Chapter 4
OCEANIC GEOGRAPHY AND THE EARTH
I. Major Divisions of Earth
A. atmosphere: all the air held by Earth’s gravity; formed by outgassing of gases from
volcanoes
1. sections: troposphere (has weather), stratosphere (contains ozone layer), mesosphere,
thermosphere
2. the early atmosphere was composed mostly of hydrogen and helium
3. major constituents of dry atmosphere (excludes water)
a. Nitrogen (N2) – 78.1%
b. Oxygen (O2) – 21.0%
c. Argon (A) – 0.93%
d. Carbon dioxide (CO2) – 0.03%
B. biosphere: all the living organisms on Earth
C. hydrosphere: all the water on Earth; oceans cover 70.8% of Earth’s surface
1. water distribution on Earth:
a. oceans (saltwater): 97.2%
b. freshwater: 2.8%
i. ice caps and glaciers: 2.15%
ii. groundwater
iii. surface water: the Great Lakes hold 22% of all surface freshwater
2. formation of the oceans: oceans have been around for 4 billion years
a. Theory #1: ocean water came from outgassing from volcanoes that spew water
vapor from the mantle, which then condensed; some dissolved salts also may have
come from the mantle, but they also come from weathering of rocks
b. Theory #2: comets with ice constantly bombard Earth and fill up basins with
water; this theory is probably wrong because most comets have a different isotope
of hydrogen than most of earth’s water
3. ocean surface area (largest to smallest): Pacific, Atlantic, Indian, Arctic
a. Antarctic Ocean [Southern Ocean] is not considered an ocean because there are
no boundaries
4. overall average ocean depth is 3729 m; from deepest to shallowest, the oceans are
Pacific, Atlantic, Indian, Arctic
5. ocean volumes have changed over time due to temperature changes and the amount of
water in ice caps; thermal expansion of seawater increases global sea level 60 cm for
every 1°C increase in temperature
II. Inside Earth
A. Earth’s Compositional Layers: classified by what type of rock is present
1. Crust: silicates (silicon, oxygen); quartz [SiO2] is the most common mineral;
hardened early in Earth’s history
a. Oceanic Crust: thickness = ~8 km; denser basalt (3 g/mL)
b. Continental Crust: thickness = ~35 km; less dense granite (2.7 g/mL)
2. Mohorovicic discontinuity [the Moho]: boundary between crust and mantle where
seismic waves experience drastic changes in velocity
3. Mantle: iron and magnesium silicates
4. Core: iron and nickel
B. Earth’s Mechanical Layers: based on the combined effects of pressure and temperature
1. lithosphere (thickness = ~100 km): solid layer, includes crust and upper mantle
2. asthenosphere: plastic ductile layer; lithosphere floats and moves on it; highly viscous
3. mesosphere: solid
4. outer core: liquid
5. inner core: solid
III. Bathymetry: mapping and charting the sea floor by measuring its depth
A. sounding: measuring depth; depth recorders or echo sounders do this using sound; in
early times, a piano line with a weight on the end was dropped until it touched the bottom
1. echo sounder: sends a ping, a sound signal, which bounces off the bottom and is
detected by the device; the time difference allows it to calculate depth
2. precision depth recorders (1950s)
3. multibeam echo sounder: Seabeam
B. side-scan sonar: can map a 60-km wide swath of sea floor
a. GLORIA, Sea MARC: two examples of side-scan sonar systems
C. the seafloor can be mapped by satellites with altimeters that detect the height of the
water; the height of the water is altered by underwater features, the gravitational
attraction of which pulls water into a mound over them; this allows the satellites to
indirectly measure the large underwater features
IV. Marine Provinces
A. Continental Margin: extends from the coast to the continental rise; includes shelf, slope,
rise
1. continental shelf [neritic zone]: flat part extending from shoreline; shallow ocean
water; average width: 70 km; takes up 8% of the total ocean area
a. shelf break (average depth: 135 m): edge of continental shelf
2. continental slope: sloped part
a. steeper in Pacific Ocean due to active margins
3. continental rise: area of turbidite (rocks that tumbled down the slope) deposition at
the base of the continental slope; result of merging deep-sea fans
4. submarine canyon: steep-sided valley engraved in the continental slope caused by
erosion from turbidity currents; found at the mouth of a sediment-carrying river
5. Associated Terms
a. turbidity current: rapid movement of sediment-laden water down the continental
slope spurred by an earthquake; cause submarine canyons by erosion
b. graded bedding: layer of turbidites at the continental rise; larger, heavier rocks are
on the bottom and light ones on top in each layer, but there are multiple layers, so
the true pattern is large, small, large, small, large, small and so on
c. western boundary undercurrent [WBUC] or slope current: strong current flowing
at the base of the continental slope along the western boundary of the Atlantic
Ocean that helps shape continental rises; starts in Norwegian Sea and flows south
B. Ocean Basin (ocean basin + continental slope = oceanic zone): contains abyssal plains,
mid-ocean ridges/rises, submarine fans, hydrothermal vents, seamounts, abyssal hills, and
trenches
1. abyssal plain: deep, flat area that accounts for most of ocean’s floor
a. suspension settling: forms flat abyssal plains and buries short features; deposition
of sediments from the water column over millions of years
2. deep-sea fan [abyssal fan, submarine fan]: fan-shaped accumulation of sediment from
turbidity currents that is deposited at the base of a submarine canyon
a. Ganges fan: found in the Bay of Bengal, east of India; largest submarine fan;
deposits come from Ganges River
b. Amazon cone: one of the largest deep-sea fans; extends 700 km
V. Other
A. snowball events: there is evidence that the ocean froze to a depth of 2000 m in some
places, and the earth was completely covered by ice about 580-750 million years ago
B. ice ages occur every 100,000 years; the last ended 10,000 years ago, shifting from the
Pleistocene to Holocene epoch; ice caps grow in ice ages and lower sea level
C. Data about the earth
1. Earth is 4.6 billion years old
2. the circumference is 40,000 km
D. Chicxulub crater, on the Yucatan Peninsula along the Gulf of Mexico, is where the
meteorite crashed that may have caused the dinosaur extinction
E. Hypsographic curve: a graph of elevation versus the area of land above that elevation