4 Marine Geology

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Marine Geology

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1. Basic Morphology of the Seaoor

2. Plate Tectonic Theory Development supporting evidence/observations

formulation of the tectonic concept

3. Details of Plate Boundaries divergent convergent transverse

4. Mantle hot spots

5. Marine Sediments

O utline

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Measuring Ocean Bathymetry=Bottom Depth Sonar Methods

Single-Beam Sonar Creates a trackline of bottom depth as the

ship moves forward

Multi-Beam Sonar

Creates a swath of bottom depth as theship moves forward

Towed Multi-Beam Sonar Creates a high-resolution swath of

bottom depth as the ship moves forward

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Global Ocean Bathymetry (Artists Rendition)

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Bathymetry of SatelliteMeasurements

gravity gravity

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Ocean Bathymetry From Satellite Altimetry

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Main Sea Floor Features (besides mid-ocean ridges)

Continental Shelf Abyssal Plain

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Development of Plate Tectonic Theory

1. The development of Plate Tectonic theory took its rst stepsforward when Alfred Wegener proposed the Theory ofContinental Drift in 1915 His theory of continental drift was not well accepted at the time because while the evidence

was overwhelming that continental drift had occurred, the forcing mechanism he proposed(i.e., tidal action of moon) was certainly wrong. The concept of moving tectonic plates wasnot known at this time

2. Plate Tectonic Theory was nally accepted after Sea-Floor

Spreading and Sea-Floor Subduction were discovered in Post- WWII exploration. This was the correct forcing mechanism that Alfred Wegener had missed when he rst

proposed continental drift theory

where we are heading in the coming slides...

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Before We Begin Earth Composition

1. Rigid Outer Crust

Thin and Rigid and oats on the higher density mantle

2. Plastic Upper Mantle

Although solid, the high temperatures cause thematerial to be sufcientlyductile to ow on very long

timescales

4. Molten Outer Core

5. Solid Inner Core

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Numerical Simulation of Mantle Convection

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Part 1:

Continental Drift

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1.1 Fit of ContinentsMany naturalists hadnoticed the apparent

t of the continents ,includingEnglishman FrancisBacon and Italiangeographer AntonioSnider-Pellegrini,

whose 1858 map isshown above.

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1.2. Paleobiogeography

Alfred Wegener noted in 1915 thatin addition to the apparent good t ofcontinental boundaries, thedistribution of fossil and mineral

belts made sense if continents were joined together in the past.

He noted specically that: it was as ifpieces of a torn newspaper were placedback together and all the letters lined up

- one could not help but conclude thatthe pieces at one point in time formed awhole page.

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Part 2:Seaoor Spreading

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2.1 Mid-Ocean Ridges(Discovered with WWII Sonar)

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Some indication of seaoor spreading atmid-ocean ridges was inferred based

purely on observed morphology of thefaulting* patterns along the ridge center

* Faulting = Movement which produces relative displacement of adjacent rock masses along a fracture in the rock

classic pattern of extensional faulting

2.2. Extensional Faulting

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Strong Evidence of Seaoor Spreading Was ProvidedBy Magnetic Anomaly Patterns in Ocean Crust Along

the Ridge

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The Earths Magnetic Fieldnot as stable as you might think

Earths Magnetic Field Past, Present and FutureMagnetic North Pole Location

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The Earths magnetic eld has undergonemany reversals over geologic time

Dark bands = normal polarity

White bands = reversed polarity

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2.3. Magnetic Anomalies that are Symmetric Across the Mid-Ocean Ridge

Magnetic anomalies are a proxy measure of geologic time - each anomaly can be assigneda specic geologic date. Consequently, observing symmetric magnetic banding is really around-about way of observing the rock getting symmetrically older as you move away from

the ridge axis - and this is strong evidence that the ridge is slowly spreading away from theridge axis in opposite directions.

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2.3b. Age of Ocean Crust is Youngest at Ridges and Symmetrically Older Off-Axis

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Part 3:

Seaoor Subduction

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3.1 Discovery of Deep Sea Trenches , and Associated Seismic Activity, Helped to Explain The Eventual Loss Of Ocean Crust that was Initially Formed at Mid-Ocean Ridges.

This really sealed the deal!Ocean Bathymetry

Major Ocean Trenches

Subduction Zone

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4.0 - Finally, Seismic Activity Around the World Delineates Plate

Boundaries (both spreading and subduction boundaries)

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Plate Tectonics putting it all together

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Major Tectonic Plates of the World and their Relative Motion

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Hot molten rock rises to the surface atdivergent boundaries,spreads laterally,cools and gets moredense and then sinksand is pulled bygravity back into the

mantle at subduction boundaries

The Driving Mechanism Convection in the Mantle and Slab Pull

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Plate Tectonic Movement

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1. The earths outer crustis is composed of manyindividual crustal platescalled tectonic plates

that move relative toone another

2. Tectonic Plates can becomposed of both oceancrust and continentalcrust

3. When a Tectonic Platemoves so too does thecontinent that is a partof that plate

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1. Continents are thicker and much less dense than mantle material and so they oat muchhigher in the mantle and create a topographic high spot (dry land)

2. Ocean crust is thiner and only slightly less dense that mantle material so it oats deeper in the mantle and creates a topographic low spot for ocean water to rush in to form ocean basins

Oceanic and Continental Crust

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Tectonic Plate Divergence and Convergence

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Divergent Boundaries

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Fig. 3.24Divergent plate boundaries occur where plates are moving apart .

Most of these boundaries

are mid-oceans ridges , less commonly they arecontinental rifts as in thisexample.

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East Africa Rift Zone( A New Divergent Plate Boundary )

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Mid-Ocean Ridge(A Mature Divergent Boundary)

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Hydrothermal Vent Systems (found at mid-ocean ridge spreading centers )

1. seawater is drawn down through cracks in the off-axis region

2. seawater is heated by, and reacts with, hotrock material

3. mineral elements are extracted from the

rock and dissolved into the hot seawater

4. heated seawater that is rich in rockminerals rises to ocean oor along the ridgeaxis

5. rock minerals in the hot seawater precipitate when the hot water hits the

surrounding cold seawater

6. dissolved rock minerals supportmicrobial life that forms the base of

large vent ecosystems

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Convergent PlateBoundaries

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Three Possible Convergent Boundaries1. Oceanic Crust & Continental Crust

Because ocean crust is more dense than continent crust, this is the plate that issubducted under the continent crust and pushed back down into the mantle

2. Oceanic Crust & Oceanic Crust The plate that gets subducted is often the one that is furthest from its respective

spreading center and thus older, colder, and more dense that the newer plate

3. Continental Crust & Continental Crus t

This is a battle of titans because neither wants to subduct under the other. Mountain-building is a common result.

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Figure 2.22

Examples of Convergent Boundaries

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1. dense ocean crust slides under less dense continental crust 2. generates a deep ocean trench

3. forms explosive volcanoes

4. Examples: North Cascade Mountains in Washington State and/or AndesMountains in Chile

Ocean Crust Colliding with Continental Crust

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Mount Saint HelensExample of Explosive Volcano Caused fromOcean Crust Colliding with Continental Crust

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Ocean Crust Colliding with Ocean Crust

1. Deep oceanic trench (but sometimes lled with sediment)

2. Chain of volcanoes called an Island Arc . 3. Examples: Aleutian Islands, Japan, and Indonesia.

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Island Arcs in the PacicExample of an Island Arc Formedfrom Ocean Crust Colliding withOcean Crust

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Mt. Everest

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Transform Boundary Plates Slide Laterally Relative to One Another

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The San Andreas Fault of aTransform Plate Boundary

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Mantle Hot Spots

(Hawaiian Islands)

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Stationary Mantle Plume Underlying A Moving Tectonic Plate

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Figure 2.22

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Generation of Seamount Chains

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Marine Sediments

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Marine Sediments Outline

1. Brief Overview of Sediment Accumulation

2. Qualitative Look at Marine Sediments Thickness Composition

Rate of Deposition

3. Deep Sea Drilling Programs paleo-temperatures and biological productivity from analysis of sediments fossil species chemical isotope ratios

4. Example of Applying Sediment Core Analysis to Reconstructing Resultsfrom a Meteor Impact with Earth about 65 Million Years Ago

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Marine Sediment Thickness,Composition, and Deposition Rate

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Marine Sediment Types

BiologicalBiological

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Sedimentation Rates

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Summary of Sediment Accumulation

1. Wide range of material raining down continental dust, biological material, river-born sediments

2.Sediment thickness is high near coast due to river runoff of terrigenous sediment , and high productivity that leads to

high rain rate of biological material

3. Red Clays found in open-ocean: Slow rain of continental dust (and very low biological addition) creates red clays

4.Calcareous or Siliceous Sediments found In high biological productivity regions(and in absence of river outows containing terrigenous material):

Downward rain of biological material cause red clays to be ooded/diluted by

biologically-derived calcareous or siliceous material

5.Rate of accumulation very-very slow and a 10 meter sediment core can represent arecord of up to a million years of earth history

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Deep-Sea DrillingProcess

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Remains of planktonicorganisms contained withinsediment cores reveal

information about growthconditions of the overlyingocean

For Example surface water temperatures

biological productivity

Proxy Measurements of Past Ocean Conditions

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Ocean Proxy Temperatures in the Geologic Past

1. Many microorganisms shells found in sediment cores are constructed of calcite ( Ca C O 3 and sometimes MgC O 3).

2. All oxygen atoms have 8 protons and Mos t oxygen atoms also have 8 neutrons giving it a molecularmass of 16 (8 protons + 8 neutrons): 16O

3. Some oxygen atoms have 10 neutrons giving it a molecular mass of 18(8 protons + 10 neutrons): 18O

4. Indirect (proxy) estimates of the temperature of seawater can be obtained from calcite fossils using the isotope ratio 18O: 16O preserved in the oxygen atoms of their shells.

18O is preferentially removed from the water and incorporated into the calcite shells during growth.

This effect diminishes as temperature increases so that, all other things being equal, calcite in shells grown in colder temperatures will have more 18O than calcite in shells grown at warmer temperatures .

Important additional information of Ca/Mg ratio, which is also temperature sensitive, adds to the precision/accuracy of the paleotemperature estimates derived from standard 18O proxy methods .See: Baker et al. Quaternary Science Reviews 24:821-834, 2005

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An Example of Reconstructing EarthHistory Using Sediment Core Analysis

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Of all species that have existedon Earth, 99.9 percent are nowextinct. Many of them perishedin ve cataclysmic events.

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According to a recent poll,seven out of ten biologists

think we are currently in the throes of a sixth massextinction. Some say itcould wipe out as many as90 percent of all species

living today. Yet otherscientists dispute such dire

projections.

American bisonskull heap

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Summary1. Deep-Sea Sediments Accumulate at Very Slow Rates

On the order of centimeters per thousand years

2. Deep-Sea Sediment Cores Provide a Time Series of Ocean EventsDating Back Nearly 200 Million Years

3. Sediment Cores Can Record the Past Conditions of Ocean PrimaryProduction and Ocean Temperatures

4. Sediment Cores Can Also Record Past Extinction Events in theOcean

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4 Marine Geology