Unit 2: Earth's Interior, Systems, and Processes

Interior of the Earth

We cannot directly observe the Earth's interior. Our knowledge comes from indirect evidence, primarily the study of seismic waves (waves from earthquakes) as they travel through the Earth. Changes in the speed and direction of these waves reveal boundaries between different layers.

Layers of the Earth

The Earth is composed of three main chemical layers: the Crust, the Mantle, and the Core.

Layer	Sub-layer	Approx. Depth	Composition	State
Crust	Continental Crust	0 - 70 km (avg. 35 km)	Granitic (lighter, rich in Si, Al - "SIAL")	Solid
	Oceanic Crust	0 - 10 km (avg. 7 km)	Basaltic (denser, rich in Si, Mg - "SIMA")
Mantle	Upper Mantle	~10 - 660 km	Peridotite (ultramafic rock, rich in Fe, Mg)
	Lower Mantle	660 - 2900 km
Core	Outer Core	2900 - 5150 km	Iron (Fe) and Nickel (Ni) - "NIFE"	Liquid
	Inner Core	5150 - 6371 km		Solid (due to immense pressure)

Important Discontinuities

These are boundaries between layers, identified by abrupt changes in seismic wave velocity.

• Mohorovičić Discontinuity ("Moho"): The boundary between the Crust and the Mantle.
• Gutenberg Discontinuity: The boundary between the Mantle and the Outer Core.
• Lehmann Discontinuity: The boundary between the Outer Core (liquid) and the Inner Core (solid).

Earth's Spheres and their Interrelationship

Earth science is often viewed as the study of four interconnected "spheres." These systems are not isolated; they constantly interact, transferring energy and matter.

1. Lithosphere (or Geosphere):
   • What it is: The solid, rocky part of the Earth. It includes the crust and upper mantle (as defined above).
   • Contains: Rocks, minerals, soil, landforms (mountains, volcanoes, etc.).
2. Hydrosphere:
   • What it is: All the water on Earth in all its forms.
   • Contains: Oceans (which hold 97% of Earth's water), rivers, lakes, groundwater, glaciers (cryosphere), and water vapor.
3. Atmosphere:
   • What it is: The envelope of gases surrounding the Earth.
   • Contains: Nitrogen (78%), Oxygen (21%), and other trace gases. It protects us from solar radiation and regulates temperature.
4. Biosphere:
   • What it is: All living organisms on Earth.
   • Contains: Plants, animals, fungi, bacteria. It overlaps with the other three spheres.

Interrelationships (Examples)

No sphere works alone. A change in one sphere triggers changes in the others.

• Volcanic Eruption (Lithosphere): A volcano (Lithosphere) erupts, releasing gases and ash into the Atmosphere. This ash can block sunlight, cooling the planet. The gases (like SO₂) can mix with water (Hydrosphere) to form acid rain, which can harm plants and animals (Biosphere).
• Water Cycle (All Spheres): Water evaporates from the Hydrosphere into the Atmosphere. It condenses and falls as rain, eroding rocks (Lithosphere). This water is then used by life (Biosphere).
• Photosynthesis (Biosphere, Atmosphere): Plants (Biosphere) take CO₂ from the Atmosphere and release oxygen.

Endogenic and Exogenic processes of the Earth

The Earth's surface is in a constant state of change, shaped by two opposing sets of forces.

Endogenic Processes (Internal)

These processes originate from within the Earth. They are driven by the Earth's internal heat (from radioactive decay and residual heat from formation). They are constructive processes that build up landforms.

• Tectonics (Diastrophism): The large-scale movement and deformation of the Earth's lithosphere (plates). This includes:
  • Folding: Bending of rock layers (forming mountains like the Himalayas).
  • Faulting: Breaking and sliding of rock layers (causing earthquakes).
• Volcanism (Igneous Activity): The eruption of molten rock (magma) onto the surface (lava), or its intrusion into the crust. This forms volcanoes and igneous rocks.
• Earthquakes (Seismicity): The sudden release of energy in the Earth's lithosphere that creates seismic waves.

Exogenic Processes (External)

These processes originate at or near the Earth's surface. They are driven by external energy, primarily from the sun and gravity. They are destructive processes that wear down landforms.

The general term for this "wearing down" is denudation, which includes weathering and erosion.

• Weathering: The in-situ (in-place) breakdown of rocks and minerals at the Earth's surface.
  • Physical (Mechanical) Weathering: Breaking rocks into smaller pieces (e.g., frost action, thermal expansion).
  • Chemical Weathering: Altering the chemical composition of the rock (e.g., rust/oxidation, dissolution by acid rain).
• Erosion: The transportation of weathered material (sediment) from one place to another. The main agents of erosion are:
  • Water: Rivers, rain, ocean waves.
  • Ice: Glaciers carving out valleys.
  • Wind: Blowing sand in deserts.
  • Gravity: Mass movements like landslides and creep.
• Deposition: The "dropping" or settling of eroded sediment when the agent of transport (like a river) loses energy. This builds up landforms like deltas, beaches, and sand dunes.