geology and history of the oceans - western …myweb.facstaff.wwu.edu/shulld/esci...

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Patterns and observations?

Observations? Patterns?

Geology and geometry of the ocean floor

Learning objectives

• Recognize the geological processes that shape the ocean

• Understand the distribution of mountains, trenches, abyssal plains, island chains and seamounts

• Learn the primary causes and consequences of plate tectonics

• Observe the overall geometry of the ocean and how it varies with geological processes (plate tectonics, sediment accumulation and erosion)

Earth’s interior, lithosphere and asthenosphere

• Lithosphere = upper mantle + crust (rigid)

• Asthenosphere: Deformable, partially melted, “plastic” layer

• Continents are supported isostatically (they “float”)

• Oceanic crust is denser than continental crust

Rigid{Deformable

More solid

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Patterns and observations?

Observations? Patterns?

Puzzle-like fit improves when you use the edge of

the continental shelf to outline the continents

Edward Bullard plot – 60’s

Fossil distributions only make sense if continents were joined in the past Age of oceanic crust (dated by paleomagnetism)

Seafloor is relatively young compared to the age of the earth

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Sediment thickness throughout the ocean

Whittaker et al. 2013

Distribution of earthquakes and volcanoes

Plate-tectonics explanation of these data

• The lithosphere is made of plates

• New seafloor is created at mid-ocean ridges (spreading centers)

• Old seafloor is destroyed at trenches (subduction zones)

• Mountain ranges, on land and in the ocean, trenches, and so on are the result of plate movements

The big picture

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Driving force: Gravity

- Leading edges of plates pulled deep into

the mantle at trenches (slab pull)

- Trailing edge of plates slide off the ridges

(ridge push)

- Convection cells inside the earth

How are mountains formed?

Divergent plate boundary: “spreading center” where crustal formation occurs

Convergent plate boundary: “subduction zone” where crust is destroyed

Transform plate boundary: “transform faults” where plates move past each other (intense seismic activity)

Plate boundaries off the coast of Washington

Diverging plate boundaries:

Juan de Fuca Ridge

Gorda Ridge

Explorer Ridge

Transform plate boundaries

Sovanco Fracture Zone

Blanco Fracture Zone

Converging plate boundary:

Cascadia Subduction Zone

“Hot spots” and island chains

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Hawaiian Islands and Hawaiian and Emperor Seamount chains The big picture

Patterns and observations?

Observations? Patterns?

Geometry of the ocean

Continental Continental Basin Mid-ocean Seamount Trenchshelf slope rise (Abyssal plain) ridge

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Actual bathymetry (with vertical exaggeration) Geometry of the ocean

Continentalslope

Continental slope and submarine canyons Active margin Passive margin

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Summary

• Seafloor is created at spreading centers and is destroyed at subduction zones

• Processes at plate boundaries lead to the creation of mountains, trenches, abyssal plains, and earthquakes

• Gravity (slab pull, ridge push) is the primary driver of plate tectonics and generate convection cells within the mantle

• The Atlantic ocean is growing, the Pacific Ocean is shrinking, and the shape of the ocean is changing over very long time scales.

• The shapes of the ocean basins are the result of plate tectonics and the deposition and erosion of sediment

Patterns and observations?

Observations? Patterns?