins is ~3800 meters (12,500 ft).


• Satellite measurements• Echo sounding profiles• Side‐scan sonar• Seismic profiles using sound waves• Manned and unmanned submersibles• Drilling and collecting core samples

MAPPING THE SEAFLOOR


EARTH’S SEAFLOOROn June 27,1667, Captain John Wood, on the H. M. Speedwell, recorded that he saw the bottom while sounding at a depth of 80 fathoms (480 feet). His position was approximately 74°30'N, 63°E (to the east of Novaya Zemlya). He records that he "could see the shells at the bottom very plain." His observation is considered by many to be valid, making this the record for ocean water visibility. EOS, March 1, 1994.

Misconception: The ocean floor is flat, except where islands protrude above the surface.

Fact: The ocean floor has far more varied topography than the land areas, with higher and longer mountain ranges, deeper canyons and trenches.


EARLY MAP AND PERSPECTIVE

PLATO, ATLANTIS, AND AMERICA CIRCA 1664


CORE SAMPLE

Scientific study of the ocean basins began in the late 1700's. Today, research ships investigate the sea floor by drilling, echo sounding, and seismic profiling. They sample the sea floor using clamshell samplers and piston corers.


SEAFLOOR DEPTH• In the 1920’s depth was first determined using an Echo sounder (sonar)

• Primary instrument for measuring depth • Reflects sound from ocean floor

• Multibeam sonar• Employs an array of sound sources and listening devices

• Obtains a profile of a narrow strip of seafloor

first measured by lowering weighted lines overboard


MODERN METHODS OF STUDYING THE SEAFLOOR


EARTH’S SEAFLOOR

Drilling core samples through the ocean floor and other remote sensing devices like seismographs are used to map the subsurface geology below the floor.


EARTH’S SEAFLOOR


EARTH’S SEAFLOOR Deep sea drilling and studies of isolated exposures on active margins of continents and within ancient mountain ranges indicate that the sequence of oceanic rock types from top to base is: basaltic pillow lava, sheeted basalt dikes, and gabbro.


DEEP‐SEA‐DRILLING PROJECT (DSDP) AND OCEAN‐DRILLING PROJECT (ODP) DRILL CORESITES


EARTH’S SEAFLOOR


EARTH’S SEAFLOOR (topography)


EARTH’S SEAFLOOR


EARTH’S SEAFLOOR

2 31 2 1


MID‐OCEAN RIDGES ‐ the site of sea‐floor spreading, are found in all major oceans and represent more than 20 % of Earth’s surface (it is where new crust is being formed).

• They nearly encircle the world but are offset at intervals by large fracture zones


MID‐OCEAN RIDGES

Most of the geologic activity on ridges occurs along a narrow region on the ridge crest—called the rift valleys, where magma from the asthenosphere moves upward to create new sections of oceanic crust (on land Iceland).


MID‐OCEAN RIDGES

• Elevated position (higher than the surrounding area)

• Oceanic ridges are long, continuous submarine mountain ranges composed of volcanic rock which has been extruded on the sea floor.

• Extensive faulting (makes sense, earth is being pushed apart here).

• Volcanic structureson newly formed mafic, oceanic crust

• Are characterized by shallow‐focus earthquakes, and basaltic volcanism.


are characterized by shallow‐focus earthquakes, and basaltic volcanism. MID‐OCEAN RIDGES

• Some ridges possess a central deep rift in which volcanism is frequent.

• Black smokers and tube worms


OCEANIC RIDGES nearly encircle the world but are offset at intervals by large fracture zones


PACIFIC OCEAN BASIN

• Seamounts and guyots are common, widespread features of the sea floor.

• Seamounts rise more than a kilometer from the ocean floor and are remnants of oceanic ridge volcanoes; Guyots have the same origin but are flat‐topped .

• Aseismic ridges consisting of seamounts and guyots extend in a perpendicular orientation from many spreading ridges.

Hawaiianseamount chain

aseismic ridges


aseismic ridges

INDIANOCEAN BASIN

abyssal plain


ATLANTIC OCEAN BASIN

mid‐ocean spreading ridges

mid‐ocean spreading ridges

abyssal plain

abyssal plain

aseismic ridge

aseismic ridge


TWO TYPE OF CONTINENTAL MARGINS

The continental margins include the continental shelf, continental slope, and in some places a continental rise.

Continental margins lying below sea level separate the continents from the ocean basins.


BLOCK DIAGRAM OF A PASSIVE MARGIN

• A broad shelf leading to a slope characterize passive continental margins

• The continental rise is typified by the overlapping deposits of submarine fans composed of turbidity current‐transported sediments.


PASSIVE CONTINENTAL MARGINS

•Void of active plate boundaries, exhibit few volcanoes and earthquakes.

•Sediments eroded from the adjacent landmass (continent) accumulate to form thick undisturbed layers.

• Submarine canyons are typical features of the continental slope, but commonly extend into the shelf.

• Some may have formed by stream erosion during the Pleistocene when sea level was lower, but others, not associated with landward streams, were probably eroded by turbidity currents.


SUBMARINE CANYONS Some may have formed by stream erosion during the Pleistocene when sea level was lower, but others, not associated with landward streams, were probably eroded by turbidity currents.


ACTIVE MARGINS

• A narrow shelf and a slope descending directly into an ocean trench characterize active continental margins.

•These margins are also characterized by volcanism and earthquakes.


ACTIVE MARGINS

• Oceanic trenches are the surface expressions of subduction zones.

• They are long, narrow, features which reach the greatest oceanic depths, and are characterized by low heat flow and negative gravity anomalies.


EARTH’S SEAFLOOR EXPOSED


Consist largely of the carbonate and siliceous skeletons of microscopic organisms (ooze), and pelagic clays which was derived from continents and oceanic islands.

SIX TYPES OF OCEANIC SEDIMENT


Consist largely of the carbonate and siliceous skeletons of microscopic organisms (ooze), and pelagic clays which was derived from continents and oceanic islands.

THREE TYPES OF SEDIMENT ON CONTINENTAL MARGINS

consist primarily of mineral grains that are weathered and transported from continental rocks into the ocean. They accumulates at very‐very slow rates— 5k to 50k years to deposit 1 cm of sediments, and are red or brown due to the oxidation of iron in sea water

TERRIGENOUS OR LAND‐DERIVED SEDIMENT

• 95% of the terrigenous material entering the ocean is delivered by rivers, and most of it is deposited on or near the continental margin.

• Only a minor amount is deposited in the deep ocean.


TURBIDITY CURRENTS AND BOUMA SEQUENCES


CONTINENTAL SHELF, MARGIN, SLOPE, AND RISE DEPOSITS

GLACIAL‐MARINE DEPOSITS


Consist largely of the carbonate and siliceous skeletons of microscopic organisms (ooze), and pelagic clays derived from continents and oceanic islands.

THREE TYPES OF DEEP‐SEA (PELAGIC) SEDIMENT

PELAGIC SEDIMENT

PELAGIC CLAY or pelagite is a fine‐grained sediment that accumulates as the result of the settling of particles to the floor of the open ocean, far from land.

• These particles consist primarily of either the microscopic, calcareous or siliceous shells of phytoplankton or zooplankton; clay‐size siliciclastic sediment; or some mixture of these with trace amounts of meteoric dust and minor amounts of volcanic ash.

• There are three main types of pelagic sediments based on composition:

siliceous and calcareous oozes, and red clays

• The composition of pelagic sediments is controlled by three main factors.

1) the distance from major landmasses, which affects their dilution by terrigenous, or land‐derived, sediment.

2) water depth, which affects the preservation of both siliceous and calcareous biogenic particles as they settle to the ocean bottom.

3) ocean fertility, which controls the amount of biogenic particles produced in surface waters.


BIOGENOUS SEDIMENT (OOZES)Foraminifera – animals

Coccolithofores – plants Radiolarian – animals

Diatoms – plants• Consist of shells and skeletons of floating animals and plants.

• Some organisms having calcareous skeletons, whereas other have siliceous (SiO2) skeletons.

• These sediments are fine grained and called oozes (very fine mud).

SiO2

CaCO3SiO2

CaCO3


CARBONATE COMPENSATION DEPTH (CCD) is the depth in the oceans below which the rate of supply of calcite (calcium carbonate) lags behind the rate of dissolution, such that no calcite is preserved.

• Calcium carbonate is essentially insoluble in sea surface waters today.

• Shells of dead calcareous plankton sinking to deeper waters are practically unaltered until reaching the lysocline where the solubility increases dramatically.

• By the time the CCD is reached all calcium carbonate has dissolved according to this equation:


HYDROGENOUS SEDIMENT• Includes minerals that crystallize directly from sea water through various chemical reactions, such as manganese nodules, that form as lumps on the seafloor and have the potential to contain large amounts of economic metals.

• They are usually 20 % manganese, with the remainder other minerals.


• Studies of this deep sea material can provide important information for comparison of the climatic histories of the land and sea.

• Research along this line has indicated that significant chemical weathering began at the Eocene‐Oligocene boundary and continued for at least 20 million years before physical weathering became dominant, in the middle or late Miocene.

• This shift appears to have been related to uplift in the Himalayas and North America (and probably elsewhere, as well). GSA Today, August, 1993.

DEEP‐SEA SEDIMENT


OCEAN BASINS GUYOUTS AND SEAMOUNTS


ATOLLS AND CORAL REEFS

• Atolls are coral islands consisting of a continuous or broken ring of coral reefs surrounding a central lagoon.

• Atolls can form on the flanks of eroding and sinking volcanic islands on the oceanic crust as it moves, where coral continue to grow and build reef complexes upward while the island slowly sinks.



• Atolls are coral islands consisting of a continuous or broken ring of coral reefs surrounding a central lagoon.


CORAL REEFS are wave resistant structures built by corals, and other organisms.

There are fringing, barrier, and atoll reefs.


CORAL REEFS



• Are constructed over thousands of years primarily from the accumulation of skeletal remains and secretions of corals and certain species of algae.

• Coral Reefs and Atolls occur in warm, sunlit waters usually in the Pacific and Indian Oceans. Corals only live in waters above 65o F. and in the photic zone above 150 ft depth below the surface.


Exclusive Economic Zones

The United States claims all rights to the natural resources within 200 nautical miles of its coastline, including petroleum, natural gas, gravel and various metals.


EARTH’S SEAFLOOR(IMPACT FEATURES?)

OCEANS, SEAFLOOR, AND CONTINENTAL MARGINS · •Features Found in the Deep‐Ocean Basins •...

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