The Oceans
I. The Ocean Basins (pp 360-361, 364-374)
Major Ocean Basins: (Fig 13.2)
Pacific Ocean - largest and deepest
Atlantic Ocean
Indian Ocean - located almost entirely in Southern Hemisphere
Arctic Ocean - smallest and most shallow
Geologic Provinces of the ocean floor: a) continental margins, b) ocean basin floor, c) mid-ocean ridge
a) Continental Margin - The region between the shoreline of a continent and the deep ocean basins where continental crust and oceanic crust are joined. Includes the continental shelf, continental slope, and continental rise. (Fig 13.9)
Passive Continental Margin - A continental margin characterized by a firm connection between continental and oceanic crust, where little tectonic activity occurs. Example: eastern margin of North America, western margin of Africa. (Fig 13.8)
Continental Shelf - A shallow, nearly level area of continental crust covered by sediment and sedimentary rocks that is submerged below seal level at the edge of the continent between the shoreline and the continental slope. Most sediment is derived from the continent (terriginous), although in warm waters thick limestone deposits (carbonate platforms) may form. Sediments and sedimentary rock are underlain by granitic continental crustal rocks. (Fig 13.9) In many places is dissected by submarine canyons, deep V-shaped canyons formed by turbidity currents - rapidly flowing submarine currents laden with suspended sediment. Result from underwater landslides which form on the continental shelf or continental slope. (Fig 13.10)
Continental Slope - The relatively steep slope between the continental slope and the continental rise. Occurs above the boundary between underlying granitic continental crust and basaltic oceanic crust. The boundary between the continental slope and the continental shelf is known as the shelf break. (Fig 13.9) Are characterized by submarine canyons and abyssal fans (submarine fan) - a fan-shaped deposit of sediment deposited by turbidity currents. (Fig 13.9)
Continental Rise - The thin layer of terriginous sediment between the continental slope and the ocean basin floor. (Fig 13.9)
b) Ocean Basin Floor -extends from the continental margin to the mid-ocean ridges. Characterized by the following features:
Abyssal Plain - A flat, largely featureless part of the ocean floor between the mid-ocean ridge and the continental rise. (Fig 13.9)
Seamount - A submarine mountain (usually volcanic) that rises 1 km or more above the sea-floor (Example: Emperor Seamount chain). Most form above a mantle plume (Fig. 8.19).
Oceanic (Volcanic) Island - A seamount that rises above sea level (Example: Hawaiian Islands). Most form above a mantle plume. (Fig 8.19)
Atoll - A circular coral reef that forms a ring of
islands around a central lagoon. Form in shallow water on the flanks of a
submerging oceanic island. (Fig
13.C)
Deep Ocean Trenches - Most occur along subduction zones. Previously
discussed under 'Plate Tectonics'.
(Fig 13.11)
c) Mid-Ocean Ridges - Previously discussed under 'Plate Tectonics' (Fig 13.15)
II. Layered Structure of the Oceanic Crust and Underlying Upper
Mantle
Ophiolite - Uplifted fragments of oceanic crust emplaced at
the edge of the over-riding plate at a subduction zone. Have a stratigraphic
sequence that is characteristic of oceanic crust, consisting of a thin cap
of sedimentary rocks overlying pillow lavas, sheeted dikes, gabbro, and
ultramafic rocks.
Layer 1 - Sediment (pp. 374-375):
Terriginous Sediment - Sand, silt ,and clay eroded from the continents and carried to the deep ocean floor by gravity and submarine currents.
Biogenous Sediment - Shells and skeletons of
marine animals and plants. Mostly produced by microscopic organisms living
in the sunlit waters near the ocean surface.
Hydrogenous Sediment - Minerals that crystallize directly
from seawater through various chemical reactions.
Pelagic Sediment - Mixture of terriginous clay and biogenous sediment.
Layer 2 - Pillow Basalt - Pillow-shaped basaltic spheroids which form when cold water comes into contact with hot lava, causing the lava to contract.
Layer 3 - Sheeted Dikes - Multiply intruded dikes which form as basaltic magma oozes toward surface through vertical cracks from underlying magma chambers.
Layer 4 - Gabbro - Form as basaltic magma within chambers beneath the mid-ocean ridge undergo crystallization.
Layer 5 - Mantle Peridotite - Ultramafic rocks underlying the oceanic crust.
III. Chemical Composition of Ocean Waters (pp. 384-386)
Salinity - (Fig 14.1) A measure of the amount of salt dissolved in ocean water. The average ocean salinity is approximately 35 o/oo (parts per mil). In tropical, high rainfall areas the salinity is slightly lower. In dry sub-tropical high evaporation regions, the salinity is slightly higher. In high latitude regions, salinity is low due to the low solubility of salts in cold water. (Fig 14.3)
Most abundant salts are: NaCl, MgCl2, Na2SO4, CaCl2, and KCl. (Table 14.1)
Sources of salt-forming ions - weathering and erosion of rocks on land, volcanic eruptions, chemical interactions between seawater and hot, newly formed oceanic crust.
Processes which decrease salinity : biological processes (e.g. formation of calcium carbonate shells), precipitation, runoff from land, melting of glaciers and sea ice.
Processes which increase salinity: evaporation, formation of glaciers and sea ice.
IV. Ocean Water Temperatures (pp. 386-388)
Three-layered thermal structure (Fig. 14.4):
Shallow-surface Zone- Water is heated by solar energy. Temperature is dependent upon amount of solar energy received. Mixing by waves and currents transfers heat downward. Extends to depths up to 300 m.
Transition Zone - Temperatures decrease with depth. Extends to a depth of approximately 1 km. The thermocline is a rapid decrease in temperature with increasing depth which occurs within the transition zone. The thermocline is not present at high latitudes where incoming solar radiation is very indirect and cannot heat the shallow surface layer.
Deep Zone - Cold water (1o - 2.5oC) at depths greater the 2 km.
Note: Layered structure does not exist in polar regions because cold surface water sinks, causing vertical mixing of waters. (Fig 14.4)
V. Tides (pp. 426-429)
Tide - The cyclic rise and fall of ocean water caused by the gravitational force of the Moon, and to a lesser extent, the Sun. (Fig 15.26)
Spring Tide - Occurs when the Earth, Sun and Moon are aligned (full moon, new moon). Results in greatest variation between high and low tides. (Fig 15.27)
Neap Tide - Occurs when the Moon is 90o out of alignment with the Sun and Earth (first quarter moon, third quarter moon). Results in smallest variation between high and low tides. (Fig 15.27)
Other factors affecting tides: shape of coastline, configuration of ocean basin, water depth. As a result, the number and elevation of tides varies from coastline to coastline.
Diurnal Tidal Pattern - Single high tide and single low tide each 24 hour period (Example: northern shore Gulf of Mexico) (Fig 15.28)
Semidiurnal Tidal Pattern - Two high tides and two low tides each 24 hour period, with each high tide being approximately the same height. (Example: Atlantic Coast of the United States) (Fig 15.28)
Mixed Tidal Pattern - Two high tides and two low tides each 24 hour period, with each hight tide and low tide being of different heights. (Example: Pacific Coast of the United States) (Fig 15.28)
VI. Waves, Currents, and Shoreline Erosion/Deposition (pp.411-425)
Waves - Develop when blowing wind transfers energy to the ocean surface. Size of a wave is dependent upon three factors: 1) wind speed, 2) length of time wind has blown, 3) distance wind has blown across the water (fetch).
Wave Crest - The highest part of the wave. (Fig 15.8)
Wave Trough - The lowest part of the wave. (Fig 15.8)
Wave Height - The vertical distance from crest to trough. (Fig 15.8)
Wave Amplitude - One-half the wave height. (Fig 15.8)
Wavelength - The horizontal distance between successive wave crests. (Fig 15.8)
Motion of water molecules within a wave is circular. Radius of circles decreases with depth. Below a depth of approximately one-half the wavelength, water molecule motion as part of the wave is negligible. (Fig 15.8, 15.9)
Waves begin to break when the depth of water is equal to one-half the wavelength. The drag on the bottom of the wave causes it to move more slowly than the upper part. As a result the wave steepens until it collapses forward. (Fig 15.10)
Sediment transport along a shoreline is largely the result of wave refraction - bending of the wave. Most waves approach the shoreline at an angle. As a result, one end of the wave encounters shallow water and slows down before the other, which remains in deeper water (Fig 15.14).
When refracted waves strike shore, they form a longshore current - a current which flows and transports sediment parallel to the coast. (Fig 15.14)
Sediment transport may also occur by beach drift . When refracted waves strike shore, sediment is pushed up and along the beach in the direction the wave is traveling. When water recedes, sediment is drawn back toward the water where it will be pushed by the next wave further along the shoreline. (Fig 15.14)
The combination of wave refraction, longshore current and beach drift result in a number of different shoreline erosional and depositional features.
Erosional features: wave-cut cliffs, wave-cut platforms, marine terraces, sea arches, sea stacks.
Wave-Cut Cliff - a seaward-facing cliff along a steep shoreline formed by wave erosion. (Fig 15.15, 15.19a)
Wave-Cut Platform - A shelf in the bedrock at sea-level, cut by wave erosion. (Fig 15.15)
Marine Terrace - A wave-cut platform that has been exposed above sea-level. (Fig 15.15)
Sea Arch - An arch formed by wave erosion when caves on opposite sides of a headland unite. (Fig 15.16)
Sea Stack - An isolated mass of rock standing just offshore, produced by wave erosion of a headland. (Fig 15.19)
Depositional Features: spit, sand-bar, tombolo, barrier island
Spit - An elongate ridge of sand that projects from the land into the mouth of an adjacent bay (Fig 15.17, 15.19c)
Sandbar - An off-shore ridge of sand. (Fig 15.7)
Tombolo - A ridge of sand that connects an island to the mainland or another island (Fig 15.19b)
Barrier Island - A low ridge of san that parallels the coast. Very common along the east and gulf coasts of North America. (Fig 15.18) Some form as spits which are severed from the mainland by erosion and/or a rise in sea-level. Others represent sand dune ridges which formed during Pleistocene glaciation, but were separated from the mainland by a rise in sea-level as glaciers melted.
Over time, erosion removes sand on the seaward side of barrier islands. As a result of such erosion, the National Park Service was forced to move the Cape Hatteras lighthouse 1600 feet from its original site to a site which is protected from wave erosion. Had the lighthouse not been moved, its base would have been undermined by the action of waves. (1600 feet is the distance between the lighthouse and the shoreline at the time it was constructed in 1870)
Hard Stabilization - Construction of structures to protect a coastline from erosion. Commonly used hard stabilization structures: groin, sea wall
Groin - A barrier built at a right angle to the beach to trap sand transported parallel to the shoreline by longshore current. A negative effect is that sand is blocked to deposition along down current shoreline areas, resulting in loss of beach sand. (Fig 15.21)
Sea wall - A wall constructed to protect buildings and roads by reflecting wave energy back toward the ocean. Commonly results in rapid erosion of beach sand on the seaward side of the sea wall. (Fig 15.11)
VII Oceanic Circulation (pp.404-409)
Current - A continuous flow of water in a particular direction. A driven primarily by wind. (Example: Gulf Stream). (Fig 15.2)
Gyre - An open ocean surface current that moves in a circular path. Rotate clockwise in the Northern Hemisphere, and counterclockwise in the Southern Hemisphere (Fig 14.2). Five main gyres: North Pacific, South Pacific, North Atlantic, South Atlantic, Indian. The centers of gyres lie at approximately 30o N or S latitude because the energy to drive them comes from descending atmospheric winds at those latitudes. Each gyre consists of separate currents (usually 4). Example: The North Atlantic Gyre consists of the Gulf Stream, North Atlantic, Canary, and North Equatorial currents.
Coriolis Effect - The deflection of air or water currents caused by the rotation of the Earth. Air and water currents rotate clockwise in the Northern Hemisphere, and counterclockwise in the Southern Hemisphere.
Deep-Sea Currents - Driving force is differences in density of ocean waters. Cold, high salinity waters have highest density and therefore sink (Example: Polar waters have both low temperature and high salinity due to freezing of fresh water). (Fig 15.6)
Upwelling - A rising ocean current that transports water from the deep ocean to the surface. Occurs predominantly along coastlines and in the equatorial region, in zones where air circulation patterns remove warm surface waters suck that cold, deep, nutrient rich waters rise upwards to replace them. (Fig 15.4)
VIII Coastal Classification
Classification of coastlines is based upon changes that occur along a coastline with respect to sea-level.
Emergent Coastline - Develops due to uplift of the coast or a drop in sea-level. Characterized by wave-cut cliffs, marine terraces, sea arches and sea stacks. Example: coastal California (Fig. 15.25)
Submergent Coastline - Develops as a result of a rise in sea level. Results in a irregular coastline because ocean water fills river valleys, forming estuaries. Ridges which separate the submerged valleys form headlands which project into the sea. Examples: Chesapeake and Delaware bays. (Fif 15.24)