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 BathymetryReview
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).
9/20/2012
2
• 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).
http://annaroseandthesea.blogspot.com/
Beam angle
Beam angle: deeper water = wider swath
9/20/2012
3
Beam angle: deeper water = wider swath
http://environmentalresearchweb.org/cws/article/yournews/49837
• Satellite measurements of gravity.
• Measure sea floor features based on gravitational bulges in the sea surface (equipotential surface).
http://www.bbc.co.uk/news/science‐environment‐12911806
VIDEO:http://www.guardian.co.uk/science/video/2011/mar/31/gravity‐map‐earth‐surface‐goce
9/20/2012
4
• Seismic Reflection Profiles– Air guns– Strong, low‐frequency sounds– Details ocean structure beneath sea floor
• Humboldt Bay Bathymetry and Topography related to tectonic deformation and sedimentation.
• Seismic Reflection Profiles reveal subsurface stratigraphy and geologic structures.
• Uninterpreted (top) vs. Interpreted (bottom) seismic reflection profiles offshore Humboldt Bay.
• 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!)
Abyssal Plains• Extend from base
of continental rise/slope
• Some of the deepest, flattest parts of Earth
• Suspension settling of very fine particles
• Sediments cover ocean crust irregularities
• Well‐developed in Atlantic and Indian oceans
http://www.pmel.noaa.gov/pubs/outstand/embl2063/structural.shtml
Mid‐Ocean Ridges• Longest
mountain chains• On average, 2.5
km (1.5 miles) above surrounding sea floor
• Wholly volcanic• Basaltic lava• Divergent plate
boundary
http://www.nasa.gov/mission_pages/cassini/multimedia/pia11138.html
Mid‐Ocean Ridges and Transform Faults
• East Pacific Rise• Saturn Moon Enceladus
9/20/2012
10
Seamount Pillow lava
Hydrothermal Vents• Sea floor hot springs, originally found by Oregon State Oceanographers in 1977.
• Foster unusual deep‐ocean ecosystems able to survive without sunlight
• Warm water vents – temperatures below 30°C (86°F)• White smokers – temperatures from 30–350°C (86–662°F)• Black smokers – temperatures above 350°C (662°F)
http://volcano.oregonstate.edu/submarine‐volcanic‐ecosystems
http://www.pmel.noaa.gov/vents/gallery/smoker‐images.html
CHAPTER 4 Marine Sediment
Classification: A. Shape, Size, VariationB. Formation Processes:• Lithogenic (rock)• Biogenic (organic based)• Authogenic/Hydrogenous
(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)
Terrigenous & Lithogenic sediments (from land)
• Rivers• Winds (aeolian)• Glaciers (ice-rafted debris, IRD)• Turbidites• Sea level changes
Terrigenous Sediments:• derived from weathering of rocks
at or above sea level (e.g., continents, islands)
• two distinct chemical compositions – ferromagnesian, or iron‐
magnesium bearing minerals – non‐ferromagnesian minerals
– e.g., quartz, feldspar, micas
• largest deposits on continental margins (less than 40% reach abyssal plains)
• transported by water, wind, gravity, and ice
• transported as dissolved and suspended loads in rivers, waves, longshore currents
River sediment loads (~109 tons/yr)
9/20/2012
13
Sediment Distribution
• Neritic– Shallow‐water deposits– Close to land– Dominantly lithogenous– Typically deposited quickly
• Pelagic– Deeper‐water deposits– Finer‐grained sediments– Deposited slowly
Neritic Lithogenous Sediments
• Beach deposits–wave‐deposited sand
• Continental shelf deposits• Turbidite deposits• Glacial deposits
– High latitude continental shelf– Currently forming by ice rafting
Pelagic Deposits
• Fine‐grained material• Accumulates slowly on deep ocean floor• Pelagic lithogenous sediment from
– Volcanic ash (volcanic eruptions)– Wind‐blown (aeolian) dust– Fine‐grained material transported by deep sea currents
• Dust (LANDSAT image).
• Dust comprise much of the fine‐grained deposits in remote open‐ocean areas (red clays)
• primary dust source is deserts in Asia and North Africa
9/20/2012
14
Distribution of Sediment on Continental Shelf by Grain Size
Continental shelf
Submarine canyons (cut into the c. slope)
Abyssal plain
Continental rise
Continental slope
Seafloor Features: Continental Margins
Abyssal plain
Glacial (Ice-rafted debris)• boulder to clay size
particles also eroded and transported to oceans via glacial ice
• glacier termination in circum‐polar oceans results in calving and iceberg formation
• as ice (or icebergs) melt, entrained material is deposited on the ocean floor
• termed 'ice‐rafted' debris
9/20/2012
15
RadiolarianDiatomsForams
Biogenic sediments(from living things)
Calcareous (CaCO3)Foraminifera -- animalsCoccolithophores -- plants
Siliceous (SiO2)Radiolaria -- animalsDiatoms -- plants
Biogenic Sediment• Two major types:
–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
Sediment Succession in Absence of Siliceous Ooze
9/20/2012
19
Cosmogenous Marine Sediments• Macroscopic meteor debris
• Microscopic iron‐nickel and silicate spherules (small globular masses)– Tektites– Space dust
• Overall, insignificant proportion of marine sediments
Marine Sediment Mixtures• Usually mixture of different sediment types• Typically one sediment type dominates in different areas of the sea floor.
• sediments thickest along continental margins, thin at mid‐ocean ridges
• coastlines – dominated by river‐borne and
wave reworked terrigenoussediments
– shelf and slope characterized by turbidites and authigeniccarbonate deposits
– glacial deposits and ice‐rafted debris common at high latitudes
– high input of terrigenoussediments 'dilutes' biogenouscomponents
• deep‐sea (pelagic) basins – abyssal clays (wind blown
deposits) common – lower quantities of biogenic
material • distribution of biogenous sediments
dependent upon three primary factors – production in surface waters – dissolution in deep waters – dilution by other sediments
types
Distribution of Marine Sediments: