Transcript The Sea Floor

The Sea Floor
Figure 2.01
The Ocean Basins
• Distribution of Oceans: 61% of the Northern
Hemisphere is ocean while 80% of the Southern
Hemisphere is ocean.
• Four large basins exist: all are connected allowing
seawater, materials and organisms to move from one
ocean to another – sometimes called the world ocean.
• Pacific – deepest and largest ocean
• Atlantic – larger than Indian, but equal in depth
• Indian
• Arctic – smallest and shallowest
Figure 2.02
The Southern Ocean refers to the
body of water that surrounds Antarctica.
Figure 2.03
Review of the Structure of the Earth
• Core – innermost layer, composed mostly of iron.
Pressure is 106 times that of the surface. Temperature
is over 4000 °Celsius. Swirling motions of the outer
core produce earth’s magnetic field.
• Mantle – layer just beyond the core, thought to be
solid and near the melting point of rock. It slowly
flows like a liquid.
• Crust – outermost layer, extremely thin - a rigid skin
floating on the mantle
Figure 2.04
The land masses on
opposite side
of the Atlantic have
coastlines
and geological features
that fit
together like pieces of a
puzzle.
Theory of Plate Tectonics
• Alfred Wegner, a German geophysicist, first proposed
the theory of continental drift in 1912
• He suggested that all of the continents had once been
joined in a supercontinent called Pangaea.
• Pangaea began to break up into our current continents
about 180 million years ago.
Oceanic Crust - (Basalt)
Continental Crust (Granite)
Density 3.0 g/cc
Density 2.7 g/cc
5 km thick
Geologically young
<200 million years old
20 to 50 km thick
Can be 3.8 billion years
Dark in color
Rich in Fe and Mg
Light in color
Rich in Na, K, Ca and Al
Figure 2.05
Major features of the sea floor.
SEA FLOOR FEATURES
• Mid-Ocean Ridge – a continuous chain of submarine
volcanic mountains that encircle our planet.
• Sea-floor rock near the ridge is young. Rock gets older as one
moves away from center.
• Bands of magnetic stripes that run parallel to the mid-ocean
ridge represent zones in which the rocks on the sea floor
alternate between normal and reversed magnetization.
• Bands are symmetric on either side of ridge.
• Magnetic anomalies is name given to these stripes.
• Importance: sea floor must have cooled from molten
material at different times.
Figure 2.09
• Sea-floor spreading is the name given to the
process during which the sea floor moves away
from the mid-ocean ridges to create new sea floor.
•
Mid-ocean ridge forms the edges of many tectonic
plates
1. It is a continuous chain of submarine volcanic
mountains that encircles the globe, the largest
geological feature on earth.
2. Mid-Atlantic Ridge runs down the center of the
Atlantic Ocean
• How does sea-floor spreading work?
As pieces of oceanic crust separate at the mid-ocean
ridges, they create a “rift”. This releases some
pressure on the mantle and causes it to melt.
The liquid mantle material rises up through the rift.
Ascending mantle material pushes up the oceanic crust
around the rift to form the mid-ocean ridge.
When this molten material reaches the surface, it cools
and solidifies to form new oceanic crust.
Figure 2.07
Distribution of earthquakes and
volcanoes.
Figure 2.06
Magnetic Reversals
1. Magnetic reversals occur about every 700,000 years
2. It takes about 5000 years for the field to reverse.
3. Cause is thought to be the movement of material in
earth’s molten outer core.
4. When rocks cool, the magnetic particles within them
align themselves with the current pole.
5. Stripes appear that are symmetrical around the ridge.
6. Called magnetic anomalies – showed that the sea
floor cooled at different times
Figure 2.08
Normal (dark) and
reversed (light)
magnetism.
stripes form bands
running parallel to
the mid-ocean ridge.
Figure 2.10
Figure 2.11
Formation of a trench by the collision of an oceanic and
continental plate.
Earthquakes are produced as the Nazca Plate descends
into the mantle. Lighter material from the plate rises as it
melts to create the Andes Mountains.
Figure 2.12
Here two oceanic plates meet and form another type of
trench. Earthquakes are produced by the descending plate
and the volcanoes have produced the Aleutian Islands.
Facts about Trenches
1. They are curved because they follow the
curvature of the earth.
2. Island arcs (Aleutian and Mariana) which
are really volcanic chains also look curved.
When two continental plates collide…..
Both are light in density and they don’t subduct, but buckle
and produce mountain chains, e.g. Himalayas formed
when India collided with Asia.
Figure 2.13
Mt. Veniaminof, an active volcano on the
Alaska Peninsula, part of the Aleutian Island
chain that formed from behind the Aleutian
Trench.
Figure 2.15
Current theory says that plates move mainly because of slab
pull, ( old, cold and dense lithosphere sinks into the mantle
and pulls the rest of the plate behind it.)
Traditional theory said that the movement
of plates is driven by large scale convection currents
in the asthenosphere and lower mantle which are driven
in turn by heat from the earth’s core.
Even though “slab pull” theory is currently accepted
Convection may still play a role in plate motion.
http://www.youtube.com/watch?v=p0dWF_3PYh4
Figure 2.16
Marine Sediments
Lithogenous sediment, derived from the physical and chemical
breakdown of rocks which are found mostly on continents.
Biogenous Sediment, consisting of skeletons and shells of
marine organisms.
1. calcareous ooze – sediment composed of CaCO3
2. siliceous ooze – made of SiO2
Microfossils are important because they tell us what organisms
lived in the ocean in the past and give clues to ocean temperatures.
Ocean temperatures are dependent upon the earth’s climate and
ocean currents.
Figure 2.17
Fossil Shell of a foraminiferan.
Climate of the earth
1. Can be determined by C-14 dating
2. Water temperature is found by the ratio of Mg to Ca
or of different isotopes of O in microfossils.
3. The ratio of Sr to Ca in coral skeletons records past
ocean temperatures.
4. Ice cores from Greenland and Antartica preserve
records of past temperatures as well as samples of our
ancient atmosphere from tiny air bubbles trapped in
the ice.
Figure 2.18
History of the earth’s climate over the past half-million years as
Determined by foraminiferan microfossils.
Red line = average sea surface temps. (Mg to Ca ratios)
Blue and white bands = major glacial periods (oxygen isotopes)
Figure 2.19
• Continental Margins
-boundaries between continental crust and oceanic crust
-sediment from the continents settles to the bottom after
reaching the sea and accumulates on the margins
-sediment may be 6 miles thick!
Continental margins consist of a shallow, gently sloping
shelf , a steeper slope and a gently sloping region –
the continental rise at the base of the slope.
- Varies from 1 km on the Pacific coast of S.A. to 750
km on the Arctic coast of Siberia.
- Ends in the “shelf break” where it abruptly gets
steeper.
Continental Slope
- This is the closest thing to the edge of the continent,
beginning at the shelf break and descending
downward to the deep sea floor.
• Continental Rise
- A thick layer of sediment piled up on the sea floor.
- The “deep sea fan” is like a river delta and is caused
by sediment moving down a submarine canyon and
accumulating at the canyon’s base.
Continental Shelf
• This is the shallowest part of the margin, making up
8% of the ocean’s surface and is the richest part of the
ocean!
• It is composed of the continental crust and is really
part of the continent that’s under water.
• Submarine canyons are canyons formed by the
erosion by rivers and glaciers that are now under
water.
Figure 2.22
Active and Passive Margins
Active margins are trenches with intense geological
activity, such as earthquakes and volcanoes.
Characterized by rocky shorelines, narrow continental
shelves and steep continental slopes and lacking a
well developed continental rise. ( the west coast of
N.A.)
Passive margins have flat coastal plains, wide shelves
and gradual continental slopes. Sediment accumulates
at the base of the slope and they have a thick
continental rise.
Figure 2.20
Figure 2.24
Figure 2.23
• Deep-Ocean Basins
The sea floor is almost flat – the abyssal plain.
It is dotted with submarine volcanoes called seamounts
and volcanic islands.
Guyots are flat topped seamounts and are common in the
Pacific. The abyssal plain also has plateaus, rises and
other features.
The Central Rift Valley is the depression at the center of
the mid-ocean ridge. It is extremely hot and dotted by
hydrothermal vents or deep-sea hot springs.
Figure 2.25
• Hot water dissolves a variety of minerals mostly
sulfides.
• Black Smokers are one type of mineral deposit found
at hydrothermal vents – chimney like structures that
build up around a vent as minerals solidify.
Figure 2.21
Figure 2.26
Figure 2.27
Text Art 2.01
Text Art 2.02
TABLES
Table 2.01
Table 2.02