Ch. 13 - Origin and Evolution of the Ocean Floor

Class: GEOL-101

Notes:

13.1 An Emerging Picture of the Ocean Floor

Define bathymetry and describe the various bathymetric techniques used to map the ocean floor.

Mapping the Seafloor

Presentation content:

• From 1872 to 1876, the HMS Challenger expedition collected oceanographic data
  • Measured the depth to the seafloor by lowering weighted lines overboard
    • Deepest spot measured is now called the Challenger Deep
      • 10,994 m deep
      • Measured in 1875
• Modern bathymetric techniques
  • The topography (shape) of the ocean floor is called bathymetry
  • Sonar, using sound energy, is now used to measure the depth to the ocean floor
  • Early bathymetric profiles were created using echo sounders, which bounce a sound off an object to determine the distance
• Modern bathymetric techniques
  • After World War II the US Navy developed sidescan sonar
  • Sidescan sonar images a horizontal region above the seafloor
  • High-resolution multibeam instruments send out a fan of sound and record reflections
    • Produced first photograph-like images of seafloor
  • Only about 5% of the seafloor has been mapped in detail
• Mapping the ocean floor from space
  • Uses satellites equipped with radar altimeters
  • Massive underwater structures exert stronger than normal gravitational attraction
    • Water piles up over these features
  • Satellite radar altimeters can detect subtle changes (a few cm) in elevation of the ocean surface

Notes from the lecture:

• HMS Challenger
• How do we know depth of the sea floor?
  • Firs scientific voyage with this purpose was the HMS Challenger expedition
    • 1872-1875
    • ~200 miles apart
    • Deepest measurements 8222 m, 27000' at south end of Marianas Trench, called the "Challenger Deep" (the deepest part of the Marianas Trench)
  • Modern bathymetric techniques I
    • Use sound waves post WWI
    • Echo sounder
  • Modern bathymetric techniques II
    • Sidescan sonar post WWII gives an image seafloor, but does not give accurate depths
    • Multibeam sonar developed in 1960's
    • Offshore seismic surveys developed in the 1950s give accurate depths
    • Sonar mapping is slow work boats go ~5-12
  • Satellite measurements
    • Satellite radar measures the time it takes for a wave to go from the satellite to the floor of the ocean
    • Mountains on sea floor have extra pull of gravity, draw more water around them, bulging the sea surface.
    • Mountain must be mile high and several wide to see

Provinces of the Ocean Floor

Presentation content:

• Three major areas of the ocean floor based on topography
  • Continental margins: Outer margins of the continents and the transition to oceanic crust
  • Deep ocean basins: Between the continental margins and the oceanic ridge
  • Oceanic ridges: A broad, linear swell at a divergent plate boundary

Notes from the lecture:

• Subdividing by elevation

13.2 Continental Margins

Compare a passive continental margin with an active continental margin and list the major features of each.

Passive Continental Margins

Presentation content:

• Geologically inactive regions (not associated with plate boundaries)

• Found along most coastal areas that surround the Atlantic Ocean

• Experience little volcanism and few earthquakes

• A continental shelf is a gently sloping, flooded portion of the continent

  • Varies greatly in width
  • Gently sloping (average one-tenth of a degree slope)
  • Contains important mineral and oil deposits
  • Some areas contain extensive glacial deposits
  • Some areas contain shelf valleys

• A continental slope is a steep structure that marks the boundary between the continental and oceanic crust

  • Inclination varies but on average is 5 degrees
    • The slope in some areas is as high as 25 degrees

• A continental rise is a thick accumulation of sediment from the continental slope

  • These sediments are typically carried by turbidity currents (mixtures of sediment and water) down submarine canyons
  • When a turbidity current emerges onto the relatively flat ocean floor, the sediments spread out in a fan shape called a deep-sea fan
  • The continental rise is composed of multiple deep-sea fans

Notes from the lecture:

• Passive margin:
  • wide shelf
  • not geologically active, since it is not a plate boundary
  • trailing edge of continent on divergent plates
• Where is the boundary of the North American plate?
  • At the ridge
• Active margin:
  • narrow shelf
  • Near convergent boundaries
• Continental shelf is important for oil and gas
• Incised valleys formed during glacial lowstands Sea level with ~400' lower during last ice age.
• Continental rise is the accumulation of sediment that has come off the sea floor
  • Really important for petroleum geologists

• Examples:
  • East Coast US -> New Jersey Passive Margin
    • We have kept this coast and other parts untampered
    • 51 wells drilled 1975-1984
    • 3 wells noncommercial gas, 1 well minor oil
    • Why doesn't it work? Insufficient source, reservoir, trap
    • Future potential?
  • Passive margin with salt: Gulf of Mexico
    • Salt is less dense than sediments
    • Squeezed upward and outward by the weight of overlying sediments
    • It is less dense than the sediments that are on top of it so it will flow like fluid
    • Plenty of oil and gas
      • Nowadays exploration: looking underneath the salt layer

Active Continental Margins

Presentation content:

• Where the oceanic lithosphere is being subducted beneath the continent

  • Often associated with deep-ocean trenches
  • Located primarily around the Pacific Ocean

• Sediments and rocks can be scraped from the descending plate and accumulate on the continental plate as an accretionary wedge

• Subduction erosion occurs when the subducting plate scrapes the bottom of the overriding plate

  • Effective when the angle of descent is steep

13.3 Features of Deep-Ocean Basins

List and describe the major features of deep-ocean basins.

Features

Presentation content:

• Features include:
  • Deep-ocean trenches
  • Abyssal plains
  • Seamounts and guyots
  • Oceanic plateaus

Notes from the lecture:

• Are largely around the pacific ocean and are associated with deep ocean trenches
• Anatomy of a convergent boundary:
  • Active margin has a narrow shelf, offshore trench

• Trench = surface expression of a subduction zone

Deep-Ocean Trench

Presentation content:

• Long narrow creases that represent the deepest part of the seafloor
  • Challenger Deep, in Mariana trench, is the deepest spot in the ocean (10,994 meters below sea level)
  • Surface expression of a subduction zone
  • Associated with volcanic activity
    • Volcanic island arcs
    • Continental volcanic arcs
  • Mostly found in the Pacific Ocean

Notes from the lecture:

• Demo: When you chop/cut an apple without fully cutting it the left over part creates an arch
• There is no trench in the Juan de Fuca Plate
• two types
• Back arc basins formed by slab roll back
  • Something found all along the western side of the pacific ocean
  • You get this trench that is being overwritten by a subduction zone

Abyssal Plains

Presentation content:

• Flat features of the ocean floor
  • Likely the most level places on Earth
• Sites of thick accumulations of sediment
  • Fine sediments from turbidity currents
  • Minerals precipitated from seawater
  • Shells of marine plankton
• Found in all oceans
  • Most extensive in the Atlantic Ocean

Volcanic structures on Ocean Floor

Presentation content:

• Seamounts and volcanic islands

  • Submarine volcanoes are called seamounts
    • Over a million seamounts exist
    • Found in all ocean floors but most common in the Pacific
    • Many form near oceanic ridges or over a hot spot
  • A seamount may grow large enough to emerge as a volcanic island
    • Examples include Easter Island, Tahiti, Bora Bora, and the Galapagos Islands

• Guyots

  • Submerged, flat-topped seamounts
    • After the volcano is extinct, it eventually erodes to sea level where waves flatten the top of the structure
    • As plates carry the structure away, it eventually sinks into the ocean

• Oceanic plateaus

  • Vast outpourings of basaltic lavas on the ocean floor
  • Resemble continental flood basalts

Notes from the lecture:

• Hawaii is one of them

Explaining Coral Atolls - Darwin's Hypothesis

Presentation content:

• Atolls
  • Ring-shaped structures that extend from slightly above sea level to depths of several thousand meters
  • Charles Darwin was one of the first to formulate a hypothesis on the origin of ringed-shaped atolls
  • Darwin’s hypothesis is that, in addition to being lowered by erosional forces, many volcanic islands gradually sink.
    • later became a theory

Notes from the lecture:

• In recent years, we know more about them
• What the volcanics are doing is not closely related to what the atoll is doing/looks like

Shallow seas on the continents

Notes from the lecture:

• Most of North America was underwater
• 450 Ma, North American Great Ordovician carbonate bank covered by extensive carbonate platform, passive margin on west and south, collision to east
• Cretaceous 85 Million years ago
  • Between the Artic ocean and the gulf of Mexico

Formation of Rift and Passive Margins

Notes from the lecture:

• Restricted basin (salt/source rock deposition) sediments cover rift faults
  • Example: Brazil
• Very important for oil and gas
• Tension Normal faults

Rift formation

Notes from the lecture:

• Triple junction
• Two sides spread, one side fails to spread or spreads more slowly "aulocogen"
• Example failed rift in North America, late Precambrian 1 billion years ago
  • Exposed in norther MI Grav-mag anomaly elsewhere
• There is also a rift in Texas/Oklahoma

What causes spreading?

Notes from the lecture:

• Mantle plumes, associated flood basalts
  • Massive outpourings of basalt

13.4 The Ocean Ridge System

Summarize the basic characteristics of oceanic ridges.

Mid-ocean ridges

Presentation content:

• An oceanic ridge, or mid-ocean ridge, or rise is a broad, linear swell along a divergent plate boundary
  • The longest topographic feature on Earth
  • Width varies from 1000 to 4000 km
  • Occupy elevated positions
  • Segments are offset by transform faults

Notes from the lecture:

• Mid ocean ridges are high because of heat

Seafloor Spreading

Presentation content:

• This concept was formulated in the early 1960s by Harry Hess
• Seafloor spreading occurs along the crests of oceanic ridges
  • Newly formed melt (from decompression melting of the mantle) slowly rises toward the surface
    • Consistent chemical composition
  • Most melt solidifies in the lower crust, but some escapes to the sea floor and erupts as lava

Ocean Ridge Topography

Presentation content:

• Newly created lithosphere is hot and less dense than surrounding rocks
• As the newly formed crust moves away from the spreading center, it cools and increases in density
• Oceanic ridges with slow spreading rates have well-developed rift valleys and rugged topography
• Oceanic ridges with intermediate spreading rates have subdued rift valleys and topography
• Oceanic ridges with fast spreading rates generally do not have a rift valley and have a shallow profile

Oceanic crust composition

Notes from the lecture:

• Based on pieces of ocean floor thrusted into continent in CA, Oman, Cyprus
• We can see that at the very top we have sediment
• In order:
  • Sediment
  • Multiple lava flows, water circulation alteration
  • Dikes feeding volcanics to sea floor
  • Solidifying magma chamber
    • Coarse grained/slow cooling

13.5 The Nature of Oceanic Crust

List the four layers of oceanic crust and explain how oceanic crust forms and how it differs from continental crust.

Oceanic Crust in Four Layers

Presentation content:

• The sequence of four layers composing the oceanic crust is called an ophiolite complex
  • Layer 1: consists of deep sea sediments and sedimentary rocks
  • Layer 2: consists of pillow lavas
  • Layer 3: consists of numerous interconnected dikes called a sheeted dike complex
  • Layer 4: consists of gabbro

How Does Oceanic Crust Form?

Presentation content:

• Basaltic magma originates from partially melted mantle peridotite

• The magma rises through the upper mantle in tiny cracks until it reaches a lens-shaped magma chamber beneath the ridge crest

• As the pressure in the chamber increases, the rock about the chamber periodically fractures

• Magma ascends through these fractures, cools, and solidifies to form a sheeted dike complex

• 10–20 percent of the magma reaches the seafloor, where it quickly solidifies, forming large tube-shaped protuberances known as pillow basalts

Interactions Between Seawater and Oceanic Crust

Presentation content:

• Permeable and highly fractured crust allows seawater to penetrate the crust by 2–3 km
• Seawater is heated as it circulates through the crust, altering the basalt by hydrothermal metamorphism
• Hot groundwater dissolves ions of various metals from the rock and precipitates them on the seafloor as particle-filled clouds called black smokers
  • Thermal springs known as black smokers are often associated with oceanic ridges, where hot water containing dissolved minerals gushes from the seafloor.

Notes from the lecture:

• Crust allows water to penetrate
• Changes from being a basalt to a serpentine

13.6 Continental Rifting: The Birth of a New Ocean Basin

Outline the steps by which continental rifting results in the formation of new ocean basins.

Evolution of an Ocean Basin

Presentation content:

• A new ocean basin begins with the formation of a continental rift
  • (an elongated depression where the lithosphere is stretched and thinned)
• When the lithosphere is thick and cold, rifts are narrow
  • Examples include the East African Rift, the Rio Grande Rift, the Baikal Rift, and the Rhine Valley
• When the lithosphere is thin and hot, the rift can be very wide
  • Examples include the Basin and Range in the western United States

Examples

Presentation content:

• East African Rift
  • Continental rift extending through eastern Africa
  • Consists of several interconnected rift valleys
  • Normal faulting led to grabens (down-faulted blocks)
  • Area has expensive basaltic flows and volcanic cones
• Red Sea
  • Formed when the Arabian Peninsula rifted from Africa beginning about 30 million years ago
  • Fault scarps surrounding the Red Sea are similar to structures seen in the East African Rift
  • If spreading continues, the Red Sea will grow wider and develop an elongated mid-ocean ridge
• Atlantic Ocean
  • After tens of millions of years, the Red Sea will develop into a feature similar to the Atlantic Ocean
  • As new oceanic crust was added to the diverging plates, the rifted margins moved further from the region of upwelling
  • These margins cooled and subsided below sea level
    • Eventually become passive continental margins

Failed rifting

Presentation content:

• A failed rift valley extends from Lake Superior into Kansas
• Formerly active rift valley is filled with basalt and clastic sedimentary rocks
• Why rifts fail or succeed is not fully understood

Mechanisms for Continental Rifting

Presentation content:

• The supercontinent cycle is the formation and dispersal of supercontinents

  • At least two supercontinents have existed in the geologic past
    • Pangaea—most recent
    • Rodinia
  • Involves major changes in the direction and nature of the forces that drive plate motion

• Mantle plumes and hot spots

  • Regions of hotter than normal mantle rise, experience decompression melting, create basalts that triggers hot-spot volcanism on the surface

    • Mantle plumes concentrate under the thick continental crust, which traps heat in the mantle
    • Hot mantle plumes eventually cause the overlying crust to dome and weaken
    • Decompression melting can trigger hot spot volcanism
      • Flood basalts can precede a rifting event

  • Doming of the crust can produce three rifts that join in the area above the rising mantle plume called a triple junction

    • Continental rift usually occurs along two of the arms
      • The third arm becomes a failed rift

  • Mantle plumes do not always lead to rifting

    • Example: Columbia River Basalts in the Pacific Northwest

• Role of tensional stress

  • When the crust is thin and hot, small stresses are sufficient to initiate spreading
    • Example: Basin and Range region
  • Slab pull from subducting plates can create sufficient tensional stress to initiate rifting

13.7 Destruction of Oceanic Lithosphere

Compare and contrast spontaneous subduction and forced subduction.

Why Oceanic Lithosphere Subducts

Presentation content:

• Subduction is complex
• Fate of oceanic crust is still debated
  • Pile up at the boundary between the upper and lower mantle
  • Subduct to the core–mantle boundary
• Overall density must be greater than underlying asthenosphere in order to undergo subduction

Notes from the lecture:

• Movement of the oceanic crust is Largely based on density.

• The Angle of Plate Subduction Depends on its Density

Spontaneous & Forced subduction

Presentation content:

• Spontaneous subduction
  • Very old, thick, dense lithosphere sinks to the mantle by its own weight
  • Results in descending angles of nearly 90 degrees
    • Example: Mariana trench
  • Lithospheric mantle is what drives subduction
• Forced subduction
  • Younger, less dense lithosphere is forced beneath the overlying plate by compressional forces
    • Results in frequent earthquakes
    • Can fold and thicken upper plate
  • Descends at shallow angles
    • Example: Peru–Chile trench

Notes from the lecture:

• Spontaneous subduction: occurring at a deeper angle
• Forced subduction: occurring at a more shallower angle but a lot of compressional force
  • It has a lot more of deformation
  • Crust is a little bit thiccer

Subducting Plates: the Demise of Ocean Basins

Presentation content:

• If a plate subducts faster than it is produced at a spreading center, the plate will get smaller until it completely subducts
  • Example: Farallon Plate and replacement of subduction zone with San Andreas transform boundary

End of Chapter 13 - Concept Checks

13.1 An Emerging Picture of the Ocean Floor

1. Define bathymetry.
2. Describe how satellites orbiting Earth can determine features on the seafloor
   without being able to directly observe them beneath several kilometers of
   seawater.
3. List the three major provinces of the ocean floor.
   13.2 Continental Margins
4. 2. List the three major features of a passive continental margin.
      Describe the differences between active and passive continental margins.
      Where is each type found?
5. How are active continental margins related to plate tectonics?
   13.3 Features of Deep-Ocean Basins
6. 2. Explain how deep-ocean trenches are related to convergent plate boundaries.
      Why are abyssal plains more extensive on the floor of the Atlantic than on the
      floor of the Pacific?
7. 4. How does a flat-topped seamount, called a guyot, form?
      Using Darwin’s hypothesis, place these coral reefs in order from youngest to
      oldest: barrier reef, atoll, and fringing reef.
      13.4 The Oceanic Ridge System
8. Briefly describe oceanic ridges.
9. What is the primary reason for the elevated position of the oceanic ridge
   system?
10. Compare a slow spreading center such as the Mid-Atlantic Ridge with one that
    exhibits a faster spreading rate, such as the East Pacific Rise.

13.5 The Nature of Oceanic Crust

1. 2. 3. Briefly describe the four layers of the ocean crust.
         How does a sheeted dike complex form?
         How does hydrothermal metamorphism alter the basaltic rocks that make up the
         seafloor? How is seawater changed during this process?
2. What is a black smoker?
   13.6 Continental Rifting: The Birth of a New Ocean Basin
3. 2. 3. Name a modern example of a continental rift.
         Briefly describe each of the four stages in the evolution of an ocean basin.
         What role do hot spots and mantle plumes play in the breakup of a supercontinent?
         13.7 Destruction of Oceanic Lithosphere
4. Compare spontaneous subduction and forced subduction. Provide examples of places
   where each operates.
5. 3. What role do mineral phase changes play in plate subduction?
      Explain what happened when the spreading center that generated the Farallon plate
      collided with the North American plate.