Unit 5: Ocean Floor Structure and Plate Tectonics



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 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.

|[pic] |

|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. |

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 mid-ocean ridges, old areas are dragged down, or subducted, into the Earth's mantle, which explains why the older rocks cannot be found.

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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

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!!!!!!

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.

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 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

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.

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Guyot

Inner core

Outer core

Crust

Upper Mantle

Lower Mantle

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Continental Plate – Granite Rock

Oceanic Plate -Basalts

Upper Mantle

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.

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