Week 3: Volcanism and Earthquakes

Earth is a dynamic planet where internal forces constantly reshape its surface. Among these forces, volcanism, earthquakes, and tsunamis are some of the most powerful and destructive natural phenomena. These events are mainly driven by plate tectonics, which is the movement of Earth's lithospheric plates.

What is Volcanism?

Volcanism refers to the movement of molten rock (magma) from the Earth's interior to its surface, where it erupts as lava, along with gases and volcanic ash. This process plays a crucial role in shaping landscapes and influencing climate and life on Earth.

Causes of Volcanism

1. Tectonic Plate Movements:
• At divergent boundaries (e.g., Mid-Atlantic Ridge), magma rises to fill the gap between separating plates.
• At convergent boundaries (e.g., Pacific Ring of Fire), one plate subducts beneath another, melting into magma that rises to the surface.
2. Hotspots:
• In some regions like Hawaii, magma rises from deep within the mantle, forming volcanic islands over time.

Types of Volcanoes

Classification on the basis of eruption period:

1. Active. – Barren Island, India
2. Dormant – Mount Kilimanjaro, Tanzania
3. Extinct – Mount Kenya

Classification on the basis of nature of eruption and forms developed at the surface (Extrusive):

1. Shield Volcanoes
• Large, broad, and dome-shaped.
• Formed by highly fluid basaltic lava.
• Example: Hawaiian Volcanoes- Mauna Loa, Kilauea
2. Composite (Stratovolcanoes)
• Steep-sided, explosive eruptions.
• Layers of lava and ash accumulate over time.
• Example: Mount St. Helens (USA).
• Light House of the Mediterranean- Mt. Stromboli
• Mt Etna (Sicily, Italy),
3. Cinder Cone Volcanoes
• Smallest type, composed of ash and cinders.
• Erupts explosively.
• Example: Paricutin (Mexico).
• Mt Nuovo (Naples, Italy)
4. Caldera Volcanoes
• Extremely explosive.
• Forms a large depression (caldera) when the magma chamber empties and collapses.
• Example: Yellowstone (USA).
• Kilauea, Halemaumau
• Caldera lakes- Lake Toba (Sumatra, Indonesia)
5. Flood Basalt Provinces
• Large outpourings of lava that create extensive plateaus.
• Example: Deccan Traps (India).
6. Mid-Ocean Ridge Volcanoes
• Occur at underwater spreading centres.
• Example: Iceland's volcanic system.

Intrusive Rocks	Extrusive Rocks
Also known as plutonic rocks.	Also known as volcanic rocks.
Formed when molten material cools and solidifies beneath the Earth's surface.	Formed when lava cools and hardens on the Earth's surface.
Characterized by a coarse texture with large, visible crystals due to slow cooling.	Characterized by a fine-grained, smooth texture due to rapid cooling.
Example: Granite	Example: Basalt

Intrusive forms:

Batholiths	Large dome of cooled magma chambers which are exposed due to denudational factors. Granitic in nature
Laccoliths	Large dome body with a level base a conduit connecting from below similar to domes of composite volcano.
Lopoliths	Saucer shaped concave body formed when lava flows in horizontal direction within the crust
Phacoliths	Wave like structure with a conduit to magma chambers beneath
Sills	Near horizontal bodies. Thin- sheets. Thick- sills
Dykes	Lava flowing from cracks solidify almost perpendicular to the ground. Wall-like structure

What are released in a volcanic eruptions?

Ans. Lava, gases like carbon dioxide, Hydrogen sulphide, Nitrogen, chlorine and other volatile substances

Types of lava:

• Acidic Lava
• Basic Lava

Properties :

Acidic Lava	Basic Lava
Higly viscous	Highly fluid
High melting point
Light coloured	Dark coloured
High percentage of silica	Poor in silica, high in basalt, rich in iron and magnesium
Slow flowing	Flow readily
Steep sided cone	Spread out as thin sheets over long distances
Explosive (throws out pyroblasts/volcanic bombs	Gentle sloping forming flattened shield

Distribution of Volcanoes

Most volcanoes are found along tectonic plate boundaries:

• Pacific Ring of Fire: The most seismically and volcanically active zone.
• Mid-Atlantic Ridge: A divergent boundary with underwater volcanoes.
• Hotspots: Isolated volcanic activity (e.g., Hawaii, Yellowstone).

Effects of Volcanism

• Positive Effects
• New land formation (e.g., Hawaii).
• Fertile soil for agriculture.
• Geothermal energy sources.
• Negative Effects
• Loss of life and property.
• Air pollution from ash clouds (e.g., 2010 Eyjafjallajökull eruption disrupted air travel in Europe).
• Climate changes due to released gases (e.g., 1815 Tambora eruption caused the "Year Without a Summer").

What is an Earthquake?

An earthquake is the sudden shaking of Earth's surface caused by the release of seismic energy due to the movement of tectonic plates. The point where the earthquake originates is called the focus (hypocenter), and the point directly above it on the surface is the epicenter.

Causes of Earthquakes

1. Tectonic Plate Movements
• Convergent Boundaries: Subduction zones create powerful megathrust earthquakes (e.g., 2004 Indian Ocean earthquake).
• Divergent Boundaries: Moderate earthquakes occur at mid-ocean ridges.
• Transform Boundaries: Strike-slip faults cause large earthquakes (e.g., San Andreas Fault).
2. Human-Induced Earthquakes
• Reservoir-induced seismicity (e.g., 1967 Koyna Dam earthquake, India).
• Mining and underground explosions.

Types of Earthquakes

• Tectonic Earthquakes – Caused by plate movement.
• Volcanic Earthquakes – Associated with volcanic activity.
• Collapse Earthquakes – Due to cave-ins or mine collapses.
• Explosion Earthquakes – Resulting from nuclear or chemical explosions.

Measuring Earthquakes

Richter scale	Mercalli scale
By Charles F. Richter	By Giuseppe Mercalli
Magnitude	Intensity
Relates to energy released	Visible damage caused
Range 0-10	Range 1-12

 

Seismic Zones in India

India is divided into four seismic zones (Zone II to Zone V), with Zone V being the most earthquake-prone:

• Himalayan belt – Most seismically active.
• Northeast India, Gujarat, and Delhi – High-risk areas.

Effects of Earthquakes

• Ground shaking – Causes infrastructure damage.
• Soil liquefaction – Unstable ground can sink buildings.
• Tsunamis – Underwater quakes can generate massive waves.
• Landslides – Triggered in hilly areas.

What is a Tsunami?

A tsunami is a series of ocean waves caused by sudden underwater disturbances like earthquakes, volcanic eruptions, or landslides.

Characteristics of Tsunamis

• High Speed – Can travel at 800-900 km/h in deep water.
• Shoaling Effect – Waves grow in height as they approach the shore.

Effects of Tsunamis

• Destruction of coastal regions.
• Massive loss of life (e.g., 2004 Indian Ocean Tsunami killed 230,000+ people).
• Economic devastation.

Tsunami Early Warning Systems

• Pacific Tsunami Warning System (PTWC).
• Indian Ocean Tsunami Warning System (IOTWS).
• Uses seismic sensors and buoys to detect and predict tsunamis.

Case Study: 2004 Indian Ocean Tsunami

What Happened?

• Magnitude 9.1–9.3 earthquake near Sumatra.
• 230,000+ deaths across 14 countries.

Lessons Learned

• Better warning systems needed.
• Importance of preparedness and infrastructure planning.

Remember

• Seismic hazards are inevitable, but preparedness can save lives.
• Building earthquake-resistant infrastructure is crucial.
• Awareness and early warning systems play a vital role.