- PLATE TECTONICS:
The Earth's crust is formed of a dozen or so slowly moving Tectonic plates. Most plates are the size of continents.
North America, South America, Eurasia (without India), Africa, and Antactica each occupy a separate Plate.
India and Australia were once part of the same Plate but they are sometimes considered separate.
The crust under the Pacific Ocean forms a Pacific Plate.
Smaller plates include the Caribbean Plate, Cocos Plate, Scotia Plate, Sandwith Plate, Arabian Plate, Philippine Plate, and Fiji Plate.
Each plate includes both the crust and the upper, rigid layer of the Mantle, above the Athenosphere.
Slow, plastic flow within the Athenosphere causes each plate to move slowly with respect to the other plates, causing Continental Drift.
For example, the North American and Eurasian Plates are moving apart at a rate of about 1 centimeter per year, about as fast as your fingernail grows.
Convection currents within the Asthenosphere causes the crustal movements responsible for Plate Tectonics.
- DIVERGING PLATE BOUNDARIES:
The East African Rift Valleys probably represent an early stage in the formation of new plate boundaries.
Tension within the Earth's crust creates a parallel series of tension faults on either side of a long Rift Valley.
The East African Rift Valleys run North to South, with steep sides on either side of the widening valley.
Lake Malawi and Lake Tanganyika are rift valleys that have
flooded and filled with water. So is the Red Sea.
The Arabian peninsula was once part of the African Plate, but the growing rift of the Red Sea has now made it a separate plate.
Mid-Ocean Ridges are mature divergent boundaries where different plates are spreading apart and convection brings new rock material to the surface.
The Mid-Atlantic Ridge has been studied most intensively. Sediments just on either side of the Ridge are very recent. Sediments further from the Ridge
form parallel bands of
increasing age, with the youngest bands close to the Ridge and successively older bands at successively greater distances.
On opposite sides of the ridge, the bands run parallel but in the
opposite order, with the youngest bands always closest to the Ridge.
Convection currents within the asthenosphere rise upward along Mid-Ocean ridges, then go away from the Ridge in opposite directions.
Mid-ocean ridges are interrupted every so often by Transform Faults running perpendicular to the Ridge.
- COLLIDING or CONVERGENT PLATE BOUNDARIES:
Wherever Tectonic Plates converge or collide, a Convergent Plate Boundary is formed.
Convergent Plate Bouondaries are usually marked by lots of tectonic activity in the form of earthquakes and volcanoes.
Some convergent boundaries are areas of Uplift, such as the Himalayas in Asia (where the Indian Plate is converging
with the Eurasian Plate),
or the earthquake-prone, mountainous regions of Italy, Greece, Turkey, and Iran, or the Andes Mountains of South America.
Other convergent boundaries are Subduction zones, where one plate plunges deep beneath another, usually forming a Deep Sea Trench.
The Mariana Trench, for example, is a subduction zone formed by the Pacific Plate plunging beneath the smaller Philippine Plate.
The Tonga Trench and Aleutian Trench are other examples.
- HOT SPOTS:
An area of persistent volcanic activity often forms a Hot Spot beneath the Earth's crust and gives rise to multiple volcanoes, such as those
of the Canary Islands in the Atlantic or the Galapagos Islands in the Pacific.
If a Tectonic Plate moves over a Hot Spot, a string of volcanoes or volcanic islands may form. The Hawaiian Islands are the best example of this.
As the Pacific Plate moves westward toward Japan, volcanic eruptions originating from the Hawaiian Hot Spot first formed the
westernmost of the Hawaiian Islands (Niihau and Kauai), about 8 million years ago,
then successively younger islands (in order, west to east),
and finally the youngest and easternmost island (the "Big Island" of Hawai'i), where volcanoes are still active today.
- PLATE TECTONICS in the REMOTE PAST:
If you examine the Atlantic coastlines of South America and Africa, you will notice that their shapes match up.
The match is even more perfect if we include the Continental Shelf of each continent, up to the margins of each Ocean basin.
Geological formations in northeastern Brazil often match up with similar geological formations in West Africa.
All this is evidence that South America and Africa were once together, before the Atlantic Ocean was formed.
Certain fossils on opposite sides of the Atlantic show similarities between African and South American animal faunas and plant floras.
Glossopteris is an extinct type of seed-fern that occurs on various southern continents: South America, Africa, India, Australia, and Antarctica.
Along with various other fossils, Glossopteris shows that the several southern continents were once all connected.
In many places, the floras (plant assemblages) containing Glossopteris were followed by glaciation that also extended across various southern continents.
All this is evidence that the several southern continents were once connected into a large southern landmass known as Gondwanaland,
named after a formation in India where the evidence is strikingly well-preserved.
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