Marine Science
Chapter 2 – The Sea Floor
FOCUS QUESTIONS
I. What are the major physical
features of the Earth’s surface?
II. What explanation have scientists
developed for the existence and
distribution of these features?
III. How do these features affect the
Earth’s oceans?
The Water Planet
• 71% of earth covered in ocean
• Oceans regulate climate and atmosphere
• Four large basins:
–
–
–
–
–
Pacific – largest and deepest
Atlantic
Indian
Arctic – smallest and shallowest
(Southern ocean – continuous body of water
surrounding Antarctica)
• Large basins connected by smaller, shallow
seas
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
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
• 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
See Table 2.2 on page 24
… than Oceanic crust.
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.
• Hypothesis not widely accepted
• http://www.wiley.com/college/strahler/0471
480533/animations/ch13_animations/animat
ion3.html
• http://education.sdsc.edu/optiputer/flash/pan
gea_4.htm
• http://vimeo.com/14258924
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
• Plates
“float” on the asthenosphere.
There are areas where
plates spread apart…
Mid-ocean Ridges/ Rifts
…and areas where
plates collide.
Subduction zones/ Trenches
Types of Plate Boundaries
http://pubs.usgs.gov/publications/text/Vigil.html
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
• 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
• 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
• 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
and Transform Faults
• 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
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
• Hydrothermal vents – deep water hotsprings that dissolve minerals in the Earth’s
crust (mainly sulfides)
• Hydrothermal Vents:
http://oceanexplorer.noaa.gov/edu/learning/player/lesson05.html
• Black smokers – chimney-like structures
that build up deposits of minerals and
“smoke” a cloud of mineral particles.
Major Features of the Sea Floor
World Distribution of Volcanoes &
Earthquakes
Seafloor Topography
• Continental shelf
• Continental edge /
Shelf break
• Continental slope
• Abyssal plain
• Ocean ridge and
rise systems
• Trenches
• Seamount
Global Seafloor Topography
Seafloor Topography
Geological Provinces of the Ocean Floor
• Based on previous knowledge and logic, label the
following parts of the map:
–
–
–
–
–
–
–
–
Abyssal plain
Continental rise
Continental shelf
Continental slope
Guyot
Seamount
Shelf break
Trench
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.
E. Abyssal Plain – The nearly flat region of the deep-sea floor.
F. Guyot – A flat topped- seamount
G. Trench – A narrow deep depression in the sea floor.
H. Seamount – A submarine volcano in the abyssal plain
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
Comparative Fossil/Sediment Data
Geologists noted the similarities between the fossils and
sediment deposits found on opposite sides of the Atlantic Ocean.
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
• 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 Anomalies
• Geologists have
thus discovered a
pattern of
magnetic banding
parallel to the
mid-ocean ridge.
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
Although this model has been
contested, it describes a stationary
magma source over which plates slide.
References
• 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
http://pubs.usgs.gov/publications/text/understanding.html
• Watson, J.M. (1999) “Understanding plate motions.”
USGS. Retrieved on September 27, 2004 from
http://pubs.usgs.gov/publications/text/hotspots.html
• http://www.ngdc.noaa.gov/mgg/fliers/97mgg03.html
Global projection of Earth showing tectonic
boundaries and areas of active seismic and vulcan
activity.
2002-2003 Annual Seismic
Activity at Mt. St. Helens, WA
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.
Further Evidence of Earth’s
Changing Nature
• Sediments
• Sea Level Changes
• Greenhouse Effect
Origin of the Atmosphere
• Originally free oxygen not present in
the atmosphere.
• 600 mya – O2 produced by singlecelled photosynthetic organisms
(cyanobacteria)
• Stromatolites
Photo credit
http://www.discoverwest.com.au/australiaimages/l741b.jpg
Stromatolites – Shark Bay, WA
Photo credits
http://www.ldeo.columbia.edu/~small/personal/Images/Stromatolites.jpg
http://www.discoverwest.com.au/album/northwest/north_west_map.gif
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!
Sediments
• 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
• Calcareous ooze – skeletons composed of
calcium carbonate (CaCO3)
• Siliceous ooze – skeletons composed of
silica (SiO2).
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
• 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.