The Sea Floor
Chapter 2
The Water Planet
Habitats are shaped by geological processes
o
Form of coastlines
o Depth
o Type of bottom – sandy, muddy, rocky
Case in Point
Satellite images of Aceh Province, Indonesia before (left) and after (right) the
December 2004 tsunami destroyed coastal villages. Note areas denuded of
vegetation along the coast.
The Water Planet
Earth has lots of liquid water
Most other planets have little water
Oceans play major role in weather
Without water life would not be possible
71% of the earth is covered by water
2/3 of the land on earth is found in the Northern
Hemisphere
5 Major Ocean Basins
Pacific – deepest and largest (as big as the others
combined)
Atlantic – 2nd Largest
Indian - similar depths to Atlantic
Arctic – smallest, shallowest
Southern – 4th Largest – not universally recognize
as an ocean
The World’s Oceans
World Ocean
The oceans are interconnected – not separate
The connections allow sea water, materials, and
some organisms to move from one to another
Big Bang
Earth is about 4.6 billion years old
Big Bang Theory - 15 billion years ago
Materials settled according to their density
Density
Density is mass of a given volume
Which has more volume, a pound of lead or a
pound of feathers?
Density
Denser materials tend to sink
Densest material flowed to the center of the
earth
Lighter material floated on the surface.
Light material cooled and formed a thin crust
Core
Innermost layer – core
o Mostly iron alloys
o Solid inner, Liquid outer core
O High pressure
o 4000 degrees Celsius
Mantle – outside core
* solid
very
hot
flows like liquid, but much slower
Crust
Outermost
Very
thin
Like rigid skin floating on the mantle
Minerals
Oceanic crust – minerals called basalt
Continental – granite
Oceanic crust is denser
Oldest oceanic rocks are less than 200
million years old
* Continental rocks – up to 3.8 billion years
Continental Drift
1620
Francis Bacon
Noted the continents fit together like pieces of a puzzle
Coal deposits and other geological formations match up
on opposite sides of the Atlantic
Fossil evidence also supports this idea
Continental Drift
Alfred
Wegener – proposed “continental drift”
proposed that all the continents were once joined in a
single “supercontinent”
Pangea
Started breaking up 180 million years ago
Plate Tectonics
What causes the continents to drift?
A process called plate tectonics
Mid Ocean Ridge
Sonar surveys revealed a mid ocean ridge system
Continuous
chain of volcanic submarine mountains that
circle the earth
Largest geological system on the planet
At regular intervals – transform faults
Sometimes the ridges are so high they break the surface
like iceland
Mid-Atlantic Ridge – runs down the middle of the
Atlantic Ocean
Ridges
Ridges & Trenches
Great
deal of activity around the ridges
Earthquakes around the ridges
Volcanoes at the trenches
Scientists
found that the farther you get from the
ridge, the thicker the sediments get and the older
the rocks
Earth’s Magnetic Field
The
earth’s magnetic field reverses a few times
every million years
During the shift, a compass would point to the
magnetic “north” – that is what is now the south
pole.
Cause of the reversals is unknown, but thought to
be related to changes in movement of material in
the outer core
Magnetic Anomolies
Many rocks are magnetic. If they are in the molten outer
core, they are free to point to magnetic north
When the rocks cool, they stay in the same position
Geologists found a pattern of magnetic stripes or bands in
the mid ocean ridge system.
Symmetrical around the ridge – mirror images
The magnetic bands are called “magnetic anomalies”
A – 5 mya B – 3 mya C- present day
What did that show?
Shows
the sea floor was not formed all at once, but
in strips along the mid-ocean ridge
Creation of the Sea Floor
Magnetic anomalies together with other evidence led to
the understanding of plate tectonics
At the ridges, large pieces are separating
As the pieces separate, they form a crack or “rift”
This releases pressure and some material rises through
the rift
The ascending material pushes up and forms a ridge
The process is called sea-floor spreading
Sea Floor Spreading and Plate
Tectonics
Lithosphere
Broken
– “Rock Sphere”
into plates called lithospheric plates
Plates
The plates are about 100 km thick
Plates contain oceanic crust, continental crust or
both
The Mid-Ocean Ridges form the edges of many of
the plates
How Fast Do They Move?
The plates move at between 2 and 18 cm per year
Human fingernails grow at 6 cm per year
Subduction
As lithosphere is created at the ridges, it must be
destroyed somewhere else
It is destroyed at the trenches
Trenches are formed when 2 plates collide and
one sinks below the other
The downward movement is called subduction
Trenches are sometimes called “subduction
zones”
???
Oceanic plate always descends into the
mantle when colliding with a continental
plate. Why?
Answer
Because the continental plate is less dense
A Record in the Marine Sediments
2 major types of sediments are found in the sea.
Lithogenous sediment
- Physical and chemical breakdown or
weathering of rock
Biogenous Sediment
Skeletons and shells of marine organisms
Calcium carbonate – calcareous ooze
Silica – siliceous ooze
A Record in the Marine Sediments
Most sediment is in the form of microfossils
Tell what kind of organisms lived there in the past
Clues to environment – cold, warm for example
Guyot
Shelf Break
Deep Ocean Floor
plain – flat, 3000 to 5000 feet deep
Dotted with seamounts – volcanoes
Flat topped seamounts – guyots
Hydrothermal vents – areas where heated seawater
are forced up through the crust
Abyssal
Hydrothermal Vents
Water
is up to 350º C (660º F)
Mainly sulfides are dissolved in the water and form
black smokers
Black smokers are chimney like structures that build up
as minerals solidify
Rich marine life
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