University of Colorado GEOLOGY 1010

Class Note 19

The Sea Floor

The oceanic crust is thin (5-10km) basaltic, dense (3.0-3.2), and young (< 250Myrs).

Studies of the Ocean Floor over the past 40 years have led to major discoveries about how the Earth works.

The Oceans cover more than 70% of the planet. If there were no continents and the Earth's surface were perfectly even, the water depth would be about 2km.

The ocean basins have been studied using a great variety of techniques, including deep sea drilling, magnetometer studies, sonar bathymetry, and radar imaging from aircraft and satellites.

The continents have shallow margins called shelves that are underlain by continental crust and are generally less than 200m deep.

Outboard of the shelf is the continenetal slope extending into deep water (>1000m).

Submarine canyons formed by turbidity currents (currents of dense, sediment-laden water) and submarine landslides are prominent features of the continental shelf and slope.

Most of the ocean basin is relatively featureless abyssal plains with depths > 2000m.

There are ocean trenches extending along convergent plate boundaries. These trenches are sinuous deeps with depths as great as 12,000m (~38,000ft).

There is also a long, sinuous ridge, the mid-ocean ridge, that extends more than 80,000 km around the globe. The mid-ocen ridge is marked by a central rift valley which is the principal site of sea-floor spreading. The ridge is also offset at irregular intervals by fracture zones.

Other features of the sea floor include volcanic islands, annular reef islands called atolls, undersea mountains called seamounts and flat-topped undersea mountains called guyots.

An atoll is formed by a sinking volcanic island that is surrounded b y a reef. If the reef growth is able to keep up with the sinking, the result is an atoll or anular island. If the atoll is submerged the result is a guyot. Guyots and atolls are tropical features. Why?

Chains of guyots may form aseismic ridges that extend for thousands of km across the ocean floor.

Sea floor sediments may be either terrigenous (derived from continental land masses), or pelagic (derived from ocean organisms, dust, and airborne volcanic ash).

Occasionally, slabs of oceanic crust may be tectonically emplaced onto continental margins, where the whole sequence of marine sediments, pillow basalts, (subaqueous), and sheeted gabbros may be exposed. These complexes are called ophiolites, and prominent examples occur in northern California, Cuba, the Italian Apls, Cyprus, and Oman.

High thermal gradients are observed in some ocean basins (up to 25 C/km). However average heat flow is about the same in ocean basins as in continents.

GEOL 1010 Syllabus

Class Note 20

Class Note 18

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