Earth’s Interior Structure and Composition

Posted on Dec 6, 2024 in Geology

Study of Earth’s Interior

Much of our knowledge about the Earth’s structure and composition comes from indirect studies, as direct observation is limited to approximately 10km deep within the crust.

Direct Methods

These methods involve analyzing material obtained through boreholes.

Indirect Methods

These methods rely on analyzing and interpreting the Earth’s physical characteristics, including seismic wave behavior, gravity, density, magnetism, internal heat emission, and meteorites.

Direct Methods

Based on the analysis of materials extracted through drilling.

Indirect Methods: Analysis of Meteorites

  • Iron Meteorites (Siderites): Composed of 80-90% iron and the remainder nickel, these meteorites are thought to represent the Earth’s core.
  • Stony-Iron Meteorites (Siderolites): Composed of a mixture of silicates and metallic iron and nickel, these are believed to reflect the composition of the mantle.
  • Stony Meteorites: Primarily composed of sodium, potassium, and calcium silicates, similar to the rocks in the Earth’s crust.

Analysis of Earth’s Internal Heat

The crust’s temperature increases by about 3°C per 100m of depth (geothermal gradient). The Earth’s center is estimated to be around 4600°C and is solid.

Analysis of Seismic Waves

Seismic waves originate at the hypocenter. Primary waves (P-waves) are faster and travel through both solids and liquids. Secondary waves (S-waves) are slower than P-waves and cannot travel through fluids. Wave propagation speed increases with material rigidity and density.

Earth’s Structure

Chemical Model

  • Crust: The thinnest and outermost layer, ranging from 7km beneath the ocean floor to 70km under mountain ranges.
  • Mantle: The intermediate layer between the crust and the core, extending to a depth of 2900km. It is divided into the upper mantle (crust to 660km) and the lower mantle (660km to 2900km).
  • Core: The innermost layer, divided into the outer core (2900km to 5150km) and the inner core (5150km to the Earth’s center at 6371km).

Physical Model

  • Lithosphere: The rigid, outermost layer with an average thickness of 100km.
  • Asthenosphere: The plastic layer beneath the lithosphere, extending to 660km. It has a highly plastic upper zone with magma currents and a transition zone where rigidity increases with depth.
  • Mesosphere: Located between the asthenosphere and the core, reaching a depth of 2900km.
  • Endosphere (Core): Lies between the mesosphere and the Earth’s center, divided into the outer core and inner core.

Lithosphere and Asthenosphere

Lithosphere

The Earth’s rigid, solid surface layer, composed of the crust and the uppermost part of the mantle. It is divided into tectonic plates that float and move on the asthenosphere.

Asthenosphere

The plastic layer beneath the lithosphere, characterized by a highly plastic upper zone with magma currents and a transition zone with increasing rigidity down to 660km.

Faults and Fractures

Faults and fractures occur in rocks when stress exceeds their deformation limit, causing them to break.

Fractures

Cracks in rocks where no significant displacement has occurred.

Faults

Fractures in rocks where there is relative displacement between the two blocks formed.

Folds

Folds are bends in rock strata caused by tectonic stresses when the rocks exhibit plastic behavior.

Folds Based on Layer Arrangement

  • Anticlines: Folds where the oldest strata are at the core.
  • Synclines: Folds where the youngest strata are at the core.

Fold Types

  • Upright: Axial plane is perpendicular to the horizontal.
  • Inclined: Axial plane forms an angle between 45° and 90° with the horizontal.
  • Overturned: Axial plane forms an angle between 0° and 45° with the horizontal.
  • Recumbent: Axial plane is parallel to the horizontal.

Earthquakes and Volcanoes

Volcanoes are structures formed around openings where magma erupts due to weaknesses in the crust. Magma is molten rock at around 1000-1200°C, containing abundant gases. Upon reaching the surface, magma loses its gas and becomes lava.

Types of Eruptions

  • Point Eruptions: Magma flows through a single vent.
  • Fissure Eruptions: Magma erupts through a long fissure or crack.

Products of Volcanic Eruptions

  • Pyroclasts: Solid fragments formed by cooling and consolidation of magma. Classified by size as ash, lapilli, and volcanic bombs.
  • Lava: Liquid rock formed when magma loses its gaseous components.
  • Gases: Released from magma during eruptions.