Transcript The Ocean Floor and the coast

The Ocean Floor and
the coast
Section 1: Plate tectonics and the
ocean basin
The Ocean and Seas
• Oceans cover 71% of earth’s
surface
– Southern hemisphere = 80%
– Northern hemisphere = 61%
• 84% deeper than 2000m
• Greatest depth ~ 11,000 m
in Marianas Trench
• Seas (marginal seas)
– Close to coasts with limited
connection to ocean
– Found mostly in Atlantic ocean
– Examples: Gulf of Mexico,
Mediterranean Sea
Ocean zones and features
• Based upon depth and habitat
• Intertidal zone (also known as the littoral
zone): covers area of the coast from high
water mark to lowest part of the shore
permanently submerged (think high and low
tide)
• Continental shelf: area between high water
mark and edge of continental shelf
(approximately 10 km-300 km from shore)
• Continental slope: steep slope beginning at
the edge of the continental shelf
• Pelagic zone: open ocean
– Epipelagic: top 200 m
– Mesopelagic: 200-1,000 m
– Bathypelagic: 1,000-4,000 m
– Abyssopelagic: 4,000-6,000 m
• Benthic zone: seafloor
FIG. 1.13 A cross section of the ocean from the shoreline to the deep sea, showing the
location of major marine habitats.
• Deep-sea features:
– Submarine canyons: steep sided valley
located in the continental slope
– Trench: deep depression in the ocean floor
sometimes found near volcanic islands
• Deep-sea features:
– Mid-Ocean ridge: underwater mountain
ranges
– Seamount: steep-sided formation
• Seamount with a flat top worn down by wave action =
guyot
– Abyssal plains: flat bottom of ocean floor
Another view
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Continental shelf (1° slope)
Continental slope (2.9° slope)
Continental Rise
Abyssal Plain
Submarine Canyons
Topographical features
(w/ measurements)
FIG. 2.2 Two examples of continental margins showing various topographic features and also topographic features of
the deep-sea floor (vertical slope is exaggerated).
How do we know the depth of these features?
Brainstorm
• Where did all of these features come
from?
• Which regions of the world have more
ridges, trenches and canyons?
Theory of Plate tectonics
• Earth’s surface is divided into large sections
called plates.
• Plates are bordered by ridge systems,
trenches and faults.
• Fault – break in Earth’s crust
• Movement of plates causes underwater
features and natural disasters (earthquakes,
tsunamis)
FIG. 2.3 The ocean floor showing plate boundaries, oceanic ridges, where new oceanic crust is created by volcanism (red lines with thin arrows), fault and
fracture zones (red lines without thin arrows), and trench zones (thick, dark blue bands).
Map of earth with features
Why are the plates moving?
Plate boundaries
• Convergent
– Two tectonic plates collide
– Plates can buckle, shift
upward or bend
• Divergent
– Two tectonic plates move
away from each other.
– Magma rises to surface
• Transform
– Two tectonic plates slide
past each other
Continental Drift Theory
• 1912: Alfred Wegener first to suggest theory
– One land mass (Pangea) slowly separated
• 1960’s: theory revised and accepted
– Outer crust of Earth (lithosphere) floats on top of a
layer called the asthenosphere
– Lithosphere contains dense ocean crust and lighter
continental crust
– Continents move as ocean floor moves
Why do we have different plate
movement?
Evidence to support
continental drift theory
1. Coastlines of present
day continents “fit”
together
2. Distribution of fossil
species
3. Magnetic stripes in
sea floor
1. Coastlines of present day
continents “fit” together
2. Distribution of fossil species
• Fossils from same
or similar species
found on continents
separated by
oceans today.
3. Magnetic stripes in sea floor
• Evidence found at
divergent plate
boundaries
• As divergent
plates move
apart, new
oceanic crust
forms as magma
rises to the
surface of the
lithosphere.
Seafloor spreading
• Minerals in the
magma crystallize in
direction of earth’s
magnetic field
Ocean floor and land features
• What are the visible effects of tectonic
plate movement?
– Trenches, mid-ocean ridges, volcanoes,
hydrothermal vents, earthquakes & tsunamis
Trenches
• Formed by subduction
• Subduction: dragging
of oceanic crust
downward causing
melting into mantle.
• Occurs at convergent
plates
• Challenger Deep is
approximately 11,020
m deep and found in
Pacific ocean
Mid-ocean ridges
• Formed at divergent
plates
• Magma rises slowly
forming a series of
small “mountains” as it
cools
Volcanoes
• Formed by opening in Earth’s crust, which
then allows magma and gases to escape
• How do they form?
– Thinning crust at divergent plates (mostly under
sea)
– Convergent plates (mostly land)
Volcanoes
• Effects of volcanic activity on seafloor
– Formation of new seafloor
– Formation of islands that rise above sea-level
– Release of minerals and gases for use by
deep-sea organisms
• Ring of fire – area with most
volcanic activity in world
Hydrothermal vents
• Openings that look like chimneys found near
mid-ocean ridges in deep ocean (typically
around 2,000m depth)
• Cold sea water seeps into cracks in ocean floor
near divergent zones, is heated by magma,
returns to the surface and is released through
vents
Hydrothermal vents
• The depth creates high
pressure, which can heat water
up to 340oC
• Minerals dissolve in heated water
coming out of hydrothermal vents:
– Contain iron, copper, zinc and sulfur
– The minerals precipitate out of
solution as the water cools
(sometimes adding to the vents
structure)
– Environment provides energy source
for chemosynthetic organisms
Earthquake
• Caused by a release of large
amounts of energy from the
earth’s crust.
– Convergent plates and
transform plates may be unable
to slide past each other.
– Strain on plates builds up
pressure and energy
– Sudden movement of these
plates produces burst of energy
• What can underwater
earthquakes cause?
Tsunami
• Large wave traveling at high speed
• Caused by earthquakes from
converging and transform plates on
seafloor.
• In open sea the wave produced is
long but short in depth
• As the wave approaches shallow
water, the land slows the wave but
causes water to pile up and form
taller waves
Review of underwater features
Brainstorm
• We have discussed plate tectonics in
relationship to the deep-sea and its
features. Let’s watch a review video and
then answer these questions.
– What affect do moving plates have on the
continental shelf?
– What happens to moving continental shelves?
Continental shelves
• Many continental shelves were above sea level
during ice ages
– Sea level was lower (due to ice sheets)
– Weathering caused changes in formation of land
• Erosion, sediment deposited by rivers, waves
• Melting glaciers and sea level rising covered
continental shelves.
• Depth of seawater over continental shelf relies
upon isostasy and sea level
Isostasy produces shallow seas in
continental shelves
• Isostasy = ‘weighing the same’
• Layers of the earth’s crust float on each
other according to density.
• Which layer is thicker: continental or oceanic
crust? Which is more dense?
Isostasy and Collisions
• Continents can collide
with each other when
two plates with
continental crust
converge.
• Causes mountain
ranges
• Why does oceanic crust
sink below the
continental crust?
Shallow seas of Continental
shelf
• How does isostasy govern the depth of
seawater over a continental shelf?