Volcanicity, Earthquakes & Tsunamis

1. Volcanicity

Volcanicity represents the process by which magma and associated gases rise through the Earth's crust and are extruded onto the surface or intruded into the crust.

Causes of Volcanism

• Decompression Melting: At divergent boundaries (mid-ocean ridges), the upward movement of the mantle reduces pressure, lowering the melting point and generating magma.
• Flux Melting: At convergent boundaries (subduction zones), water from the subducting oceanic plate lowers the melting temperature of the overlying mantle.
• Mantle Plumes / Hotspots: Stationary areas of intense heat in the mantle create localized volcanism far from plate boundaries (e.g., Hawaii, Yellowstone).

Types of Volcanoes

• Shield Volcanoes: Formed by very fluid basaltic lava (low silica). They are broad, gently sloping cones (e.g., Mauna Loa, Hawaii).
• Composite Volcanoes (Stratovolcanoes): Steep-sided cones built by alternating layers of viscous, granitic lava and volcanic ash (pyroclastic material). Highly explosive (e.g., Mt. Fuji, Mt. Vesuvius).
• Calderas: The most explosive volcanoes. The magma chamber empties so rapidly that the summit collapses into it, forming a massive depression (e.g., Yellowstone Caldera).
• Flood Basalt Provinces: Massive outpourings of highly fluid lava that cover vast areas, forming plateaus (e.g., Deccan Traps in India).

Intrusive Landforms

When magma cools within the crust, it forms plutonic bodies:

• Batholiths: Massive granitic domes exposed by erosion.
• Laccoliths: Mushroom-shaped intrusions pushing up overlying strata.
• Sills: Horizontal sheets of magma injected between layers.
• Dykes: Vertical wall-like intrusions cutting across rock layers.

2. Earthquakes

Earthquakes are sudden tremors or shaking of the Earth's surface caused by the rapid release of energy in the lithosphere, which creates seismic waves.

Causes Mechanism: Elastic Rebound Theory

Rocks on either side of a fault undergo stress and accumulate strain energy. When the stress exceeds the rock's strength, it ruptures (faulting), releasing the stored energy rapidly as seismic waves. The point of origin is the focus (hypocenter), and the point vertically above it on the surface is the epicenter.

Distribution of Earthquakes

• Circum-Pacific Belt (Ring of Fire): Accounts for ~81% of the world's largest earthquakes, mostly related to subduction zones.
• Alpine-Himalayan Belt: Accounts for ~17%, running from Java through the Himalayas and Mediterranean, primarily associated with continental collisions.
• Mid-Oceanic Ridges: Associated with divergent boundaries and transform faults; generally shallower and less severe.

Measurement

• Richter Scale: Measures the magnitude (amount of energy released). It is a base-10 logarithmic scale (a 7.0 magnitude earthquake releases 31.6 times more energy than a 6.0).
• Mercalli Scale: Measures the intensity (the visible damage caused). It ranges from I to XII.

3. Tsunamis

A tsunami (Japanese for "harbor wave") is a series of massive ocean waves typically caused by underwater earthquakes at subduction zones, volcanic eruptions, or submarine landslides.

• Generation: A sudden vertical displacement of the tectonic plates displaces the huge column of water above it.
• Propagation: Over the deep ocean, tsunamis travel incredibly fast (up to 800 km/h) but have a very low amplitude (wave height), making them barely noticeable.
• Shoaling Effect: As the tsunami approaches shallow coastal waters, its speed drastically decreases, but the wave energy is compressed, causing a massive increase in wave height, resulting in catastrophic inundation.
• Warning Systems: Rely on a network of seismographs to detect the earthquake and ocean buoys (DART sensors) to measure changes in sea level.