Earth Science for Civil Engineers - Detailed Notes

Structure of the Earth

• The Earth is an oblate spheroid comprising three main concentric layers:

  • Core: Approximately 3486 km in radius.

  • Mantle: Approximately 2885 km thick, surrounds the core.

  • Crust: The outermost layer that floats on the mantle.

• Each layer of the Earth is subdivided into different layers based on chemical composition and physical properties.

Core

• The core constitutes 16.2% of the Earth's total volume.

  • Composed of two layers: inner core (solid, primarily iron (Fe) and nickel (Ni)) and outer core (molten, contains iron, sulphur (S)).

  • The outer core does not transmit seismic S-waves, indicating its liquid state.

Mantle

• Comprises about 82.3% of the Earth's volume.

  • Main composition is believed to be peridotite, rich in olivine and pyroxene.

  • Divided into:

  • Upper Mantle: Solid and does not absorb shear waves, includes lithosphere (upper 100 km).

  • Asthenosphere: Underlying layer that behaves plastically, extends to about 200 km.

Crust

• Constitutes only 1.5% of the Earth's volume.

  • Two types:

  • Oceanic Crust: Thin (5-15 km), composed of basaltic rocks (SIMA - silicate and magnesium), density ~3.2 g/cm³.

  • Continental Crust: Thicker (up to 70 km), composed of granitic rocks (SIAL - silicate and aluminium), density ~2.7 g/cm³.

Other Spheres

• Hydrosphere: All water on Earth’s surface and subsurface.

• Biosphere: Habitat for all living organisms.

• Atmosphere: Gases enveloping the Earth.

Isostasy

• The concept that Earth's crust floats on the denser mantle.

  • When weight increases (e.g. mountain building), the crust sinks (I).

  • When weight decreases (e.g. erosion), the crust rises (II and III).

  • Example: Scandinavia's land rising post ice cap melting.

Plate Tectonics

• The movement of Earth's lithosphere described by the theory of plate tectonics, evolving from the earlier continental drift theory.

• Major plates: several large and small slabs making up the outermost layer.

• Plates move due to convection currents in the asthenosphere: typical movement rates of 1-15 cm/year.

Types of Plate Boundaries

1. Constructive or Divergent Boundaries:

   • Plates move away from each other, new oceanic crust forms.

   • Examples: Mid-Atlantic Ridge, East Pacific Rise.

2. Destructive or Convergent Boundaries:

   • Plates collide, causing subduction:

   1. Oceanic-Oceanic: One plate subducts forming island arcs (e.g., Japan).

   2. Oceanic-Continental: Oceanic plate subducts under continental, forming mountains and volcanic activity (e.g., Andes).

   3. Continental-Continental: Both plates collide, forming large mountain ranges (e.g., Himalayas).

3. Conservative or Transform Boundaries:

   • Plates slide past one another, leading to earthquakes (e.g., San Andreas Fault).

How Plate Tectonics Works in a Nutshell

1. Lithosphere: Divided into continental (lighter) and oceanic (denser) types.

2. Asthenosphere: Sem-solid rock layer beneath lithosphere, allows plates to float.

3. Convection Currents: Heat and pressure in the mantle can cause softening, creating currents that drive plate movement.

4. Divergent Boundaries: Occur when convection currents pull plates apart, leading to rift formation and ocean basin development.

5. Subduction Zones: Formed where oceanic plates sink beneath others, generating volcanic activity and mountain ranges.

Evidence for Plate Tectonics

• Fit of Continental Margins: How continents align.

• Match of Continental Geology: Similar rock types across oceans.

• Paleobiogeography: Distribution of fossils aligning with plate movements.

• Magnetic Evidence: Magnetic readings show shifts in plates.

• GPS Measurements: Current movements of tectonic plates are continuously monitored.