Transcript Historical Geology

Chp 11: Sea Floor-ancient mariners
Chp 11: Sea Floor
Map showing 4 major oceans, sea floor topography=bathymetry
Chp 11: Sea Floor
ALVIN submersible used to survey the sea floor
Sampling the
sea floor:
a. Clamshell
sampler taking
a sea floor
sample
b. Piston core
falls to sea floor
penetrates
sediment, then
is retrieved with
sediment in
Chp 11: Sea Floor
JOIDES Resolution vessel-drills in deep oceans to recover
Sediments and basalts to study history of Earth
Chp 11: Sea Floor
DSDP core from ocean drillingcrosses the Cretaceous-Tertiary
Boundary; the dark gray layers
in middle are the sediments
produced in middle of ocean as
result of asteroid impact at end
of Cretaceous Period.
Chp 11: Sea Floor
Continental Margins and deep sea:
a.
Continental Margins- subdivided by bathymetry (depth)
1. Continental shelf: down to 200m
2. Continental slope: steep slope, down to 2500m?
3. Continental rise: gentle slope, down to 3000m?
b. Deep Ocean sea floor- essentially the deep ocean
1. Abyssal Plain- generally flat lying, central ocean plains
2. Oceanic trench- where subduction occurs, deepest portion
of ocean.
3. Mid-ocean Ridges: subsea mountain ranges, volcanic centers,
where molten lava extruded on sea floor, generates new
oceanic crust.
-where Submarine hydrothermal vents are found. Known as ‘black smokers’
a unique ecosystem exists here-bacteria, animals, plants….now hypothesized
as possible means by which life began on Earth in PreCambrian-more than 600MYA
Generalized Profile of Sea Floor
showing features of margins also
Continental Margin
Continental Margin
Continental Shelf
Continental Slope
Sea Level
Continental Shelf
Continental Slope
Continental Rise
Oceanic trench
Abyssal Plain
Mid Ocean ridge
Chp 11: Sea Floor architecture
Mid ocean ridges
Trenches
Chp 11: Sea Floor
Chp 11: Sea Floor
Continental Margins:
a. Two types of Continental margins:
1. Passive: within a plate, lack volcanic activity (US E Coast)
2. Active: leading edge of continental plate-where subduction
occurs (along western edge of S America)
b. Sedimentary features common along continental margins
1. Submarine canyons- deep, steep sided canyons on shelves and
slopes. Most can be related to sea level fall during last Ice Age
when rivers extended to edge of present day shelf.
2. turbidity currents- sediment-water mixtures that are denser than
sea water; flow down canyons onto slope where form ‘fans’
3. Submarine fans- deposition by turbidity currents of overlapping
wedge shaped sediment piles referred to as ‘submarine fans’
Chp 11: Sea Floor
Chp 11: Sea Floor
Note thickest sediments
occur offshore major deltas
today, or where
major deltas
existed in
geologic
past.
(yellow-red-orange is
thickest)
Age of Sea Floor: oldest sea floor is in blue (Jurassic)
youngest sea floor is along mid ocean ridges
Sea floor fractures: where mid ocean ridges are
offset perpendicular to ridge itself
Profile thru mid ocean ridgevertical succession of layers=ophiolite
Chp 11: Sea Floor
Deep Sea Sedimentation: see Fig 11-19 in book!!
a. Most deep sea sediment is fine grained material- silt and clay sized-because most transport
mechanisms can move coarse material (sand and gravel) far from shore.
1. most material is wind blown dust and ash from continents and islands, or shells of microscopic organisms that fall to sea floor after organism dies- referred to as Pelagic sedimentsmeaning the settled from suspension far from land.
Several types of Pelagic sediment are common:
1. Pelagic clay: red to brown, composed of clay sized particles from continents or islands.
covers most of the deep ocean basins.
2. Ooze: composed mostly of shells of microscopic marine organisms and plants
- calcareous ooze: Calcium carbonate skeletons of tiny marine organisms (forams)
- siliceous ooze: silica skeletons of single celled animals (radiolarians) or plants (diatoms)
b. coarse grained sediments are delivered to deep ocean in several waysa. ice rafting: icebergs transport coarse material into ocean, deposit it as ice melts
b. turbidites: high energy flows transport coarse material far from continental shelf edge
Chp 11: Sea Floor
Imaging the sea floor and beneath the sea floor:
1. Imaging the sea floor: side-scan sonar
2. Imaging shallow depth beneath sea floor: sub bottom profiler
(up to 100 m or so….)
3. Imaging greater depths beneath sea floor: seismic reflection
data (up to several kilometers)
Chp 11: Sea Floor-Side Scan sonar
‘fish’ towed behind vessel,
emits sound waves that are
reflected back to sensor,
produces ‘picture’ of sea
floor surface.
Chp 11: Sea Floor-side scan sonar
deployment
Side scan sonar – sediment waves, escarpment
Chp 11: side scan sonar-note central no data zone
Side Scan sonar-iceberg marks
Chp 11: Sea Floor
Sub bottom or Echo Profile- reflects off
sea floor, returns to vessel, gives profile of depth
but actually measured in TIME
Chp 11: Sea Floor
Chp 11: Sea Floor
Chp 11: Sea Floor
Chp 11: Sea Floor
Chp 11: Sea Floor
Chp 11: Sea Floor topography
Mid-ocean ridges: where new crust forms, extrusive igneous
Trenches: deepest portions of ocean-where subduction occurs
(oceanic plate dives beneath continental plate or other oceanic
plate).
Guyots and Seamounts:
-guyot-eroded, flat topped volcanic island. Due to erosion at
sea level….
-seamount-never reached sea level, has sharp, cone profile
Atolls: coral reefs that grow around edges of guyot, form
circular islands
a. trenches
b. other features??-guyots
-seamounts
Chp 11: Sea Floor
Chp 11: Sea Floor
Chp 11: Sea Floor
Chp 11: Sea Floor-Atolls, Tahiti
Chp 11: Sea Floor- Reefs
Most reefs are constructed in warm,
tropical climates by algae.
Some cold water reefs are known today also.
Chp 11: Sea Floor-Deep Sea Resources
Resources from the ocean:
a. Continental shelf-slope-rise:
-drilling for hydrocarbons (oil and gas)
-shelf: dredging for diamonds etc off river mouths
-methane hydrate: gas in form of ‘ice’- undeveloped resource
-phosphorites: phosphorus deposits in shallow water
b. Abyssal plain:
-manganese nodules: round, softball size nodules that are
concentric layers around a central object; precipitate rare elements
out of sea water into these layers. Not economic to date….
A significant legal question….Economic Exclusion Zones declared by
countries with ocean boundaries….200 nautical miles from shore….
- sulphide deposits at midocean ridges: deep water, not feasible now….
Chp 11: Sea Floor-Summary
1. Profile of sea floor: continental shelf down slope onto
-abyssal plain
- mid-ocean ridges
-trenches: deepest spots in oceans
2. Sampling of sea floor-oceanography
-coring apparatus retrieve actual sediment samples
-submarines
-dredging
-Deep sea drilling: academic research
3. Age of oceans- result of plate tectonics; oldest sediment
is Jurassic (approx 180 MYA).
Chp 11: Sea Floor
Chp 11: Sea Floor- Summary
Imaging the sea floor and beneath the sea floor:
1. Side scan sonar: imaging sea floor
2. sub bottom profiler- imaging shallow depths beneath sea floor
(up to 100+m or more)
3. Seismic Reflection- imaging sea floor to depths in kilometers
beneath the sea floor.
Sea floor topographic features:
-guyots
-seamounts
-atolls
-reefs
Economic uses: drill for oil and gas- shelf to slope
dredge shallow water offshore rivers for minerals, ores…
Chp 11: Sea Floor- Summary
Deep sea sedimentation:
Several types of Pelagic sediment are common:
1. Pelagic clay: red to brown, composed of clay sized particles from continents or
islands. covers most of the deep ocean basins.
2. Ooze: composed mostly of shells of microscopic marine organisms and plants
- calcareous ooze: Calcium carbonate skeletons of tiny marine organisms (forams
- siliceous ooze: silica skeletons of single celled animals (radiolarians) or plants
(diatoms)
b. coarse grained sediments are delivered to deep ocean in several waysa. ice rafting: icebergs transport coarse material into ocean, deposit it as ice melts
b. turbidites: high energy flows transport coarse material far from continental shelf
edge
Chp 11: Sea Floor- Summary
Continental margins:2 types recognized
1. Passive: intra plate, rare volcanic activity; US E Coast
2. Active: plate margin where subduction occurs, typified by volcanic
activity; west coast of S America
Continental Margins- subdivided by bathymetry (depth)
1. Continental shelf: down to 200m
2. Continental slope: steep slope, down to 2500m?
3. Continental rise: gentle slope, down to 3000m?
b. Abyssal Plain- essentially the deep ocean
1. Abyssal Plain- generally flat lying, central ocean plains
2. Oceanic trench- where subduction occurs, deepest portion
of ocean.
3. Mid-ocean Ridges: subsea mountain ranges, volcanic centers,
where molten lava extruded on sea floor, generates new
oceanic crust.
-where Submarine hydrothermal vents are found: high temperature, volcanic gases