The Sea Floor
Chapter 2
Continental Drift
• Theory proposed by Alfred Wegner (German
geophysicist),
• Stated that all the continents were joined
together at one time in to a supercontinent
called “Pangea.”
• Pangea began to break apart 180 million years
ago.
Pangea
Theory of Plate Tectonics
• Theory that provides the explanation for
continental drift.
• It is the process involved in the movement of
large plates on the earth’s crust.
Mid Ocean Ridge
• Discovered from sonar images after WWII.
• It is a continuous chain of volcanic mountains
that encircles the globe like a baseball’s
seams.
• It is the largest geological feature on Earth!!!
• The formation was made when transformation
faults were displaced by cracks in the Earth’s
crust.
The Sea Floor
• The sea floor is geologically distinct from
continental land masses.
• Geological processes sculpt the shoreline,
determine water depth, control if the sea
bottom is muddy, sandy, or rocky.
• Geological processes create new islands and
undersea mountains for colonization and
determine specific marine habitats.
Earth is a water planet.
• Oceans cover 71% of the globe.
• Oceans regulate Earth’s climate and
atmosphere.
• The Northern Hemisphere is 61% covered by
oceans.
• The Southern Hemisphere is 80% covered by
oceans.
Ocean Classification
• Oceans are divided into four basins.
1)
2)
3)
4)
Pacific Basin- deepest and largest
Atlantic Basin- second largest
Indian Basin- third largest
Arctic Basin- smallest and shallowest
Connected to basins are seas
.
• Examples of seas are: Mediterranean, Gulf of
Mexico, South China, Red, Dead, Baltic, etc.
• All oceans are interconnected and
compromise a single “world ocean” referred
to as Panthalassa.
• The Southern Ocean is a continuous body of
water that surrounds Antarctica.
Internal Earth Structure
The Core
• The innermost portion of Earth.
• The inner core is made of solid iron.
• The outer core is made of liquid iron that has
a swirling motion.
• This motion is responsible for Earth’s magnetic
field.
The Mantle
• Found outside the core.
• Most is solid and very hot, near the melting
point of most rocks.
• Some flows very slowly like a liquid.
The Crust
• Outermost layer made of thin, rigid rock that
floats on top of the mantle.
1) Continental Crust- mostly made of granite and is
light in color. It is less dense than oceanic crust
so it floats on the mantle. Approx. 3.8 million yr.
old.
2) Oceanic Crust- mostly made of basalt and is
denser and thinner than continental crust. It
does not float as high on the mantle. Aprox. 180
million years old.
Mid Ocean Ridge
Classification
• In the Atlantic- Called Mid Atlantic Ridge
• In the Pacific- Called East Pacific Rise
Trenches
• Deep depressions in the sea floor especially
common in the Pacific.
Significance of Mid Ocean Ridge
• Lead to the discovery that from time to time
the Earth’s magnetic field has reversed
direction.
• This happens about every 700,000 years.
• Reversal takes 5000 years.
• Reversal occurs from the movements of
materials in the Earth’s core.
Magnetic Anomalies
• Geologists found striped patterns of magnetic
bands in the sea floor.
• Show alternate periods of normal and
reversed periods of earth’s magnetic field.
The Sea Floor
• Created by oceanic crust separated at the midocean ridge allowing some mantle to rise
through the rift.
• When the mantle rises through the rift it cools
and solidifies to form new oceanic crust.
Sea Floor Spreading
• Process of making new oceanic crust.
• What do you remember about the oceanic
crust?
Lithosphere
• Litho = “Rock”
• “Rock sphere”
• Composed of plates of crust and upper
mantle.
• These plates can contain continental/oceanic
crusts or both.
• The lithosphere floats on the denser, more
plastic athenosphere.
Plate Movement
Subduction
• The “downward” plate movement into the
mantle at a subduction zone to form a
“trench.”
• This movement causes earthquakes and when
some molten material rises to the surface it
may form volcanoes.
• Ex. When oceanic crust subducts under
continental crust the Andes mountains of
South America were formed.
Subduction
• When two oceanic crusts collide one will
subduct below the other and form trenches
that lead to the formation of volcanic island
chains called “island arcs.”
• Ex: The Aleutian and Marianas Islands
• When two continental plates collide both tend
to float and buckle and neither is subducted.
• This forms mountains ranges such as the
Himalayas.
• What properties of the continental crust make
this happen?
Shear Boundary
• When two plates slide past each other causing
extreme friction.
• This stress can cause earthquakes.
• Ex: A shear boundary formed the San Andreas
Fault in California.
Shear Boundary
Convection
• When heat from the mantle swirls the plates.
• This used to be the main hypothesis of plate
tectonics.
• Now it is believed to be an auxiliary form of
plate motion.
Convection
Geologic History
Pangea
• 200 million years ago Pangea was surrounded
by Panthalassa.
• What was Panthalassa?
• Panthalassa is thought to be the ancestor to
the Pacific ocean.
• Tethys Sea- separated Eurasia from Africa (It is
the modern Mediterranean Sea.
• Sirius Borealis- is the modern Arctic Ocean.
180 million years ago. . .
• A new rift formed between North America and
combined the continents of South America and
Africa.
• This rift formed the early Mid-Atlantic Ridge and
the North Atlantic Ocean.
• Pangea broke into two continents:
1) Laurasia- North America and Eurasia
2) Gondwana- South America, Antarctica, India, and
Australia. A rift split at this time forming the Indian
Ocean.
Marine Sediments
• Lithogenous- sediments derived from physical
and chemical weathering of rocks.
• These are most common.
• Ex: red clay on the open ocean floor.
• Biogenous- made of skeletons, shells of
marine organisms such as diatoms,
radiolarians, foraminiferans, and
coccolithophorids.
• Composed of calcium carbonate- sometimes
referred to as “calcareous ooze.”
Diatoms
Foraminiferans
• Microfossils- microscopic preserved remains
of marine organisms that give clues about the
type of water the organism lived in such as
whether it was cold or warm.
Microfossils
Radiometric Dating
• The use of radioactive isotopes to determine
the exact age of a fossil.
• Also referred to as carbon dating.
Climate and Changes in Sea Level
• Earth’s climate fluctuated between warm
interglacial periods and cold ice ages.
• During ice ages the sea level falls because
water is trapped as glaciers.
Pleistocene Epoch
• Two million years ago- was the last major
period of glaciation.
• Sea level has risen over the past 3,000 years
due to greenhouse gases such as carbon
dioxide and methane.
• These gases are said to cause global warming
therefore melting glaciers and subsequently
causing a rise in sea level.
Geology of the Sea Floor
The sea floor is divided into two main
regions:
• Continental margin- are boundaries between
continental and oceanic crust.
• They consist of shallow, gently sloping
continental shelf, a steeper continental slope,
and a gently sloping continental rise.
Continental shelf
• Shallowest part of the continental margin.
• Makes up only 8% of the ocean’s surface area
but are the most biologically rich portion of
the ocean.
• When the sea level rises- submarine canyons
have formed here.
• The shelf ends at the “shelf break” where the
slope gets steeper 120-400m.
Continental Shelf
Continental Slope
• Begins at the shelf break and descends to
deep sea floor.
• Submarine canyons cause its depth to reach
3000-5000m.
• The continental slope channels sediments to
the sea floor.
Continental Slope
Continental Rise
• When sediment from the submarine canyons
accumulates and deep sea fans (deposits that
are similar to a river delta) piles on the sea
floor.
Continental Rise
Active and Passive Margins
Active Margins
• Continental margins with intense geological
activity.
• Earthquakes and volcanoes are characteristic
here.
• Have steep, rocky shorelines, narrow
continental shelves, steep continental slopes,
and lack a developed continental rise.
Active Margin
Passive Margin
• Inactive geologically.
• Flat, coastal plains with wide continental
shelves and gradual slopes, leading to a thick
continental rise.
Passive Margins
Deep Ocean Basins
• Most of the deep sea floor lies at a depth of
3000-5000m (10,000-16,500 ft).
• Abyssal plain- deep sea floor, rises at gentle
slope toward mid-ocean ridge.
• Relatively flat but can contain submarine
channels, abyssal hills, plateaus, rises and
other features.
Abyssal Plain
Seamounts
• Comprised of volcanic islands and submarine
volcanoes.
• Ex: Guyots- flat-topped seamounts that are
common in the Pacific.
• These areas have a great diversity of marine
life.
Trenches
• Areas where plates descend into mantle.
• Are the deepest parts of the ocean.
• The deepest is the Mariana Trench in the
Western Pacific at 11, 022m (36,153ft).
Mariana Trench
Mid-Ocean Ridge and
Hydrothermal Vents
• The Mid-Ocean Ridge contains a gap called
“the central rift valley.”
• Formed when fractures and crevices let
seawater seep downward where it is then
heated to a very high temperature by the
mantle.
• This heated water is forced back through the
crust via hydrothermal vents or deep sea
hotsprings.
Mid-Ocean Ridge
Hydrothermal Vents
Deep Sea Hot Springs
• Water from hydrothermal vents can be warm,
10-20C (50-68F) or blistering 350C (660F)!
• This hot water dissolves minerals such as
sulfides. These minerals cool and form
deposits around the vents.
Black Smokers
• One type of deposit. They look like a chimney.
• Made of solidified minerals.
• The smoke the boils from them is a dense
cloud of mineral particles.
Black Smokers
• Deep Sea hot springs are of interest to
geologists and biologists.
• They are an abundant source of marine life.
• Many marine organisms are found around
these vents.
Tube worms around a vent.