Lecture 12

Abyssal plains

-Flat features of the ocean floor • Likely the most level places on Earth -Sites of thick accumulations of sediment • Fine sediments from turbidity currents • Minerals precipitated from seawater • Shells of marine plankton -Found in all oceans • Most extensive in the Atlantic Ocean

Ophiolite Complex Sequence of four layers composing the oceanic crust

-Layer 1 —consists of deep sea sediments and sedimentary rocks -Layer 2 —consists of pillow basalts -Layer 3 —consists of numerous interconnected dikes called sheet dikes -Layer 4 —consists of gabbro

Active Continental Margins

- Where the oceanic lithosphere is being subducted beneath the continent • Often associated with deep-ocean trenches • Located primarily around the Pacific Ocean

Evolution of an Ocean Basin

-A new ocean basin begins with the formation of a continental rift (Elongated depression where the lithosphere is stretched & thinned)

Oceanic plateaus

Vast outpourings of basaltic lavas on the ocean floor

• Mountains that display faulted and folded rocks are ________

compressional mountains -Display visual evidence of compressional forces • Plate tectonics provides a model for orogenesis -Earth's major mountains have formed along convergent plate boundaries

Deep Ocean Trenches

- Created when oceanic lithosphere bends as it descends into the mantle - Trench depth is related to the age of the subducting lithosphere • Old lithosphere is cold and dense -Plates subduct at a steep angle, producing a deep trench • Young lithosphere is warm and buoyant -Plates subduct at a shallower angle and produce shallower trenches (if at all)

Passive Continental Margins

- Found along most coastal areas that surround the Atlantic Ocean - Not associated with plate boundaries - Experience little volcanism and few earthquakes

Deep-Ocean Trench

- Long narrow creases that represent the deepest part of the seafloor • Challenger Deep, in Mariana trench, is the deepest spot (10,994m below sea level) • Surface expression of a subduction zone • Associated with volcanic activity -Volcanic island arcs -Continental volcanic arcs • Mostly found in the Pacific Ocean

Why Are Ocean Ridges Elevated?

- Newly created lithosphere is hot and less dense than surrounding rocks - As the newly formed crust moves away from the spreading center, it cools and increases in density

Mechanisms for Continental Rifting • Mantle plumes and hot spots

- Regions of hotter than normal mantle rise, experience decompression melting, create basalts triggering hot-spot volcanism on the surface Mantle plumes concentrate under the thick continental crust, which traps heat in the mantle Hot mantle plumes eventually cause the overlying crust to dome and weaken

Mechanisms for Continental Rifting supercontinent cycle

- The supercontinent cycle is the formation and dispersal of supercontinents • Two supercontinents have existed in the geologic past -Pangaea - most recent -Rodinia • Involves major changes in the direction and nature of the forces that drive plate motion

Seafloor Spreading

- This concept was formulated in the early 1960s by Harry Hess - Seafloor spreading occurs along the crests of oceanic ridges • Newly formed melt (from decompression melting of the mantle) slowly rises toward the surface • Most melt solidifies in the lower crust, but some escapes to the sea floor and erupts as lava

Convergence & Subducting Plates Major Features of Subduction Zones

-Volcanic arc -Deep-ocean trench -Forearc region -Back-arc region

Active Continental Margins

-Where the oceanic lithosphere is being subducted beneath the continent • Often associated with deep-ocean trenches • Located primarily around the Pacific Ocean

- Oceanic ridges with slow spreading rates have ________ rift valleys and _____ topography - Oceanic ridges with intermediate spreading rates have ________ rift valleys and _________ topography - Oceanic ridges with fast spreading rates generally ________ rift valley and have a _______ profile

-well developed, rugged -subdued, subdued -do not have a, shallow

Anatomy of the Oceanic Ridge

An oceanic ridge, or mid-ocean ridge, or rise is a broad, linear swell along a divergent plate boundary -The longest topographic feature on Earth -Occupy elevated positions -Segments are offset by transform faults -Extensive faulting and earthquakes -A rift valley (a deep, down-faulted structure) exists on the axis of most ridges

Features of the Deep-Ocean Basin

Features include: - Deep-ocean trenches - Abyssal plains - Seamounts and guyots - Oceanic plateaus

Isostasy

The Principle of Isostasy • Less dense crust floats on top of the denser rocks of the mantle How is it related to elevation changes? If weight is added or removed from the crust isostatic adjustment will take place as the crust subsides or rebounds

Subduction and Mountain Building

• Andean-Type Mountain Building - Subduction beneath a continent rather than oceanic lithosphere - Exemplified by the Andes Mountains • Starts with a passive continental margin -Thick platform of shallow-water sedimentary rocks

Provinces of the Ocean Floor - Three major areas of the ocean floor based on topography

• Continental margins -Outer margins of the continents and the transition to oceanic crust • Deep ocean basins -Between the continental margins and the oceanic ridge • Oceanic ridges -A broad, linear swell at a divergent plate boundary

What Causes Earth's Varied Topography?

• Mantle Convection: A Cause of Vertical Crustal Movement -Uplifting whole continents • Mantle plumes can elevate a region on continental crust -Crustal subsidence • The slabs of oceanic lithosphere will detach from the trailing lithosphere • A downward flow is created as the detached slab continues to sink, pulling down the crust into a basin structure

Seamounts and volcanic islands

• Submarine volcanoes are called seamounts -Over a million seamounts exist -Found in all ocean floors but most common in the Pacific -Many form near oceanic ridges or over a hot spot - A seamount may grow large enough to emerge as a volcanic island

Guyots

• Submerged, flat-topped seamounts -After the volcano is extinct, it eventually erodes to sea level where waves flatten the top of the structure -As plates carry the structure away, it eventually lowers into the ocean

Modern bathymetric techniques

• The topography (shape) of the ocean floor is called bathymetry • Sonar, using sound energy, is now used to measure the depth to the ocean floor • Early bathymetric profiles were created using echo sounders, which bounce a sound off an object to determine the distance