Structure and Physiography

Earth's radius is 6,371 km. No direct physical sampling is possible beyond the shallowest crust. Information is deduced from two source categories:

A. Direct Sources:

• Surface Rocks & Deep Mining: Gold mines in South Africa (Mponeng) reach depths of 3 to 4 km. Beyond this, intense heat and crushing pressure prevent human descent.
• Deep Ocean Drilling Projects: Deep Sea Drilling Project (DSDP) and Integrated Ocean Drilling Program (IODP). The deepest borehole on Earth—the Kola Peninsula Borehole in Russia—reached a record depth of 12.2 km.
• Volcanic Eruptions: Magma ejected during active volcanic eruptions provides direct laboratory specimens of molten rock originating from depths of 60 to 100 km (upper mantle).

B. Indirect Sources:

• Temperature, Pressure & Density Gradients: Temperature increases with depth at an average rate of $1^\circ\text{C}$ per 32 meters in the upper crust. Density increases from $2.7 \text{ g/cm}^3$ at the surface to over $13 \text{ g/cm}^3$ at the center.
• Meteors: Formed from the same primordial solar nebula as Earth; when meteorites fall to Earth, analyzing their dense iron-nickel core reveals Earth's internal composition.
• Gravitational Anomalies: Gravity ($g$) is greater at the poles and less at the equator (due to equatorial bulge). Deviations from expected gravity values (Gravity Anomalies) reveal uneven distribution of mass in the crust.
• Seismic Waves: The most authoritative, scientifically precise source of information regarding Earth's interior.

A. Earthquake Mechanics:

• Focus (Hypocentre): The exact point deep within the crust/mantle where stored tectonic strain energy is suddenly released, generating seismic shockwaves.
• Epicentre: The point on the Earth's surface vertically directly above the focus; the first surface point to experience seismic tremors.
• Measurement Scales:
  • Richter Scale: Measures earthquake magnitude (absolute energy released) on an open logarithmic scale of 0 to 10 (each whole number represents a 32-fold increase in energy!).
  • Mercalli Scale: Measures earthquake intensity (visible destruction and human impact) from 1 to 12.

B. Types of Seismic Waves:

C. The Earthquake Shadow Zones:

Seismographs located around the globe reveal specific zones where seismic waves fail to register:

• S-Wave Shadow Zone (Beyond $105^\circ$): S-waves do not appear beyond $105^\circ$ from the earthquake epicenter (covering over 40% of the entire globe!). Reason: S-waves cannot travel through liquids. This proves conclusively that Earth's Outer Core is molten liquid!
• P-Wave Shadow Zone ($105^\circ$ to $145^\circ$): P-waves appear up to $105^\circ$, vanish between $105^\circ$ and $145^\circ$, and reappear beyond $145^\circ$. Reason: P-waves are severely refracted (bent) as they enter and exit the dense liquid outer core, leaving a shadow band between $105^\circ$ and $145^\circ$.

1. The Crust (0 to 30–70 km):

• The brittle outer skin of the Earth. Forms barely 1% of Earth's volume.
• Oceanic Crust (Sima): Thin ($5 \text{ km}$ average), dense ($3.0 \text{ g/cm}^3$), composed of heavy basaltic rocks rich in Silica and Magnesium (Sima).
• Continental Crust (Sial): Thick ($30 \text{ km}$ under plains, up to $70 \text{ km}$ under the Himalayas), lighter ($2.7 \text{ g/cm}^3$), composed of granitic rocks rich in Silica and Aluminium (Sial).

2. The Mantle (30 to 2,900 km):

• Forms 84% of Earth's total volume. Composed of dense ultramafic peridotite rock rich in iron-magnesium silicates (density $3.4 - 5.5 \text{ g/cm}^3$).
• The Asthenosphere (Upper Mantle, 100 to 400 km): A semi-molten, ductile, plastic zone of partial melting. It is the primary magma source feeding volcanic eruptions.
• The Lithosphere: The rigid outer crust PLUS the uppermost solid mantle (depth: 10 to 200 km). Broken into tectonic plates that float atop the asthenosphere!

3. The Core / Barysphere (2,900 to 6,371 km):

• Forms 15% of Earth's volume. Composed of heavy Nickel and Iron (NiFe):
  • Outer Core (2,900 to 5,100 km): In a molten liquid state (confirmed by S-wave disappearance). Convection currents of molten iron generate Earth's protective Geomagnetic Dynamo.
  • Inner Core (5,100 to 6,371 km): In a solid crystalline metallic state due to tremendous crushing pressure ($3.6 \text{ million atmospheres}$), with an astonishing density of $13 \text{ g/cm}^3$ and temperature exceeding $6,000^\circ\text{C}$!

Key Seismic Discontinuities:

• Mohorovicic Discontinuity (Moho): Boundary separating Crust from Mantle.
• Gutenberg Discontinuity: Boundary separating Mantle from Outer Core (2,900 km).
• Lehmann Discontinuity: Boundary separating Liquid Outer Core from Solid Inner Core (5,150 km).

A. Major Types of Volcanoes:

1. Shield Volcanoes: Largest volcanoes on Earth (e.g., Hawaiian volcanoes like Mauna Loa). Composed of fluid, low-viscosity basaltic lava that travels long distances before solidifying, creating broad, gentle, sloping domes. Non-explosive unless water enters the vent.
2. Composite Volcanoes (Stratovolcanoes): Steep conical peaks built of alternating layers of viscous, silica-rich andesitic lava, pyroclastic ash, and cinders (e.g., Mount Fuji in Japan, Mount Vesuvius). Eruptions are violently explosive.
3. Calderas: The most explosive volcanoes on Earth. During eruption, they collapse into their own emptied magma chamber rather than building a tall cone, creating a massive collapsed crater depression (Caldera).
4. Flood Basalt Provinces: Gigantic outpourings of highly fluid basalt lava covering thousands of square kilometers. Exemplified by the Deccan Traps of India (covering 500,000 sq km, formed 65 million years ago).

B. Intrusive Volcanic Landforms (Plutonic Rocks):

Formed when molten magma cools and solidifies deep underground beneath the crust:

• Batholith: Colossal granitic magma chambers cooling deep in the crust, forming the massive core of mountain ranges.
• Laccolith: Large dome-shaped intrusive bodies with a flat horizontal base and arched top, connected by a pipe-like conduit from below (looks like a mushroom).
• Lopolith & Phacolith: Lopolith is a saucer-shaped shallow basin; Phacolith is a wavy lens-shaped mass at the crest of an anticline or trough of a syncline.
• Sill: Horizontal sheet of solidified magma parallel to bedding planes of sedimentary rock.
• Dyke: Near-vertical wall-like fissure intrusion cutting across rock strata.