9/20/2012 1 Figure 2.10 Review: What Drives Plate Motions: (1) Density vs. Gravity: causes oceanic crust to sink in subduction zones, causes crust to extend at spreading ridges (called ridge push, but the ridge is not pushing, the crust is pulling as it sinks into subduction zones)… (2) Thermal Convection: exerts drag force to base of crust, circulates heat and mantle material... Crustal Age Bathymetry Review Plate Boundary Motion •Ocean Basin Structure • Bathymetry • Topography • Plate Boundaries Review CHAPTER 3 Ocean Basins • Bathymetric Mapping (echo sounding, sonar, satellite gravimetry): measuring submarine topography. • Sea floor physiography driven by plate tectonic processes. • Abyssal Plain, Ridges, Basins, Continental Margin (Slope / Shelf).
19
Embed
ocean 10 lecture 3.ppt - science.earthjay.com• California’s transform active margin has a continental borderland. Continental Slope • Steep slope between the shelf and the deep
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
9/20/2012
1
Figure 2.10
Review: What Drives Plate Motions:(1) Density vs. Gravity: causes oceanic crust to sink in
subduction zones, causes crust to extend at spreading ridges (called ridge push, but the ridge is not pushing, the crust is pulling as it sinks into subduction zones)…
(2) Thermal Convection: exerts drag force to base of crust, circulates heat and mantle material...
• Echo Soundings– Echo sounder or fathometer: Reflection of sound signals– German ship Meteor identified mid‐Atlantic ridge in 1925
• Lacks detail
Modern Acoustic Instruments• Side scan SONAR
– GLORIA (Geological Long‐range Inclined Acoustical instrument), Sea MARC (Sea Mapping and Remote Characterization): use the properties of acoustic reflection to characterize the seafloor material properties.
• Multi‐beam Echosounders/SONAR– Pole mounted, towed, or hull mounted.– Collect Bathymetric data, as well as acoustic data that can be
processed to characterize the seafloor material properties (eg. for habitat classification for the US Territorial Sea).
• Seismic Reflection Profiles reveal subsurface stratigraphy and geologic structures.
Hypsographic Curve: Shows relations between elevation of land and ocean
• 70.8% of Earth covered by oceans• Average ocean depth is 3729 meters • Average land elevation is 840 meters• Uneven distribution of areas of different depths/elevations• Variations suggest plate tectonics at work
9/20/2012
5
Three Major Ocean Provinces:Continental margins: Shallow‐water areas close to shoreDeep‐ocean basins: Deep‐water areas farther from landMid‐ocean ridge: Submarine mountain range
• Passive or ActiveMargins• Passive
–Not close to any plate boundary–No major tectonic activity–Example: East coast of United States
• Active–Associated with convergent or transform plate boundaries
–Much tectonic activity
Convergent or Transform• Convergent Active Margin
– Oceanic‐continent convergent plate boundaries– Active continental volcanoes– Narrow shelf– Offshore trench– Example: Western South America
• Transform Continental Margin– Less common– Transform plate boundaries– Linear islands, banks, and deep basins close to shore– Example: Coastal California along San Andreas Fault
9/20/2012
6
Continental Shelf
• Flat zone from shore to shelf break– Shelf break is where marked increase in slope angle occurs
• Slope generally <5°• Average width is 70 km (43 miles) but can extend to 1500 km (930 miles)
• Average depth of shelf break is 135 meters (443 feet)
Margins are dominated by sedimentation through glaciostatic sea‐level fluctuations.
Eel River has the largest annual sediment discharge (per km^2) in the continental US.
9/20/2012
7
• The type of continetnal margin determines the shelf features.
• Passive margins have wider shelves.• California’s transform active margin has a continental borderland.
Continental Slope
• Steep slope between the shelf and the deep sea• Topography similar to land mountain ranges• Steeper slope than continental shelf
– Averages 4° but varies from 1–25° gradient
• Marked by submarine canyons
Turbidity Currents• Submarine Landslides • Sediment from continental shelf and slope• Move under influence of gravity and bouyancy driven flow• Sediment deposited at slope base
9/20/2012
8
Prothero, 1989
Mosher, et.al. 2008
Cross Section
Adams, 1990; Goldfinger, et.al. 2009
RR0705‐96PC Submarine Canyons• Narrow, deep, v‐shaped in profile
• Steep to overhanging walls• Traverse the slope to the base• Carved by turbidity currents
9/20/2012
9
Continental Rise
• Transition between slope and abyssal plain• Marked by turbidite deposits from turbidity currents
• Deposits generate deep‐sea/submarine fans• Distal ends of submarine fans transition to flat abyssal plains (e.g. Bengal fan!)
(precipitated from water)• Volcanic• Cosmogenic (outer space)
9/20/2012
11
Fluid velocitydetermines thesize of theparticles thatcan be moved
Sediment Transport Sediment Texture
• Grain size sorting– Indication of selectivity of transportation and deposition processes
• Textural maturity– Increasing maturity if
• Clay content decreases• Sorting increases• Non‐quartz minerals decrease• Grains are more rounded (abraded)
9/20/2012
12
Sediments• Reflect composition of rock from which derived• Coarser sediments closer to shore• Finer sediments farther from shore• Mainly mineral quartz (SiO2)
–Macroscopic• Visible to naked eye• Shells, bones, teeth
–Microscopic• Tiny shells or tests• Biogenic ooze
• Mainly algae and protozoans
m = micron = millionth of a meter! m = micron = millionth of a meter!
9/20/2012
16
m = micron = millionth of a meter! m = micron = millionth of a meter!
Biogenic Sediments:
• composed primarily of marine microfossil remains
• median grain size typically less than 0.005 mm (i.e., silt or clay size particles)
• characterized as CaCO3(calcium carbonate) or SiO2(silica) dominated systems
• sediment with biogenic component less than 30% termed calcareous, siliceous clay
• calcareous or siliceous 'oozes' if biogenic component greater than 30%
• siliceous oozes (primarily diatom oozes) cover ~15% of the ocean floor – distribution mirrors regions of high productivity
– common at high latitudes, and zones of upwelling
– radiolarian oozes more common in equatorial regions
9/20/2012
17
• calcareous oozes (foraminifera, coccolithophores) cover ~50% of the ocean floor
• level below which no CaCO3 is preserved is the 'carbonate compensation depth‘ (CCD)
• This change in dissolution rate is called the lysocline.Below the lysocline, more and more calcium carbonate dissolves, until eventually, there is none left. The lysocline typically occurs at a depth of 3000 to 4000 m
Sediment AccumulationCalcium Carbonate Content in Pelagic
Oceanic Sediment
9/20/2012
18
Rates of Deposition of Marine Sediment
Temporal Succession of Pelagic Sediment at Spreading Centers