1. The ocean formed from the condensation of gasses released by degassing
of the earth as it cooled. Salt content was added from the gasses and chemical
weathering (Box 3.1).
2. Continental shelves are shallow, submarine platforms of variable widths
inclined very gently seaward covered by young sediments that become finer
offshore. Wide shelves may have gravel at their outer edges that was deposited
during lower Pleistocene sea levels. Continental slopes are steeply inclined
toward ocean depths.
3. Submarine canyons are erosional features that cross continental shelves
and slopes caused by a combination of down-canyon sand movement, bottom currents,
river erosion during low sea level, and turbidity currents. Abyssal fans are
formed of land-derived sediment moved down the submarine canyon by turbidity
currents. Turbidity currents were responsible for the 1929 Grand Banks cable
breaks, and they produce graded bedding.
4. A passive continental margin includes a continental shelf, slope and rise,
and lacks earthquakes, volcanoes, and young mountain belts. The continental
rise is a wedge of sediments deposited by turbidity currents (deposits exhibit
graded bedding) moving down the continental slope and contour currents (not
graded and wedge shaped) moving along the slope. Abyssal plains are exceedingly
flat, covered by graded sediments deposited by turbidity currents that cover
irregularities on the sea floor.
5. Active continental margins lack continental rises and abyssal plains,
but possess oceanic trenches associated with Benioff zones, volcanoes, low
heat flow, and negative gravity anomalies.
6. Mid-oceanic ridges have rift valleys along their crests, except for the
Pacific Ocean ridge. Other geologic activity includes shallow-focus earthquakes,
high heat flow, basaltic eruptions, hydrothermal activity, and exotic organisms.
7. Fracture zones cross and may offset the mid-oceanic ridges. They produce
shallow-focus earthquakes and along segments called transform faults.
8. Conical seamounts are extinct volcanoes, sometimes forming islands. Guyots
are flat-topped seamounts, cut by wave activity, that have subsided to their
present depths. Aseismic ridges link guyots and seamounts on the sea floor.
9. Reefs are wave-resistant ridges formed by coral and other calcareous organisms.
Three types include: fringing - attached directly to shore, barrier - separated
from land by a lagoon, and atoll - circular reefs that rim lagoons that form
around subsiding volcanoes.
10. Terrigenous sediments on the sea floor were derived from land and deposited
by turbidity and/or contour currents. Pelagic sediments are clays and fine-grained
skeletons of microorganisms that settle through the ocean water. Pelagic sediments
are absent from mid-oceanic ridges.
11. Oceanic crust is divided into three layers: 1 - marine sediment, 2 -
pillow basalt overlying sheeted dikes, 3- thought to be sill-like gabbro,
but unsampled by deep drilling at the moment. Ophiolites found in mountain
chains exhibit a similar three-layered sequence. They are interpreted as sea
floor from marginal ocean basins, but are not typical ocean floor.
12. Rocks of the sea floor are younger than 200 million years, in contrast
to continental crust that is 3-4 billion years old.
