Plate Tectonics
Geography · प्लेट विवर्तनिकी
📋Quick Overview
Plate Tectonics is the theory that Earth's outer shell (lithosphere) is divided into several large plates that float on the semi-molten asthenosphere and move due to convection currents. Alfred Wegener proposed the Continental Drift Theory in 1912, suggesting all continents were once joined as a supercontinent called Pangaea, surrounded by a super ocean called Panthalassa. The modern Plate Tectonics theory explains earthquakes, volcanoes, and mountain formation.
Wegener's evidence for Continental Drift: Jigsaw fit of continents (Africa-South America), same fossils on different continents (Mesosaurus), similar rock formations, and matching glacial deposits.
📖7 Major Tectonic Plates
| Plate | Covers |
|---|---|
| Pacific Plate | Largest oceanic plate, most of Pacific Ocean floor |
| North American Plate | North America, western Atlantic, Greenland, Iceland (part) |
| South American Plate | South America, western South Atlantic |
| African Plate | Africa, eastern Atlantic |
| Eurasian Plate | Europe, most of Asia (not India) |
| Indo-Australian Plate | India, Australia, Indian Ocean |
| Antarctic Plate | Antarctica and surrounding ocean |
Minor plates include: Nazca, Philippine, Arabian, Caribbean, Cocos, Scotia, Juan de Fuca plates.
📝Plate Boundaries
| Boundary Type | Movement | Result | Example |
|---|---|---|---|
| Convergent (Destructive) | Plates move towards each other | Mountains, trenches, subduction zones, earthquakes, volcanoes | Himalayas (Indian + Eurasian), Mariana Trench (Pacific + Philippine) |
| Divergent (Constructive) | Plates move away from each other | Mid-ocean ridges, new crust formed, rift valleys | Mid-Atlantic Ridge, East African Rift Valley |
| Transform (Conservative) | Plates slide past each other | Earthquakes (no volcanoes), no creation or destruction | San Andreas Fault (California, USA) |
📝Ring of Fire & Earthquake Zones
- •Ring of Fire = horseshoe-shaped zone around the Pacific Ocean with 75% of world's volcanoes and 90% of earthquakes
- •Richter Scale measures the magnitude (energy) of an earthquake. Developed by Charles Richter in 1935. It is logarithmic — each whole number = 10x more amplitude.
- •Seismograph is the instrument used to record earthquakes. The point of origin underground = Focus/Hypocenter. The point on the surface directly above = Epicenter.
- •India's Seismic Zones: Zone V (highest risk) — Northeast India, J&K, Himachal, Uttarakhand, Kutch. Zone IV — NCR, Bihar. Zone II (lowest risk) — parts of south India.
📝Types of Volcanoes
| Type | Shape | Eruption | Example |
|---|---|---|---|
| Shield Volcano | Wide, gentle slope, dome-shaped | Quiet, flowing lava (basaltic) | Mauna Loa (Hawaii) — largest volcano on Earth |
| Composite/Strato Volcano | Tall, steep cone | Explosive, alternating lava & ash layers | Mt. Fuji (Japan), Mt. Vesuvius (Italy) |
| Cinder Cone Volcano | Small, steep, cone-shaped | Short, explosive bursts | Paricutin (Mexico) |
Barren Island (Andaman & Nicobar) is India's ONLY active volcano. Narcondam Island is a dormant volcano.