Transcript GEOL 1e Lecture Outlines

GEOL: CHAPTER 9
The Seafloor
Learning Outcomes
LO1: Examine the history and methods of
oceanic exploration
LO2: Describe the structure and composition
of the oceanic crust
LO3: Identify the continental margins
LO4: Discuss the features found in the
deep-ocean basins
Learning Outcomes, cont.
• LO5: Discuss sedimentation and sediments
on the deep seafloor
• LO6: Explore coral reefs
• LO7: Recognize the types of natural
resources found in the oceans
Introduction
• Oceanic crust is thinner and denser than
continental crust
• Gabbro at depth and basalt at surface
• Oceanic crust produced continually
• Largest part of Earth’s surface
• Varied topography of seafloor
Exploring the Oceans
• Interconnected body of saltwater
• Oceans and seas cover 71% Earth’s
surface
• Oceans are large; seas are smaller
• 1400s and 1500s: voyages of exploration:
– Columbus 1492
Exploring the Oceans, cont.
• Scientific voyages later:
– Captain Cook in 1768, 1772, 1777
– HMS Beagle 1831-1836 with Charles Darwin –
evolution of organisms and coral reefs
– HMS Challenger 1872: voyage to sample
seawater, determine depths, collect seafloor
sediments, and classify organisms
Exploring the Oceans Today
• Echo sounder: sound waves used to
determine ocean depths
• Seismic profiling: seismic waves penetrate
seafloor and reflect from layers; helps
determine structure of oceanic crust
• Ocean ships drill into seafloor
• R/V Chikyu
• Submersibles: Alvin
Oceanographic Research Vessels The R/V Chikyu (“Earth”), a
research ship in the Integrated Ocean Drilling Program
Oceanic Crust Structure
and Composition
• Ophiolites: sections of oceanic crust and
upper mantle emplaced by subduction
zones and thrust faults in mountain ranges
• Top to bottom:
– Pillow lava and sheet lava flows
– Sheeted dike complex, basaltic
– Gabbro
Continental Margins
• Continental margin: area separating
continent portion above water from the
deep seafloor
• Continental shelf
– Gently sloping; 1 degree or less
– Between shore and steeper continental slope
– Shelf-slope break averages 135 m deep
– Pleistocene (1.8 million years ago to 10,000
years ago): much of shelf above sea level
Continental Margins, cont.
• Continental slope: begins at shelf-slope
break
• Continental rise: gently sloping area
between continental slope and abyssal
plain
– Absent in Pacific (oceanic trenches)
– Present in most of Atlantic
Continental Margins, cont.
• Shelf-slope break:
– Landward: sediments are affected by waves
and tidal currents
– Seaward: gravity transports and deposits
sediments
– Much of land-derived sediment is seaward of
shelf-slope break and covers the continental
slope and continental rise
Features of Continental Margins A generalized profile showing
features of the continental margins. The vertical dimensions of the
features in this profile are greatly exaggerated, because the
vertical and horizontal scales differ.
Turbidity Currents
• Underwater flows of sediment/water mix;
denser than seawater alone
• Reaches relatively flat seafloor
• Deposits sediments in graded beds, with
largest particles first and smallest particles
last
• Forms overlapping submarine fans
• Evidence: 1929 event breaks North
Atlantic cables
Submarine Canyons
• Best developed on continental slopes, but
also found on continental shelves
• Some connect to land rivers, but most
don’t
• Turbidity currents move through
submarine canyons, and likely are the
primary agents of their formation
Active Continental Margins
• At leading edge of continental plate where
oceanic lithosphere is subducted
• Narrow continental shelf
• Continental slope descends to trench, so
no continental rise
• South America
• Pacific Northwest
• Earthquakes and volcanoes
Passive Continental Margins
• Broad continental shelves
• Well-developed continental slopes and
rises
• Abyssal plains extend from continental rise
• Within a plate
• Overlapping submarine fans at continental
rise
Features of the
Deep-Ocean Basins
• Average 3.8 km deep; dark, cold
• Abyssal plains
• Oceanic trenches
• Oceanic ridges
• Hydrothermal vents
• Seafloor fractures
• Seamounts, guyots, aseismic ridges
Abyssal Plains
• Beyond continental rises of passive
continental margins
• Flat and cover large areas
• A few peaks up to 1 km
• Flatness from sediment deposition that
covers topography
• Not found near active margins: sediments
are trapped in oceanic trenches
Oceanic Trenches
• Long steep-sided depressions
• Near convergent boundaries
• Subduction: cool, dense oceanic
lithosphere is consumed
• Common in Pacific Ocean basin
• Earthquakes along Benioff zones
• Volcano chain on overriding plate
Oceanic Ridges
• Mostly submarine mountain system
composed of basalt and gabbro
• Found in all ocean basins
• Divergent boundary: new crust formation
• May have rift along crest
• Mid-Atlantic Ridge
• East Pacific Rise
Submarine Hydrothermal Vents
• At oceanic ridges
• Cold seawater seeps below crust, is
heated at depth, discharges as plumes up
to 400ºC
• Black smoker: from dissolved minerals
• Community of organisms: bacteria, crabs,
mussels, starfish, tube worms
– Chemosynthesis: bacteria oxidize sulfur
compounds
Submarine Hydrothermal Vents,
cont.
• Economic potential:
– Heated seawater reacts with crust
– When discharged into ocean it cools, and iron,
copper, and zinc sulfides and other minerals
precipitate
Seafloor Fractures
• Oceanic ridges terminate at fractures that
run at right angles to the ridges
• Several hundred kilometers long
• Shallow-focus earthquakes
– Transform faults (active)
• Fracture zone (inactive)
Seamounts and Guyots
• All are volcanic in origin
• Seamount: at least 1 km height
• Guyot:
– Volcano originally above sealevel
– Plate carries it away from ridge and into
deeper waters
– Waves eroded top and made it flat
• Abyssal hills: 250 m high
Aseismic Ridges
• Ridge or broad area rising up to 2-3 km
above seafloor; lacks seismic activity
• Some are microcontinents
• Form as linear succession of hot-spot
volcanoes near oceanic ridges
• Can also form in the interior of plates
– Hawaiian Islands/Emperor Seamount chain
Deep Seafloor Sediments
• Silt- and clay-sized particles
• Sources:
1. Windblown dust and volcanic ash
2. Shells of microscopic plants and animals
from near-surface waters
3. Particles from chemical reactions in seawater
4. Cosmic dust
Deep Seafloor Sediments, cont.
• Pelagic clay: particles from continents and
islands
• Calcareous ooze: calcium carbonate
skeletons of marine organisms
• Siliceous ooze: silica skeletons of some
marine organisms
Reefs
• Skeletons of marine organisms: corals,
mollusks
• Shallow tropical seas with clear water and
water temperature above 20ºC
• Corals with symbiotic photosynthetic
algae: 50 m deep maximum
Reefs, cont.
• Fringing reefs
– Attached to island or continent
– Rough tablelike surface
– Slope steeply to seafloor
• Barrier reefs
– lagoon separates reef from shore
• Atoll
– Oval/round reef surrounds lagoon
– Form around subsiding volcanic islands
Barrier Reef The white line of breaking waves marks the site
of a barrier reef around Rarotonga in the Cook Islands in the
Pacific Ocean. The island is only about 12 km long.
Resources from Oceans
• Evaporation of seawater: sodium chloride
• Seafloor deposits
– Ownership questions
– U.S. Exclusive Economic Zone – 200 nautical
miles
– Oil production
– Methane hydrate
– Manganese nodules
– Sulfide deposits at hydrothermal vents