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Answers to Study Questions Chapter 10 - Sea Floor

1. Find the number in the cross-sectional sketch below which corresponds to the following         features:

 _9_rift valley  _2_oceanic ridge
_4_continental shelf  _6_continental slope
_1_passive continental margin _10_seamount
 _3_active continental margin  _5_shelf/slope break
_13_deep-sea trench   _11 guyot
_8_abyssal plain _7_continental rise
_7_thickest turbidite layers occur here _14_accretionary wedge
_12_abyssal hills
_6_most common location of submarine canyons
_6_where turbidity currents flow fastest


          
Sea Floor


2. Active and passive continental margins both have continental shelves and continental slopes. Passive margins have continental rises, relatively wide shelves and are found mainly in the Atlantic and Arctic oceans. Active margins are bordered by deep ocean trenches and are found mainly in the Pacific.

3. The statement: "Most submarine canyons were formed during the Ice Age when rivers extended their courses seaward", is false. Some submarine canyons are cut into the continental shelf and may have formed in such a way, but most submarine canyons are found on the continental slope. Since the continental slope was not exposed during the low sea levels of the Ice Age, submarine erosion (turbidity currents and/or sand falls) must have carved them.

4. Abyssal plains more extensive on the floor of the Atlantic than on the floor of the Pacific because deep sea trenches rim the Pacific. Trenches trap sediment from turbidity currents and thereby prevent the deposition of enough sediment to cover the irregular sea floor and transform it into abyssal plains.

5. Mid-ocean ridges are where sea-floor spreading makes new oceanic lithosphere. Oceanic lithosphere steadily moves away from the ridges and in so doing ages, cools, sinks, and eventually subducts into the mantle at a deep-ocean trench. Subduction tends to pull oceanic lithosphere away from the ridges which increases the rate of spreading and broadens the profile of ridges such that they become an oceanic rise (e.g. the East Pacific Rise).

6. Darwin hypothesized that atolls represent the last stage in the evolution of coral reefs that grow around sinking volcanic islands. As the island sinks, coral growth keeps the coral at the surface, transforming fringing reefs into barrier reefs, and barrier reefs into atolls.

7. The top layer in an ophiolite sequence is usually some type of deep ocean sediment. The sediment is deposited directly on top of the second layer which is pillow basalt. A complex of sheeted dikes lies below the pillow basalt representing magma that solidified in the vertical fractures through which it flowed towards the surface from the magma chamber below the ridge. The magma chamber solidifies into various types of gabbro directly below the sheeted dike complex. Below the gabbro lies peridotite, which represents the top of the mantle.

Last Updated: 06/16/2015
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