1. Oceanic Lithosphere & Marine Sediments

2. 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

3. Structure of Oceanic Lithosphere

4. 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

5. 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 

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

7. 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

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

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

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

11. 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

12. 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

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

14. 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

15. 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

16. 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

17. 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 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)

18. 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

19. 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