Week 2: Earth’s Interior, Plate Tectonics, and Continental Drift

The Earth is a dynamic planet, constantly changing due to various geological processes. Understanding its internal structure, seismic activity, and plate movements is crucial in the study of Earth sciences.

Radius of Earth = 6370 km

Earth’s Internal Structure

Composition of the Earth

The Earth consists of several layers with distinct physical and chemical properties. These layers were formed due to the differentiation process, where heavier elements sank towards the core and lighter materials rose towards the surface.

Evolution of Lithosphere

During the process of the planet’s formation, there had been an increase in density leading to increase in temperature inside. Due to which the materials inside started separating on the basis of their densities. The heavier materials sunk towards the centre of the earth and the lighter ones moved towards the surface. As the earth cooled and solidified the outer surface developed into the crust. Along with this different layers were formed with distinct characteristic features like mantle, outer core and inner core. From the crust to the core, the density of the material increases.

Exogenic as well as endogenic processes are responsible for the physiographic character of the landscape.

Sources:

Direct Sources- rocks, mining, volcanic lava

Two major projects:

1.     Deep Ocean Drilling Project

2.     Integrated Ocean Drilling Project

Deepest drill at Kola in Arctic Ocean (reached a depth of 12 km)

Indirect Sources-

• Analysis of properties of matter (Temperature, pressure and density increases with depth)
• Meteors
• Gravitation (Highest gravity at the poles than at the equator)
• Magnetic Field (presence of magnetic materials at different locations)
• Seismic activity (earthquakes) seismograph is used to record waves reaching the surface

Major Layers of the Earth

1. Crust (Lithosphere)

• Outermost layer, brittle and solid.
• Divided into:
• Oceanic Crust: Thin (~5 km), dense (3.0 g/cm³), composed mainly of basalt.
• Continental Crust: Thick (~30-70 km), less dense (2.7 g/cm³), composed mainly of granite.

Properties	Oceanic Crust	Continental Crust
Thickness	5 km	30 km (70 km in Himalayan region)
Density	3 g/cm3	2.7 g/cm3
Rock type	Basaltic	Granitic
Minerals	Silica, iron, Magnesium (Sima)	Silica, alumina (Sial)

2. Mantle (Mesosphere)

• Extends from the base of the crust to ~2900 km depth.
• Composed of rocks rich in olivine.
• It has a density of 3.4 g/cm³.
• Highly viscous and ductile
• The asthenosphere (upper mantle) is semi-molten and allows tectonic plates to move.
• The lower mantle is solid due to immense pressure.

2. Core (Barysphere)

• Constituted of nickle and iron (nife)
• Outer Core: 
• Liquid, composed of iron and nickel, responsible for generating Earth’s magnetic field. 
• Density 9.9 g/cm³ to 12.2 g/cm³
• Inner Core: 
• Solid, composed of iron and nickel, with extremely high pressure and temperature (~6000°C). 
• Density 13 g/cm³(at 6300 km).

Discontinuities (Boundaries Between Layers)

• Moho’s Discontinuity: Boundary between the crust and mantle.
• Gutenberg Discontinuity: Boundary between mantle and outer core.
• Lehmann Discontinuity: Boundary between outer core and inner core.

Seismic Waves & Earth's Structure

Seismic waves, generated by earthquakes, provide insights into Earth's internal composition.

Types of Seismic Waves

1. Body Waves (Travel through Earth's interior)
• P-Waves (Primary Waves):
• Other names: longitudinal waves, compressional waves, pressure waves
• Fastest, travel through solids, liquids, and gases.
• Compressional, vibrating parallel to the wave direction.
• S-Waves (Secondary Waves):
• Other names: transverse waves, shear waves, distortional waves
• Slower than P-waves, travel only through solids (proving the outer core is liquid).
• Vibrate perpendicular to the wave direction.
2. Surface Waves (Travel along Earth’s surface, causing damage)
• Love Waves: Move side-to-side, fastest surface wave.
• Rayleigh Waves: Roll like ocean waves, most destructive.

How Seismic Waves Reveal Earth’s Structure

• Denser materials increase wave velocity.
• Shadow Zones: Areas where seismic waves are absent confirm layer boundaries.
• Seismographs measure seismic activity, helping us locate earthquakes.

Theory of Plate Tectonics

The Plate Tectonics Theory explains the movement of lithospheric plates on the Earth's surface, driven by forces within the mantle.

Key Concepts of Plate Tectonics

• The lithosphere (crust + upper mantle) is divided into rigid plates.
• Plates float on the asthenosphere, a semi-molten layer of the mantle.
• Convection currents in the mantle drive plate movements.
• Plate interactions cause earthquakes, volcanoes, mountain formation, and oceanic trench formation.

Forces Driving Plate Movements

1. Mantle Convection – Heat from the Earth's core causes convection currents in the mantle.
2. Ridge Push – New crust forms at mid-ocean ridges, pushing plates apart.
3. Slab Pull – Dense oceanic plates sink into subduction zones, pulling plates along.

Types of Plate Boundaries

1. Divergent Boundaries (Constructive Margins)

• Plates move apart, creating new crust.
• Example: Mid-Atlantic Ridge (Iceland).
• Forms rift valleys (e.g., East African Rift Valley).

2. Convergent Boundaries (Destructive Margins)

• Plates collide, leading to:
• Oceanic-Continental Collision → Subduction & volcanic arcs (e.g., Andes Mountains).
• Continental-Continental Collision → Fold mountains (e.g., Himalayas).
• Oceanic-Oceanic Collision → Volcanic island arcs (e.g., Japan).

3. Transform Boundaries (Conservative Margins)

• Plates slide past each other, causing earthquakes.
• Example: San Andreas Fault, California.

Continental Drift Theory

Proposed by Alfred Wegener (1912)

• Suggested that all continents were once joined in a supercontinent called Pangaea.
• 200 million years ago, Pangaea began breaking apart into present continents.

Evidence Supporting Continental Drift

1. Fossil Evidence:
• Identical fossils found on continents now far apart (e.g., Mesosaurus in Africa & South America).
2. Geological Evidence:
• Similar mountain ranges across continents (e.g., Appalachians & Caledonian Mountains).
3. Paleoclimatic Evidence:
• Glacial deposits found in tropical regions, suggesting they were once closer to the poles.

Criticism & Later Acceptance

• Initially rejected due to lack of a mechanism for movement.
• Later supported by seafloor spreading and the development of Plate Tectonics Theory.

Case Studies of Plate Movements

1. Mid-Atlantic Ridge (Divergent Boundary Example)

• The Eurasian and North American Plates are moving apart.
• Causes seafloor spreading, creating new oceanic crust.

2. Himalayas (Convergent Boundary Example)

• Formed by the collision of the Indian & Eurasian plates.
• Still rising due to continuous plate movement.
• Frequent earthquakes due to ongoing compression.

3. San Andreas Fault (Transform Boundary Example)

• The Pacific Plate slides past the North American Plate.
• Results in frequent earthquakes along California.

Conclusion & Key Takeaways

• Earth’s layered structure affects surface processes.
• Seismic waves help understand Earth’s interior.
• Plate tectonics explain geological phenomena like earthquakes, volcanoes, and mountain formation.
• Continental drift theory laid the foundation for modern plate tectonics.
• Understanding Earth’s dynamics helps in disaster prediction, resource exploration, and geological studies.