Earth Structure and Plate Tectonics Notes

Why is our earth unstable?

Structure of the Earth

• The Earth is divided into three layers: crust, mantle, and core.
• From the surface to the center, these layers are arranged as follows: crust, mantle, then core.
• The crust is the thinnest layer, while the core is the thickest.
  • Core thickness: $3,470$ km.
  • Mantle thickness: $2,900$ km.
  • Crust thickness:
    • Oceanic crust: $6-8$ km.
    • Continental crust: $25-70$ km.
• Layers with lighter, less dense minerals lie on top.
  • Core average density: $11$ g/cm\textsuperscript{3}
  • Mantle average density: $3.3$ g/cm\textsuperscript{3}
  • Crust average density: $2.7$ g/cm\textsuperscript{3}

Detailed Look at Earth's Layers

• Crust: Solid state.
• Mantle: Includes a plastic asthenosphere, a solid lower mantle and a solid uppermost part.
• Core: Features a liquid outer core and a solid inner core.

Plates and Plate Tectonics

• The lithosphere is broken into pieces called plates.
• Examples of plates include the Pacific Plate, North American Plate, Eurasian Plate, African Plate, Antarctic Plate, Indo-Australian Plate, Nazca Plate, and others.
• Hong Kong rests on the Eurasian Plate.

Characteristics of Plates

• Plates are irregular in shape and of different sizes and thicknesses.
• Some plates carry both oceanic and continental crust; some carry only oceanic crust.
• Plates move slowly in different directions.

Plate Movement

• Two major forces lead to plate movement: ridge push and slab pull.

Types of Plate Boundaries

• The border between two plates is called a plate boundary.
• There are three types of plate boundaries:
  • Divergent Plate Boundary: Plates move apart; new crustal material is formed.
  • Convergent Plate Boundary: Plates move towards each other; the denser plate subducts into the asthenosphere.
  • Transform Plate Boundary: Plates slide horizontally past each other; no crustal material is formed or subducted.

Crust Types

• There are two types of crust: continental crust and oceanic crust.

Continental Crust

• Continental crust makes up the continents and the continental shelves, which are gently sloping plains bordering the continents.
• Continental crust is thick ($25-70$ km) and light.

Oceanic Crust

• Oceanic crust is found under the oceans.
• Oceanic crust is thinner ($6-8$ km) and denser than continental crust.

Mantle

• The mantle lies below the crust and is denser.
• The mantle can be divided into three parts:
  • Uppermost part of the mantle (Lithosphere): Solid state; together with the crust, they are collectively known as the lithosphere.
  • Asthenosphere: Plastic state; lies below the lithosphere ($350-500$ km).
  • Lower Mantle: Solid state due to high pressure.

Core

• The core is the hottest and densest part of the earth.
• It can be subdivided into two parts:
  • Outer Core: Material is in liquid form due to intense heat.
  • Inner Core: Material is kept in solid form because it is under the greatest pressure.