Our Changing Earth

The Theory of Plate Tectonics:

The Earth's lithosphere (crust + upper mantle) is broken into about 12 major and several minor plates that move on the plastic asthenosphere.

Types of plate boundaries:

Convergent boundary (plates collide):

• Continental-Continental collision: Both plates crumple upward → fold mountains
  • Example: Indian plate colliding with Eurasian plate → Himalayan mountain range (still rising ~5mm/year)
  • Example: Alps (Africa-Europe collision)
• Oceanic-Continental collision: Denser oceanic plate subducts (goes under) continental plate → volcanoes and coastal mountains
  • Example: Andes mountains (Nazca plate subducting under South American plate)
• Oceanic-Oceanic collision: One subducts under other → volcanic island arcs
  • Example: Japan, Philippines

Divergent boundary (plates separate):

• Plates pull apart → magma fills gap → creates new oceanic crust
• Example: Mid-Atlantic Ridge (where plates are pulling apart; Atlantic Ocean is widening ~2.5 cm/year)
• Example: East African Rift Valley — Africa splitting apart; will eventually create a new ocean in millions of years

Transform boundary (plates slide past each other):

• Horizontal movement; no crust created or destroyed
• Cause major earthquakes
• Example: San Andreas Fault (California, USA) — most famous transform fault

Indian Ocean context:

• India was once part of Gondwanaland (supercontinent); drifted north over ~100 million years
• Collision with Eurasia began ~50 million years ago → Himalayas formed
• This is why marine fossils are found in the Himalayas (Tethys Sea sediments)

Volcanoes: Openings in the Earth's crust where magma, gases, and ash escape to the surface.

Types:

• Active: Currently erupting or recently erupted — Mt. Etna (Italy), Kilauea (Hawaii)
• Dormant: Not erupted recently but could erupt — Mt. Fuji (Japan), Barren Island (India — Andaman)
• Extinct: No eruption in historical record — Deccan Plateau (ancient eruptions)

India's volcanoes:

• Barren Island (Andaman & Nicobar Islands): Only active volcano in South Asia; last erupted 2017–2020 (ongoing activity); an uninhabited island
• Narcondam Island (Andaman & Nicobar): Dormant volcano; dense forest cover; one of India's smallest uninhabited islands; habitat of Narcondam hornbill (endemic)

Pacific Ring of Fire: ~75% of world's volcanoes and ~90% of world's earthquakes occur around the Pacific Ocean rim (where Pacific plate collides with surrounding plates). Countries: Japan, Philippines, Indonesia, Papua New Guinea, New Zealand, Chile, USA (Alaska, Pacific Northwest).

Benefits of volcanoes:

• Fertile soil: Volcanic ash enriches soil (Indonesia's Java island — most densely populated agricultural area in the world because of volcanic soil)
• Geothermal energy
• New land formation (Hawaii islands are entirely volcanic)