Chapter l8

The Sea Floor

-71% of surface is covered by salt water

-very little was known until the 1800's

-Challenger (1872) English research vessel

-Glomar Challenger ( 1968) international research vessel drilled more than 1000 roles

-Echo sounding

-Seismic profiling

Continental Margins

-zones separating the part of a continent above sea level from the deep-sea floor, consists of:

-cont. shelf: slope of <1 degree (narrow to wide)

-cont. slope: 1 to 25 degrees slope, canyons may be present

-cont. rise: more gently sloping

-turbidity currents: sediment-water mixture denser than sea water that flows downslope to the

deep-sea floor

Types of Continental Margins

-active cont. margin: at leading edge of continental plate where oceanic plate is subducted (narrow shelf)

-passive cont. margin: at trailing edge of continental plate, sites of deposition (wide shelf)

The Deep-Ocean Basin

-abyssal plains: flat surfaces covering vast areas of sea floor, beyond cont. rise, covered by sediments

-trenches: long, narrow areas at active cont. margins-where subduction occurs

-ridges: long, linear mountain chains composed of basaltic rock, rift valley in center

-fractures: cracks oriented more or less at right angles to ridge axes (transform faults)

-seamounts: mountains rising more than 1km above seafloor volcanic origin)

-guyot: flat topped seamount

-aseismic ridge: long, linear ridges and broad plateau-like features higher than 2-3 km, no seismic activity (e.g. Emperor Ridge including Hawaii)

Deep-Sea Sedimentation

mostly fine-grained material, windblown or volcanic ash, exception: drop stones

-pelagic clay: brown to reddish clay-sized particles derived from continents

-ooze: mostly shells from microscopic marine animals and plants, e.g. calcareous ooze (above CCD), siliceous ooze

Structure and Composition of Oceanic Crust

oceanic crust is usually recycled at subduction zones, in special cases oceanic crust is preserved as ophiolites on continents

ophiolite sequence: profile

deep-sea sed. (top)

pillow lava

sheeted dikes

massive gabbro

layered gabbro

peridotite (bottom)