The Ocean floor and Coast Tectonic Processes and the ocean basin. The littoral zone

Tsunami Tidal waves or Tsunamis result when a large section of the sea floor suddenly moves and therefore displaces a massive amount of water.

Tsunamis may occur at convergent plate boundaries where abrupt slippage of one plate against another In deep oceanic water, a tsunami wave travels quickly As the wave approaches shallow coastal water, it slows but forms large, destructive waves

Earthquakes Tidal waves or Tsunamis result when the low amplitude long wavelength waves reach the shallow shoreline and begin to feel the bottom of the sea floor. This Shortens the wavelength and increase the amplitude (height).

Earthquakes Location of worldwide earthquakes

Tectonic Plates on Modern Earth

Divergent boundaries: Chiefly at oceanic ridges (aka spreading centers)

continent could lead to formation of oceanic lithosphere. How rifting of a continent could lead to formation of oceanic lithosphere. e.g., East Africa Rift e.g., Red Sea e.g., Atlantic Ocean

Subduction zones form at convergent boundaries if at least one side has oceanic (denser) material. Major features: trench, biggest EQs, explosive volcanoes

Another subduction zone—this one with oceanic material on both sides. Modern example: Japan

Collison zones form where both sides of a convergent boundary consist of continental (buoyant) material. Modern example: Himalayas This probably used to be a subduction zone, but all the oceanic material was subducted.

Most transform boundaries are in the oceans. Some, like the one in California, cut continents. The PAC-NA plate boundary is MUCH more complex than this diagram shows.

Hotspots, such as the one under Hawaii, have validated plate tectonic theory.

Convergent Plates

Convergent Plates

Divergent Plates

Black: normal polarity White: reversed polarity Both: very magnetic Marine geologists found that seafloor magnetism has a striped pattern completely unlike patterns on land. Mason & Raff, 1961 Black: normal polarity White: reversed polarity Both: very magnetic

Hypothesis: Stripes indicate periodic reversal of the direction of Earth’s magnetic field. To test this hypothesis, scientists determined the eruptive ages and the polarity of young basalts using the newly developed technique of K-Ar radiometric dating.

And then (1962-1963) geologists realized that the patterns are SYMMETRICAL across oceanic ridges. The K-Ar dates show the youngest rocks at the ridge.

How magnetic reversals form at a spreading center

Age of the seafloor

Based on Composition Crust – solid, relatively low density silicate rock Mantle – Semi fluid, denser, mafic (iron and magnesium bearing) rocks Core – Liquid then solid iron and nickel with traces of heavier elements

Plate Types Oceanic plates: basalt Dark (black) and dense rock type composed of silicates, iron and magnesium Continental plates – granite and andesite Light colored (pink, white and gray) and low density rock type composed almost entirely of silicates.

Isostasy Isostasy is the vertical movement of the crust to attain “buoyancy” in the mantle. The height a block of wood floats in water depends on it’s density and thickness. The “height” of the earth’s crust also depends on it’s density and thickness. Wood is less dense than water. The earth’s crust is less dense than the underlying mantle Video

Isostasy

Crustal Density Variations in elevation are due both to thickness and density. The continents stand high because continental crust is thick and light (felsic). The ocean basins are low because oceanic crust is thin and dense (mafic).

Raised Beaches Beaches are shoreline features, they must have formed at sea level. Finding a beach at a higher elevation than current sea level is evidence of crustal uplift. Similarly, finding marine fossils at elevations above sea level also indicates that the land has been uplifted.

Continental Crust is less dense and thicker than oceanic crust, so it extends higher above Earth’s surface and deeper into the mantle

Isostacy The displacement of the mantle by Earth’s continental and oceanic crust The crust and mantle are in equilibrium with one another; meaning, the force of gravity on the mass of crust (mountain range) is balanced by buoyancy A good model for isostasy is the water line of a boat when someone boards or leaves the boat

Isostasy The term isostasy literally means ‘weighing the same’. When applied to geology, it refers to the buoyant properties of layers of rocks which float on other layers, according to their density and thickness. This explains why the Earth’s crust floats on the denser, underlying layer, just as an ice cube floats in a glass of water. The continental shelf is a gently sloping region surrounding the continents, over which the sea is relatively shallow. The continental shelf is covered with sediments, derived from erosion of the continents. The continental shelf is part of the continent itself and was above sea level during the ice age, when the sea level was lower than at present. The depth of water over the continental shelf depends upon two main factors, isostatic changes in land height and changes in sea level.

ISOSTAsY The principle of isostasy shows that the Earth’s crust is generally higher where it is thicker and less dense; lower where it is thinner and denser. The density of the continental crust is less than that of the oceanic crust. Table 6.1 shows the approximate densities of different layers of the Earth.

Questions?