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Oceanography 112
Home » People » Gary Jacobson » Oceanography 112 » Answers to Ch 4



1. Passive: Around most of the Atlantic, Indian and Arctic Oceans / shelf, slope, rise

Active: Around most of the Pacific Ocean / shelf, slope, trench

Translational: From central to Baja California / shelf, banks, troughs (basins)

2. When a supercontinent rifts, two passive continental margins are formed. As rifting widens the ocean between the margins, oceanic crust becomes older next to the margins and eventually subducts, changing the passive margins into active ones. If an oceanic ridge or rise is overridden by an active margin, the subduction stops and the margin becomes translational.

3. Sea level changes due to glaciation and tectonic processes allow wave action to erode all portions of the shelf, thereby leveling it. Sediment carried out by rivers is spread around by waves and currents and is often "ponded" behind reefs, banks, volcanoes, etc. that act as sediment dams at the edge of the shelf.

4. Submarine canyons occur mostly in the continental slope, but a few extend up onto the shelf. They form mainly by turbidity current erosion. Sand falls can also contribute to their erosion, but to a lesser degree. Canyons on the shelf may have formed by river or glacial erosion during times of lower sea level. Sediments (turbidites) accumulate at the mouths of the canyons forming deep-sea (abyssal) fans which overlap to form rises.


_9_rift valley   _2_oceanic ridge or rise
_4_continental shelf  _6_continental slope
_1_passive continental margin 10_seamount
_3_active continental margin  15_volcanic arc
14_deep-sea trench   11_guyot
_8_abyssal plain   _7_continental rise
_5_shelf/slope break 8,12_where abyssal hills are found



6. Where lateral offsets occur along a mid-ocean ridge or rise, abrupt age differences will occur in the oceanic crust formed by the offset ridges/rises. Since older oceanic crust will be deeper, the age differences are also accompanied by abrupt depth differences. Fracture zones are thus the cliff-like features that extend beyond the transform faults that occur between the offset oceanic ridges/rises.


mid-ocean ridge/rise - shallow earthquakes, basaltic volcanism, hot springs

rift valley - shallow earthquakes, basaltic volcanism, hot springs

fracture zone - shallow earthquakes occur rarely

abyssal plain - sedimentation buries irregular ocean floor

abyssal hill - In the Atlantic they are the tops of inactive volcanoes, the bases of which have been_ buried in sediment. In the Pacific, they are usually small-scale irregularities in the oceanic crust produced by faulting at the E. Pacific Rise, but are no longer active.

seamount - active or inactive basaltic volcano that was never tall enough to reach the surface

guyot - inactive basaltic volcano that at was an island leveled by erosion, but is now submerged

island/seamount chain - basaltic volcanoes, mostly inactive, but island/seamount over hot spot is active

trench - earthquakes occur in a deepening belt from the trench toward arc of volcanoes

island arc - Andesitic volcanoes with earthquakes occurring at great depths beneath them along__ subducted plate



Last Updated: 01/13/2015
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