Unit 5: Ocean Floor Structure and Plate Tectonics
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.
Continental Shelves – zones adjacent to a continent (or around an island) and extending from the
low-water line to the depth, usually about 120 m, where there is a marked or rather steep descent
toward great depths. The volume of the water in the ocean exceeds the volume of the ocean basins,
and some water spills over on to the low lying continental shelves, forming 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. The shallow seas help dissipate tides, they are often areas of high biological
productivity, and they are usually included in the exclusive economic zone of adjacent countries
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.
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
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.
Guyots – a seamount with a flat top created by wave action when the
Guyot
seamount is extended above sea level.
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.
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.
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.
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.
Basins – deep depressions of the sea floor of more or less circular or oval form.
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.
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
CONTINENTAL DRIFT
as Panthalassa.
Pangaea broke up with part of the continent drifting north
and part south. 1) The northern part split to form the North
Slowly, over millions of years, the great land
Atlantic Ocean 208-146 million years ago (mya). 2) The
mass split apart. The pieces began to move over South Atlantic and Indian oceans began to form 146-65 mya.
3) The continents continue to drift. Today the oceans are still
the Earth's surface driven by slowly churning
changing shape; the Atlantic Ocean gets wider by a few inches
currents in the molten rocks beneath the Earth's each year.
hard outer layers. The gigantic plates on the
Earth's crust move like a conveyor belt. As new areas of ocean floor form at mid-ocean ridges, old
areas are dragged down, or subducted, into the Earth's mantle, which explains why the older rocks
cannot be found.
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.
Layers of the Earth
Crust
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!!!!!!
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 asphalt-like consistancy called
the Asthenosphere. The asthenosphere is the part of the mantle that flows and moves the plates of the Earth.
The Mantle
The mantle is the layer located directly under the crust. It is the largest layer of the Earth, 1800 miles thick. The
mantle is composed of very hot, dense rock. This layer of rock even flows like asphalt under a heavy weight.
This flow is due to great temperature differences from the bottom to the top of the mantle. The movement of the
mantle is the reason that the plates of the Earth move! The temperature of the mantle varies from 1600o F at the
top to about 4000o F near the bottom!
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.
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
Important Contribution to Science
began to cool, areas of the continental and
Who? Alfred Wegener
oceanic crust began to group into larger
When? 1915
areas. Rifts formed between the thinnest
What? Developed the Theory of Continental Drift - he explain the
parts of the ocean crust, splitting it into large
presence of identical rocks on opposite sides of the Atlantic Ocean,
and tropical plant fossils in the Arctic Circle were due to the fact
plates. When the density of the oceanic and
that the continents must have shifted over time.
continental crusts became very different, the
His ideas were not accepted until seafloor spreading was
oceanic crust sank where it met the
discovered,
providing a mechanism to explain his theory.
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
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.
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.