Ocean Basins and Earth’s Composition

Defining the Marine Environment

A. 70% of the Earth’s surface is covered by the oceans – hence the “water planet”

1. 2/3 of the Earth’s land masses are in the Northern hemisphere, which is 61% ocean

2. 80% of the Southern hemisphere is ocean

B. 94% of the Earth’s ecovolume is in the ocean, much greater comparatively than the terrestrial ecovolume

C. 50% of the Earth’s photosynthesis occurs in the oceans, much greater proportionally than the tropical rainforest (which is higher per unit area)

Geography of the Ocean Basins

• Oceans are traditionally classified into four large basins (doesn’t include Southern, listed in decreasing size).

• Pacific Ocean – largest and deepest

• Atlantic Ocean

• Indian Ocean

• Arctic Ocean

• Southern Ocean - the continuous body of water which surrounds the Antarctic continent below 60°S latitude.

• average depth is about 4,000 km or 2.5 miles

• maximum depth of around 11,000 m (Mariana Trench off the coast of Japan)

Cross-Section of the Earth

• Inner core - 1,200 km radius; 4,000°C (7,200°F); solid

• Outer core - 2,300 km radius, molten, rich in iron and magnesium, thought to generate the Earth’s magnetic field

• Mantle - thought to be solid, but near the melting point for most rocks, the mantle slowly swirls and mixes, 2,900 km radius

• Crust - outermost layer, solid, floats on the mantle, continental crusts and ocean crusts differ greatly, 4-60 km

• The Earth is density stratified, meaning each

deeper layer is denser than the layer above.

Chemical Properties of Earth’s Layers

Layer Chemical Properties Continental Crust Composed primarily of granite

density = 2.7 g/cm3 Oceanic Crust Composed primarily of basalt

density = 2.9 g/cm3 Mantle Composed of silicon, oxygen, iron and magnesium

density = 4.5 g/cm3 Core Composed mainly of iron

Density = 13 g/cm3

• A cool, rigid, less dense layer (the lithosphere)

floats on a hot, slowly-flowing, dense layer (the

asthenosphere).

Physical Properties of Earth’s Layers

Layer Physical Properties Lithosphere The cool, rigid outer layer

Asthenosphere Hot, partially melted layer which flows slowly Mantle Denser and more slowly flowing than the

asthenosphere Outer Core Dense, viscous liquid layer, extremely hot

Inner Core Solid, very dense and extremely hot

Layered Earth –Isostatic Equilibrium - gravitational equilibrium between the Earth’s

lithosphere and asthenosphere

• Think for a moment about the lithosphere. Why doesn’t it sink into the asthenosphere? How are features such as mountains supported?

• The concept of buoyancy is illustrated by a ship on the ocean. The ship sinks until it displaces a volume of water equal to the weight of the ship and its contents.

(below) Earth’s lithosphere is

supported on the asthenosphere in a

similar manner, with a few important

differences. Instead of buoyancy, the

term isostatic equilibrium describes

the way the lithosphere is supported

on the asthenosphere.

Isostasy

Layered Earth - Internal Heat

• Where does the heat within Earth’s layers come from?

– Heat from within Earth keeps the asthenosphere flowing. This allows the lithosphere to keep moving. The source of this heat is radioactive decay, given off when the nuclei of unstable forms of elements break apart.

– This heat travels in convection currents in the mantle.

Layers

• What evidence supports the idea that Earth has layers?

– The behavior of seismic waves generated by earthquakes give scientists some of the best evidence about the structure of Earth.

– (above-left) S waves cannot penetrate Earth’s liquid core.

– (above-right) P waves are bent as they pass through the liquid outer core.

Oceanic versus Continental Crust

Oceanic crustal plates have a higher

density and they are:

• Thinner

• Geologically newer than continental crust

• Rich in iron and magnesium

• Basalt, dark in color

Oceanic versus Continental Crust

Continental crustal plates have a comparatively lower density and they are:

• Thicker

• Geologically older than oceanic crusts

• Rich in sodium, potassium, calcium, and magnesium

• Granite, lighter in color