1 Introduction to Oceanography

Geological Oceanography

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2-1The Earth’s Structure

Earth consists of a series of concentric layers or spheres which differ in chemistry and physical properties.The compositional layers of the Earth are the Crust, the Mantle, and the Core. The Core is subdivided into a molten outer core and solid inner core.

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Physical state is determined by the combined effects of pressure and temperature.• Increasing pressure raises the melting point of a

material.

• Increasing temperature provides additional energy to the atoms and molecules of matter allowing them to move farther apart, causing the material to melt.

• Both pressure and temperature increase toward the center of the Earth, but at different rates.

• Divisions of the Earth based upon physical state are the Lithosphere, The Asthenosphere, the Mesosphere, the Outer core, and the Inner core

2-1The Earth’s Structure

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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.

Layered Earth - Evidence Of Earth’s Layers

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Note that Earth is density stratified, that is, each deeper layer is denser than the layer above.

Layered Earth - Chemical Properties

Chemical Properties of Earth’s LayersLayer Chemical PropertiesContinental Crust Composed primarily of granite

density = 2.7 g/cm3

Oceanic Crust Composed primarily of basaltdensity = 2.9 g/cm3

Mantle Composed of silicon, oxygen, iron and magnesiumdensity = 4.5 g/cm3

Core Composed mainly of ironDensity = 13 g/cm3

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So, when we examine the chemical and physical properties of Earth’s layers, we see that a cool, rigid, less dense layer (the lithosphere) floats on a hot, slowly-flowing, more dense layer (the asthenosphere).

Layered Earth - Physical Properties

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

1Three fluid spheres surround the rocky portion of the Earth.

• Hydrosphere includes all of the “free” water of the Earth contained in the ocean, lakes, rivers, snow, ice, water vapor and groundwater.

– The ocean covers 71% of the Earth’s surface

• Atmosphere is the gaseous envelope that surrounds the Earth and is mainly a mixture of nitrogen and oxygen.

• Biosphere refers to all living and non-living organic matter.

2-1The Earth’s Structure

1Physiography and bathymetry (submarine landscape) allow the sea floor to be subdivided into three distinct provinces: continental margins, deep ocean basins and midoceanic ridges.• Continental margins are the submerged edges of the continents and consist

of massive wedges of sediment eroded from the land and deposited along the continental edge. The continental margin can be divided into three parts: the Continental shelf, the Continental slope, and the Continental rise.

• Deep Ocean Province is between the continental margins and the midoceanic ridge and includes a variety of features from mountainous to flat plains: Abyssal plains, Abyssal hills, Seamounts, and Deep sea trenches

• Midoceanic Ridge Province consists of a continuous submarine mountain range that covers about one third of the ocean floor and extends for about 60,000 km around the Earth.

2-2The Physiography of the Ocean Floor

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1Continents and ocean basins differ in composition, elevation and physiographic features.• Continental crust is mainly

composed of granite, a light colored, lower density (2.7 gm/cm3) igneous rock rich in aluminum, silicon and oxygen.

• Oceanic crust is composed of basalt, a dark coloured, higher density (2.9 gm/cm3) volcanic rock rich in silicon, oxygen and magnesium.

2-3Geologic Differences between Continents and Ocean Basins

1Isostacy refers to the balance of an object “floating” upon a fluid medium. Height of the mass above and below the surface of the medium is controlled by the thickness of the mass and its density (similar to ice floating in water).

• Greater the density of the mass, the lower it will sink in the medium.

• Greater the thickness of the mass, the higher a portion of it will rise above the medium.

• Continents are thick (30 to 40 km), have low density and rise high above the supporting mantle rocks.

2-3Geologic Differences between Continents and Ocean Basins

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1Based upon the fit of continental outlines and fossil and geologic evidence, Alfred Wegner proposed his hypothesis of continental drift. According to Wegner, the continents are sections of a past super continent called Pangea, which broke apart and the fragments plowed through the oceanic crust, to their present locations

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Continental Drift

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1Sea floor spreading demonstrates that the sea floor moves apart at the oceanic ridges and new oceanic crust is added to the edges.

• Rift valleys along oceanic ridge crests indicate tension, are bounded by normal faults and are floored by recently-erupted basaltic lava flows.

• Axis of the oceanic ridge is offset by transform (strike-slip) faults which produce lateral displacement.

• Whereas oceanic ridges indicate tension, continental mountains indicate compressional forces are squeezing the land together.

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Sea-Floor Spreading

1Seafloor spreading was an idea proposed in 1960 to explain the features of the ocean floor. It explained the development of the seafloor at the Mid-Atlantic Ridge. Convection currents in the mantle were proposed as the force that caused the ocean to grow and the continents to move.

(right) The Mid-Atlantic Ridge conforms to the shape of the adjacent continents. The inset shows the central rift.

Seafloor Spreading - A Key Idea

1The geomagnetic field is the magnetic field of the Earth.• Magnetometers detect and measure Earth’s magnetic field.

• Moving across the ocean floor perpendicularly to the oceanic ridges, magneometers alternately record stronger (positive) and weaker (negative) magnetic fields (called magnetic anomalies) in response to the influence of the sea floor rocks.

• Magnetic anomalies and the rocks causing them form parallel bands arranged symmetrically about the axis of the oceanic ridge.

• As basaltic rocks crystallize, some minerals align themselves with Earth’s magnetic field, as it exists at that time, imparting a permanent magnetic field, called paleomagnetism, to the rock.

• Periodically Earth’s magnetic field polarity (direction) reverses poles.

3-2Sea-Floor Spreading

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1Paleomagnetism: strips of alternating magnetic polarity at spreading regions.

Confirmation of the Theory of Plate Tectonics

(above-left and above-right) The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes.

1• Sea floor increases in age away from the

ridge and is more deeply buried by sediment because sediments have had a longer time to collect.

• Rates of sea-floor spreading vary from 1 to 10 cm per year for each side of the ridge and can be determined by dating the sea floor and measuring its distance from the ridge crest.

• Continents are moved by the expanding sea floor.

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Sea-Floor Spreading

1Because Earth’s size is constant, expansion of the crust in one area requires destruction of the crust elsewhere.• Currently, the Pacific Ocean basin is shrinking as other ocean

basins expand.

• Destruction of sea floor occurs in subduction zones.

• Seismicity is the frequency, magnitude and distribution of earthquakes. Earthquakes are concentrated along oceanic ridges, transform faults, trenches and island arcs.

• Tectonism refers to the deformation of Earth’s crust.

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Global Plate Tectonics

1• Benioff Zone is an area of increasingly

deeper seismic activity, inclined from the trench downward in the direction of the island arc.

• Subduction is the process at a trench whereby one part of the sea floor plunges below another and down into the asthenosphere.

3-3Global Plate Tectonics

1Earth’s surface is composed of a series of lithospheric plates. Plate edges extend through the lithosphere and are defined by seismicity.• Plate edges are trenches, oceanic ridges and transform faults.

• Seismicity and volcanism are concentrated along plate boundaries.

• Movement of plates is caused by thermal convection of the “plastic” rocks of the asthenosphere which drag along the overlying lithospheric plates.

3-3Global Plate Tectonics

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The major lithospheric plates and their direction of relative movement are shown here.

The Major Lithospheric Plates

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The lithospheric plates interact with the neighboring plates in several ways.

Divergent plate boundaries – Boundaries between plates moving apart, further classified as:

Divergent oceanic crust – for example, the Mid-Atlantic Ridge

Divergent continental crust - for example, the Rift Valley of East Africa.

Plate Boundaries

(right) Extension of divergent boundaries causes splitting and rifting.

1Convergent Plate Boundaries - Regions where plates are pushing together can be further classified as:

Oceanic crust toward continental crust - for example, the west coast of South America

Oceanic crust toward oceanic crust - occurring in the northern Pacific

Continental crust toward continental crust – one example is the Himalayas

Plate Boundaries

(right) Compression at convergent boundaries produces buckling and shortening.

1Transform plate boundaries - locations where crustal plates move past one another, for example, the San Andreas fault.

Plate Boundaries

(above) Translation at transform boundaries causes shear.

1Evidence supporting the theory of plate tectonics:

Apparent Polar wandering: plate movement causes the apparent position of the magnetic poles to have shifted.

(right) The paleomagnetic fields in the rocks would indicate a single pole until the continents drift apart.

Confirmation of the Theory of Plate Tectonics

1Hot Spots: Surface expression of plumes of magma.

Confirmation of the Theory of Plate Tectonics

(above) A volcanic island chain can form when a plate passes over a hot spot and a stationary mantle plume.

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Atolls and Guyots: Coral formations and submerged volcanic mountains.

(above) Guyots were once volcanic peaks above sea level. They were eroded by wave action as they sank beneath the surface of the water.

Confirmation of the Theory of Plate Tectonics

1Age and distribution of ocean sediments: The sediment in the ocean is thinner and younger than the age of the ocean indicates it should be.

The Oceanic ridges: Oceanic ridges are clear indicators of past events.

Terranes: Oceanic plateaus that form by uplifting and mountain building as they strike a continent.

Confirmation of the Theory of Plate Tectonics

1• Mantle plumes originate deep within the

asthenosphere as molten rock which rises and melts through the lithospheric plate forming a large volcanic mass at a “hot spot”.

3-3Global Plate Tectonics

1Through the great expanse of geologic time, this slow movement remakes the surface of Earth, expands and splits continents, and forms and destroys ocean basins.

Summary

(left) The fit of the continents at a water depth of 137 meters.