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Marine Science. Chapter 2 – The Sea Floor. FOCUS QUESTIONS. What are the major physical features of the Earth’s surface? What explanation have scientists developed for the existence and distribution of these features? How do these features affect the Earth’s oceans?. Our Fledgling Planet.

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Marine science

Marine Science

Chapter 2 – The Sea Floor

Focus questions

  • What are the major physical features of the Earth’s surface?

  • What explanation have scientists developed for the existence and distribution of these features?

  • How do these features affect the Earth’s oceans?

Our fledgling planet
Our Fledgling Planet

  • Earth – approximately 4.5 billion years old

  • Formed from an aggregation of dust clouds and gas particles that condensed into solid matter.

  • Intense heating melted many of these heavier elements in the core and mantle of the planet.

  • The uppermost of layers cooled and formed the Earth’s crust.

The structure of the earth
The Structure of the Earth

Materials have settled in the planet according to their density.

d = m/V

Lithosphere (crust – very thin)

Asthenosphere (upper mantle – fluid)

Mesosphere (mid to lower mantle – solid but very hot)

Core (mixtures of iron – inner is solid, outer is molten)

All crust is not the same
All Crust is not the Same

  • Continental crust is composed of granite.

  • Oceanic crust is composed of basalt.

  • Continental crust is:

    • lighter

    • thicker (20-50 km vs. 5 km for ocean)

    • less dense

    • geologically older

      … than Oceanic crust.

See Table 2.2 on page 24

Hypothesis of continental drift
Hypothesis of Continental Drift

  • Developed by Alfred Wegener in 1915.

  • Proposed that present continents had drifted apart after the breakup of a single supercontinent, Pangaea.

  • Pangaea animation

Theory of plate tectonics
Theory of Plate Tectonics

Plate tectonics – the study of the processes by which the lithosphere moves laterally across the asthenosphere.

  • This hypothesis is supported by:

  • presence of the mid-ocean ridge

  • comparative fossil data from different continents

  • apparent puzzle-piece shape of different continents

  • magnetic reversal and sediment patterns on the sea floor

  • distribution of geological features on the Earth

Plate tectonics in action
Plate Tectonics in Action

  • Plates “float” on the asthenosphere.

There are areas where plates spread apart…

…and areas where plates collide.

Mid-ocean Ridges/ Rifts

Subduction zones/ Trenches

Types of plate boundaries
Types of Plate Boundaries

Oceanic continental boundaries
Oceanic-Continental Boundaries

  • Heavy oceanic crust slides under continent.

  • Called subduction.

  • Trench forms.

  • Causes explosive earthquakes.

  • Molten lithosphere seeps up through continental crust and forms volcanoes.

  • Oceanic Subduction Animation

Oceanic oceanic boundaries
Oceanic-Oceanic Boundaries

  • One plate will slide under the other.

  • Trench forms.

  • Slow-flowing volcanoes form

    • sometimes break the ocean surface (island arcs).

    • Examples:

      • Aleutian Islands, AK

      • Mariana Islands

Continental continental boundaries
Continental-Continental Boundaries

  • Two continents smash against one another with force and become “welded” together.

  • Crust buckles forming mountain ranges.

  • No trenches or volcanoes.

    India Collision Animation

Californians get closer together
Californians Get Closer Together

  • In addition the Pacific Plate (which contains the city of Los Angeles) and the North American Plate (which contains San Francisco) share a shear boundary.

  • These two plates slide past each other such that these cities get 4 inches closer each year.

The mid ocean ridge
The Mid-Ocean Ridge

  • A continuous chain of submarine volcanic mountains that circles the globe.

  • Displaced at regular intervals by transform faults.

  • Occasionally breaks the surface at places like Iceland and the Azores.

  • Examples:

    • Mid-Atlantic Ridge

    • East Pacific Rise

Above: Mid-Atlantic Ridge Left: Iceland –showing ridge

Marine science

Above: The Glomar Challenger was the first research vessel specifically designed in the late 1960s for the purpose of drilling into and taking core samples from the deep ocean floor.

Rift valleys
Rift Valleys

  • A gap or depression resulting in the center of a mid-ocean ridge or on land.

    • African Rift Valley

Features of mid ocean rifts
Features of Mid-Ocean Rifts

  • Hydrothermal vents – deep water hot-springs that dissolve minerals in the Earth’s crust (mainly sulfides)

  • Black smokers – chimney-like structures that build up deposits of minerals and “smoke” a cloud of mineral particles.

Comparative fossil sediment data
Comparative Fossil/Sediment Data

Geologists noted the similarities between the fossils and sediment deposits found on opposite sides of the Atlantic Ocean.

Magnetic reversals
Magnetic Reversals

  • Rocks contain magnetic particles that align with the Earth’s magnetic field.

  • During periods called magnetic reversals, the Earth’s north pole switches to the opposite side of the globe as it is now.

  • Believed to be related to movements in the Earth’s molten core.

Magnetic anomalies
Magnetic Anomalies

  • When molten rock comes to the surface, the magnetic particles are free to align with the Earth’s magnetic pole.

  • When the rock cools, the particles are fixed in the magnetic orientation at the time.

Magnetic anomalies1
Magnetic Anomalies

  • Geologists have thus discovered a pattern of magnetic banding parallel to the mid-ocean ridge.

Geological hot spots
Geological “Hot Spots”

  • Occur in over 100 places on the globe

  • Areas where hot magma sporadically forces its way through the lithosphere to erupt in volcanic activity.

  • Examples:

    • Hawaiian Islands

    • Yellowstone National Park

Hot spot formation
Hot Spot Formation

Although this model has been contested, it describes a stationary magma source over which plates slide.

Seafloor topography
Seafloor Topography

  • Continental shelf

  • Continental edge / Shelf break

  • Continental slope

  • Abyssal plain

  • Ocean ridge and rise systems

  • Trenches

  • Seamount

Geological provinces of the ocean floor
Geological Provinces of the Ocean Floor

A. Continental shelf – The shallow gently sloping section of the continental margin that extends from the shore to the point where the slope gets steeper.

B. Shelf break – The section of the continental shelf where the slope abruptly becomes steeper, usually at a depth of 120 to 200 m (400 to 600 ft).

C. Continental Slope – The steeper, seaward section of the continental margin.

D. Continental Rise –The gently sloping area at the base of the continental slope.

  • Abyssal Plain – The nearly flat region of the deep-sea floor.

  • Guyot – A flat topped- seamount

  • Trench – A narrow deep depression in the sea floor.

  • Seamount – A submarine volcano in the abyssal plain

Continental margins
Continental Margins

  • Active Margin – a continental margin that is colliding with another plate and as a result is geologically active

  • Passive Margin– a continental margin that is located at the trailing edge of a continent and as a result shows little geological activity

Further evidence of earth s changing nature
Further Evidence of Earth’s Changing Nature

  • Sediments

  • Sea Level Changes

  • Greenhouse Effect

Origin of the atmosphere
Origin of the Atmosphere

  • Originally free oxygen not present in the atmosphere.

  • 600 mya – O2produced by single-celled photosynthetic organisms (cyanobacteria)

  • Stromatolites

Photo credit

Stromatolites shark bay wa
Stromatolites – Shark Bay, WA

Photo credits

Results of an o 2 rich atmosphere
Results of an O2 Rich Atmosphere

  • Aerobic organisms began to out-compete anaerobes.

  • Excess oxygen begins to react in the high atmosphere with ultraviolet radiation. This creates ozone (O3)

    • prevents harmful UV rays from penetrating to the earth.

    • Allows organisms to colonize the land.

  • Also, Earth ideally settled in an orbit where molecular water can exist in all three states

    • particularly the liquid state!


  • Lithogenous sediment – results from the physical and chemical weathering of rocks on land.

    • Carried by run-off and rivers to the ocean.

  • Biogenous sediment – skeletons and shells of marine organisms that have fallen to the ocean floor.

    • Diatoms, radiolarians, foraminiferans, and coccolithophorids

Biogenous sediments
Biogenous sediments

  • Calcareous ooze – skeletons composed of calcium carbonate (CaCO3)

  • Siliceous ooze – skeletons composed of silica (SiO2).

Sea level change
Sea Level Change

  • Interglacials – warm periods during which freshwater melts and sea level rises.

  • Ice ages – cold periods when large amounts of water are stored in ice caps and glaciers and sea level drops.

Greenhouse effect
Greenhouse Effect

  • Global temperatures and the rate of glacial melting are rapidly increasing.

    • Still uncertainty as to whether this is a natural phenomenon or whether it has been accelerated by humans.


  • Castro, P. & J. Huber (2005) Marine Biology, 5th ed. McGraw-Hill Higher Education, Boston, MA.

  • Watson, J.M. (1999) “Understanding plate motions.” USGS. Retrieved on September 27, 2004 from

  • Watson, J.M. (1999) “Understanding plate motions.” USGS. Retrieved on September 27, 2004 from


Marine science
Global projection of Earth showing tectonic boundaries and areas of active seismic and vulcan activity.