Oceanic Lithosphere & Marine Sediments

Origin of Oceanic Lithosphere 1960 – Harry Hess Theory of seafloor spreading At MOR plates move apart, lava wells up into the newly created fractures and generates new oceanic lithosphere New lithosphere moves away from the ridge crest Zones of active rifting are 20 to 30 km wide

Structure of Oceanic Lithosphere  

Formation of Oceanic Lithosphere Magma (liquid with crystals) originates from partially melted mantle forms gabbro layer Magma injected into fractures above the magma chambers creates the sheeted dike complex layer Pillow lavas are from basaltic lava that is flash cooled in seawater Sediments deposit on top of pillow basalts

Destruction of Oceanic Lithosphere Oceanic lithosphere subducts because its overall density is greater than the underlying mantle Subduction of older, colder lithosphere results in STEEP descending angles Younger, warmer oceanic lithosphere is more buoyant and angles of descent are SHALLOW Research indicates that parts, or even entire oceanic basins, have been destroyed along subduction zones This explains relatively young age of oceanic crust 

Marine Sediments Sediment: Solid material that has settled from a state of suspension. When lithified (consolidated or cemented) becomes a rock.

Basic Rock Types Metamorphic rocks Formed by “changing” pre-existing igneous, sedimentary or other metamorphic rocks Driving forces are increased heat and pressure Typically develop a fabric or texture Ex: gneiss and marble   Igneous Rocks A rock or mineral that has solidified from molten material (from a magma). Slower cooling times have bigger crystals. Extruded to surface or intruded into existing rock Ex: Volcanic (extrusive): Hawaii (basalt) and (Cascades) andesite Plutonic (intrusive): Sierra Nevada (granites) Sedimentary rocks: Sediment: Solid material that has settled from a state of suspension in a fluid When lithified (consolidated or cemented) becomes a sedimentary rock Derived from weathering of pre-existing rocks Can tell the geologic history of an area. Classified by grain size and source Ex: sandstone and limestone

Marine Sediments Sources a) Cosmogenous b) Hydrogenous c) Lithogenous d) Biogenous

Cosmogenous Sediments (insignificant amount) Originated from outer space Evidence for extinction of dinosaurs Example: Tektites (silica glass) Iron-nickel meteorites Iridium

Hydrogenous Sediment (uncommon) Precipitated from water Example: Manganese nodules: most in Pacific, grow very slowly Evaporite: salt NaCl

Lithogenous Sediment (terrigenous, clastic) Small pieces of broken rock transported to ocean from the land (wind, rivers, glaciers, coastal erosion, turbidity currents etc.) Generally form rapidly Can have high energy environments and coarse grain sizes. Mostly composed of quartz, very resistant to weathering Some deep ocean, very fine wind blown clays

Most lithogenous sediments are on or near a landmass Coarser sediments closer to shore, Finer sediments farther from shore As transportation distance increases, sediment becomes more mature and Clay content decreases Sorting increases Non-quartz minerals decrease Grains become more rounded   Examples: Delta, Beach, and Turbidite deposits Fine wind-blown clays

Biogenous Sediment Skeletal remains of organisms Examples: Oozes (calcareous and siliceous) Carbonate (coral) reefs

Oozes 30% biogenous material Oozes form slowly 1/2 to 2 1/2 inch per 1000 yrs Oozes are soft and mushy. Low energy environments, very fine grained (clay sized) From phytoplankton and zooplankton  

Calcareous oozes Made of CaCO3   Primarily from Foraminifera (forams), zooplankton Coccoliths: phytoplankton Fairly evenly distributed in oceans Calcium carbonate compensation depth (CCD) No CaCO3 beneath about 15,000 feet (4500 meters) Lysocline: depth at which CaCO3 begins to dissolve rapidly

Siliceous oozes Made of SiO2 Diatoms: most common plankton Phytoplankton Common in polar regions Very important for upwelling nutrients. Many uses beer filters, optical glass. Radiolarians: zooplankton, more common in equatorial regions

Carbonate (coral) reefs Must have clear, warm shallow water Contain the world's largest petroleum reserves Comprised of dead animal and plant skeletons Mostly of calcium carbonate-called "carbonates" Grow at rates of 10-30 feet per thousand years Three main stages proposed by Charles Darwin. i) fringing reef (Hawaii), ii) barrier reef (Great Barrier Reef) iii) atoll (many Pacific atolls)

Distribution of Marine Sediments Neritic sediments cover about ¼ of sea floor and are near landmasses Pelagic sediments cover about ¾ of seafloor and are mostly in deepwater Distribution controlled by proximity to sources of lithogenous sediments, ie: landmasses Productivity of microscopic marine organisms

Neritic sediments are generally: Shallow water deposits Close to land Dominated by lithogenous sources Typically deposited quickly Pelagic sediments are generally: Deepwater deposits mostly oozes and windblown clays Finer-grained sediments Deposited slowly Less lithogenous and more biogenous depending on biologic productivity