Download Unit 5: Ocean Floor Structure and Plate Tectonics

Unit 7: Part 1 - Ocean Floor Structure
Part 1 - Sea Floor Features:
Earth's rocky surface is
divided into two types:
oceanic crust, with a thin
dense crust about 10 km
thick, and continental crust,
with a thick light crust about
40 km thick. The deep,
lighter continental crust
floats higher on the denser
mantle than does the
oceanic crust. The diagram
below illustrates the basic
set up of the ocean floor.
Remember, however, that
this is just an overall view
of what exists.
A. Continental Shelves –
Most continents extend far beyond the point where ocean meets land. Their extended perimeter is referred to as the continental shelf.
Continental shelves are zones adjacent to a continent (or around an island) and extending from where the water begins to where there
is a definite steep decline to the depths of the ocean floor. The water covering continental shelves form shallow seas. Some, such as
the South China Sea, are more than 1100 km wide. Most are relatively shallow, with typical depths of 50-100 m.
A few of the more important continental shelves are: the East China Sea, the Bering Sea, the North Sea, the Grand Banks, the
Patagonian Shelf, the Arafura Sea and Gulf of Carpentaria, and the Siberian Shelf.
Continental shelves are places of great biodiversity and marine life due to the relative abundance of sunlight available in their shallow
waters. Shallow seas also help dissipate tides and they are almost always important economically for seafood production and other
economic needs to the countries connected to them. They are also valuable for oil and gas exploration.
B. Continental Slope – extends from the continental shelf at an average depth of about 135 meters . Both the continental shelf and
slope are considered structurally part of the continents, even though they are below the sea surface.
C. Ridges – Ridges are long, narrow, continuous mountain chains on the sea floor. Individually, ocean ridges are the largest features
in ocean basins. Collectively, the ocean ridge system is the most prominent feature on the Earth’s surface. The East Pacific Rise is the
largest oceanic ridge
D. Seamounts - isolated or comparatively isolated elevations rising 1000 m or more from the sea floor and with small summit area.
These are extinct underwater volcanoes that are cone shaped and often flat-topped. They rise abruptly from the abyssal plain to
heights at least 3300 feet above the ocean floor. The Indian Ocean contains the most number of seamounts than any of the other
oceans.
E. Guyots – a seamount with a flat top created by wave action when the seamount is extended
above sea level.
F. Trenches – long, narrow, and deep depressions of the sea-floor, with relatively steep sides that
form at subduction zones. As ocean plates move toward continents, they are drawn under, forming
deep trenches. There are 26 oceanic trenches in the world: 3 in the Atlantic Ocean, 1 in the Indian
Ocean, and 22 in the Pacific Ocean. Generally, the trenches mark the transition between continents
and ocean basins, especially in the Pacific basin. Trenches are also the tectonic areas.
G. Island arcs – volcanic island chains that form on the opposing edge of a subducted plate.
Examples include the Japanese islands and the Aleutian islands of Alaska.
Guyot
H. Canyons - are relatively narrow, deep furrows with steep slopes, cutting across the continental shelf and slope, with bottoms
sloping continuously downward.
V-shaped canyons cut into the continental slope to a depth of up to 1200 meters. The submarine canyons are cut perpendicular to the
running direction of the continental slope. Many canyons are associated with major rivers such as the Congo, Hudson, and others.
I. Abyssal Plains - flat, gently sloping or nearly level regions of the sea-floor. These plains may cover almost a third of Earth’s
surface -- about as much as all the exposed land combined. They’re found between the edges of the continents and great underwater
mountain ranges.
Abyssal plains consist of beds of volcanic rock topped with sediments that are up to thousands of feet thick. Most of the sediments
wash off the continents, and are carried to the depths by dense currents. Over time, the sediments spread out to provide a smooth,
level surface. Abyssal plains are most common in the Atlantic; in the Pacific, deep trenches around the continents trap most of the
sediment before it reaches the open ocean.
At depths of thousands of feet, there’s absolutely no light. The water is near freezing, and the pressure is hundreds of times greater
than at the surface. Even so, many species have adapted to the harsh conditions. They eat a “snow” of dead organic matter that falls
from the upper layers of the ocean. Because of the cold, they have slow metabolisms, so they don’t need to eat very often -- generally,
only once every few months.
J. Basins – deep depressions of the sea floor of more or less circular or oval form.
K. Hydrothermal Vents –
Hydrothermal vents are fractures (cracks) in the Earth’s surface where geothermally-heated water pulses through. Usually, a
hydrothermal vent is found where hot magma is close to the surface crust. Hot springs, fumaroles, and geysers are all examples of
geothermal vents on land. Hydrothermal vents in the ocean floor are called submarine hydrothermal vents, or black smokers. They
were discovered in 1977 around the Galapagos Islands by the NOAA using a small submersible called Alvin. They are usually
hundreds of meters wide and are formed when water, heated by geothermal activity below the crust to extreme temperatures (up to
400oC), rises through the ocean floor. The water never boils because of the extreme pressure it is under at that depth. They contain
many dissolved minerals like sulfides that usually crystallize into a chimney-shaped structure. The black color comes from the
precipitation of minerals when the cold ocean water and the superheated water collide. These black smokers exist in the Pacific and
Atlantic Oceans, usually at about 2,100 meters deep.
The areas around black smokers contain entire complex communities of organisms who survive using chemicals dissolved in the fluids
of the vents. These waters are too deep for sunlight to penetrate, so the vent communities rely on chemosynthesis instead of
photosynthesis where heat, methane, and sulfur compounds are converted into energy. At the base of the food chain are
chemosynthetic microorganisms that support entire communities of giant tubeworms, clams, and shrimp.
L. Cold seeps
A cold seep or cold vent is a vent that is not superheated, but hydrogen sulfide, methane and hydrocarbon-rich fluid still seep out into
the surrounding water. The reactions between seawater and methane create carbonate rock formations and reefs over time. Cold
seeps do not work in short, unpredictable bursts like many hydrothermal vents but are instead slow and dependable.
Light independent organisms exist in entire communities in and around cold seeps. The organisms that live in cold seeps usually live
longer than those at hydrothermal vents due to the relative stability of resources. The longest-lived noncolonial invertebrate is a cold
seep tubeworm that lives from 170-250 years. Most of the organisms living in cold seeps are reliant on bacteria that produce energy
from the chemicals and compounds in the water (chemosynthesis).
Dr. Charles Paull is credited with the discovery of cold seeps in 1984 at a depth of 3,200 m in the Gulf of Mexico, although the deepest
cold seep found is located in the Sea of Japan at a depth of 5,000-6,500 m. Since then, additional cold seeps have been found in the
Gulf of Mexico, the Sea of Japan, and in waters off the Alaska's coast.
Sub-sea features have important influences on the ocean circulation. Ridges separate deep waters of the ocean into distinct
basins separated by sills. Water deeper than the sill between two basins cannot move from one to the other. Tens of
thousands of isolated peaks, seamounts, are scattered throughout the ocean basins. They interrupt ocean currents, and
produce turbulence leading to vertical mixing of water in the ocean.
Unit 7: Part 2 – Plate Tectonics
A. The Changing Ocean
The face of the Earth is always changing and throughout geologic history oceans have been created and destroyed. Modern geologic
evidence indicates that the ocean bottom is moving at a rate from about one-half to six inches a year through a process called plate
tectonics.
Roughly 200 million years ago the Earth's surface was very
different from the familiar pattern of land we know today. All of
the land masses were grouped together into one vast
supercontinent called Pangaea. The rest of the globe was
covered by a single great ocean known as Panthalassa.
Slowly, over millions of years, the great land mass split apart.
The pieces began to move over the Earth's surface driven by
slowly churning currents in the molten rocks beneath the Earth's
hard outer layers. The gigantic plates on the Earth's crust move
like a conveyor belt. As new areas of ocean floor form at midocean ridges, old areas are dragged down, or subducted, into
the Earth's mantle, which explains why the older rocks cannot be
found.
CONTINENTAL DRIFT
Pangaea broke up with part of the continent drifting north and part
south. 1) The northern part split to form the North Atlantic Ocean
208-146 million years ago (mya). 2) The South Atlantic and Indian
oceans began to form 146-65 mya. 3) The continents continue to
drift. Today the oceans are still changing shape; the Atlantic Ocean
gets wider by a few inches each year.
By about 35 million years ago the pattern of land and sea was very
much like it is today. But the continents are still moving and as the
Atlantic and Indian oceans continue to get wider by a few inches
every year, the Pacific is slowly shrinking. At the northeast corner
of Africa we can see the start of a new ocean. For the last 25
million years, the Red Sea has been widening. If it continues at the
same rate, in 200 million years it will be as wide as the Atlantic is
today.
The Theory of Plate Tectonics states that plates make up the outer layer of the Earth and have slowly moved long distances
throughout the history. This theory explains how the continents once fit together in a single continent called Pangaea. The movement
of continents explains how animals became separated onto different continents, and it explains how mountains, volcanoes, and ocean
trenches were formed, and why earthquakes occur. The underlying theory behind plate tectonics is that the force of gravity is stronger
on a heavy, cooled ocean floor than it is on a hotter and lighter floor.
The Theory of Plate Tectonics came about in the 1960s to explain seafloor spreading and continental drift. Around 1915 a scientist
named Alfred Wegener published the first edition of "The Origin of Continents and Oceans" in which he proposed that the shapes of
the east coast of South America and the west coast of Africa indicate that may have been attached at some point in history.
In 1962, American geologist Harry Hess suggested that,
instead of continents moving through the ocean crust, an entire
ocean basin and its connected continent actually moved
together as a plate. Once the Theory of Plate Tectonics was
accepted, a multitude of questions were explained and a
scientific revolution occurred in geophysics and geology.
Important Contribution to Science
Who? Alfred Wegener
When? 1915
What? Developed the Theory of Continental Drift - he explain
the presence of identical rocks on opposite sides of the Atlantic
Ocean, and tropical plant fossils in the Arctic Circle were due to
the fact that the continents must have shifted over time.
His ideas were not accepted until seafloor spreading was
discovered, providing a mechanism to explain his theory.
B. Layers of the Earth
The Earth is composed of four different layers. Many geologists
believe that as the Earth cooled the heavier, denser materials
sank to the center and the lighter materials rose to the top.
Because of this, the crust is made of the lightest materials
(rock- basalts and granites) and the core consists of heavy metals (nickel and iron).
The crust is the layer that you live on, and it is the most widely
studied and understood. The mantle is much hotter and has the
ability to flow. The Outer and Inner Cores are hotter still with
pressures so great that you would be squeezed into a ball
smaller than a marble if you were able to go to the center of
the Earth!!!!!!
Crust
Upper Mantle
Lower Mantle
Inner core
Outer core
The Crust
The Earth's Crust is like the skin of an apple. It is very thin compared to the other three layers. The crust is only about 3-5 miles (8
kilometers) thick under the oceans (oceanic crust) and about 25 miles (32 kilometers) thick under the continents (continental crust).
The temperatures of the crust vary from air temperature on top to about 1600 degrees Fahrenheit (870 degrees Celcius) in the deepest
parts of the crust. You can bake a loaf of bread in your oven at 350o F, at 1600o F rocks begin to melt.
The crust of the Earth is broken into many pieces called plates. The plates "float" on the soft, plastic mantle which is located below the
crust. These plates usually move along smoothly but sometimes they stick and build up pressure. The pressure builds and the rock
bends until it snaps. When this occurs an Earthquake is the result!
In other words…The seven continents and ocean plates basically float across the mantle which is composed of much hotter and
denser material.
Continental Plate – Granite Rock
Oceanic Plate -Basalts
Upper Mantle
The crust is composed of two basic rock types granite and basalt. The continental crust is composed mostly of granite. The oceanic
crust consists of a volcanic lava rock called basalt. Basaltic rocks of the ocean plates are much denser and heavier than the granitic
rock of the continental plates. Because of this the continents ride on the denser oceanic plates. The crust and the upper layer of the
mantle together make up a zone of rigid, brittle rock called the Lithosphere. The layer below the rigid lithosphere is a zone of asphaltlike consistancy called the Asthenosphere. The asthenosphere is the part of the mantle that flows and moves the plates of the Earth.
Convection Currents
The mantle is made of much denser, thicker material. Because of this the plates "float" on it like oil floats on water. Many geologists
believe that the mantle "flows" because of convection currents. Convection currents are caused by the very hot material at the
deepest part of the mantle rising, then cooling, sinking again and then heating, rising and repeating the cycle over and over. The next
time you heat anything like soup or pudding in a pan you can watch the convection currents move in the liquid. When the convection
currents flow in the mantle they also move the crust. The crust gets a free ride with these currents. A conveyor belt in a factory moves
boxes like the convection currents in the mantle moves the plates of the Earth.
C. Tectonic Plates and Boundaries
What are tectonic plates?
Tectonic plates are huge fragments of Earth’s lithosphere, consisting of the crust fused with the upper mantle. They move over the
hot, fluid part of the mantle. Plate motion builds mountain ranges, but most of the effects of moving plates can be seen best on the
ocean floor, where most of the plate boundaries are formed.
How did the tectonic (crust) plates form?
When the earth was forming, after the mantle began to cool, areas of the continental and oceanic crust began to group into larger
areas. Rifts formed between the thinnest parts of the ocean crust, splitting it into large plates. When the density of the oceanic and
continental crusts became very different, the oceanic crust sank where it met the continental crust, creating subduction zones. Since
this time, the evolution of the oceans and continents has been determined mostly by plate tectonics. As the plates move, they carry the
continents with them, with oceans opening and closing in between.
As Earth’s plates have moved around, continents have come together and moved apart again. Oceans have formed, disappeared, or
have changed in size. There are multiple time periods when the continents have met and formed large “supercontinent” areas. They
were once again torn apart as the tectonic plates moved away from each other.
How is new oceanic crust formed?
The oldest rocks on the ocean floor are about 180 million years old, whereas the oldest rocks on the continents are dated from 3.8
billion years ago. This is because the ocean floor is constantly created and destroyed rather quickly. New ocean floor is created in the
mid-ocean ridges from hot magma rising in the mantle, coming out of the ridges, and then spreading away from the ridges. As it cools,
it forms new oceanic crust. As the ocean floor plates move towards the continents, it is recycled into the mantle, as it is drawn
downward in subduction zones. Trenches form at these areas where the oceanic crust is subducted into the mantle at the continental
crust. Thus, subduction zones occur where oceanic crust meets continental crust. The continental crust is less dense, so the oceanic
crust sinks (subduction) back into the mantle. This is a continual process, and explains why oceanic crust is so much newer and
younger than continental crust.
What are the 3 types of tectonic plate boundaries?
There are three types of tectonic plate boundaries
1. divergent – “pulled apart” - the crust is extended, thinned, and
fractured by the rising of hot mantle material (new crust is being
formed as magma comes out of rifts or volcanoes). Parallel ridges
emerge as new ocean floor spreads out on either side of an ocean
ridge.
2. convergent – “collide together” - occurs when plates collide,
causing volcanoes to erupt from the building pressure
3. transform – “trans = across” - occurs when plates move past each
other. Ex. San Andres Fault in California
D. Earthquakes and Tsunamis
What is the difference between an earthquake and a tsunami?
Earthquakes can occur with any type of plate boundary, but they are more frequent at convergent boundaries, such as subduction
zones. This is because stress builds up at faults in the crust until it overcomes the strength of the rock, causing the fault to slip. When
this happens, a huge amount of energy can be released in a short time.
A tsunami may be triggered if an earthquake causes the uplift or subsidence (sinking) of part of the sea floor. The water above
suddenly rises or sinks, then flows to regain equilibrium. Surface waves radiate out at 310-497 mph, and can quickly cross an entire
ocean.
E. Hotspots and the Ring of Fire
What is a plate tectonic hotspot?
A hotspot is an area of volcanoes that form far from the edges of tectonic plates. They usually form volcanic island chains in the middle
of the plate, itself. Hotspots are believed to form on the surface directly above extremely hot plumes of magma that are stationary, and
lie deep within the mantle, possibly where the mantle meets the core. As the plate moves on, the volcano on the surface is separated
from its source of magma beneath, and becomes a “dead” volcano. Dead volcanoes erode away with time, eventually sinking below
the surface of the water, forming reefs or other underwater structures. Hotspots are not limited to forming in the ocean…Yellowstone
National Park is a hotspot that occurs in the continent itself.
What is the Ring of Fire?
The "Ring of Fire" is an arc stretching from New Zealand, along the eastern edge of Asia, north across the Aleutian Islands of Alaska,
and south along the coast of North and South America. The Ring of Fire is composed over 75% of the world's active and dormant
volcanoes.
This huge ring of volcanic and seismic (earthquake) activity was noticed and described before the invention of the Plate Tectonics
Theory. We now know that the Ring of Fire is located at the borders of the Pacific Plate and other major tectonic plates.
Plates are like giant rafts of the earth's surface which often slide next to, collide with, and are forced underneath other plates. Around
the Ring of Fire, the Pacific Plate is colliding with and sliding underneath other plates. This process is known as subduction and the
volcanic and seismic active area nearby is known as a subduction zone. There is a tremendous amount of energy created by these
plates and they easily melt rock into magma, which rises to the surface as lava, forming volcanoes.
Volcanoes are temporary features on the earth's surface and there are currently about 1500 active volcanoes in the world. About ten
percent of these are located in the United States.