Notes on Internal Heat, Magma Formation, and Crustal Deformation

Internal Heat

• Geothermal gradient: the increase in temperature with depth in Earth’s layers; nonlinear.
• Earth’s core temperature:
  \sim 6000^{\circ}\mathrm{C}
  

Geothermal Gradient

• Definition: temperature rise with depth inside the Earth.
• Nonlinear profile across layers.
• Core temperature approximation: $6{,}000^{\circ}\mathrm{C}.$

Earth’s Interior Structure (simplified)

• Crust
• Upper Mantle
• Lower Mantle
• Outer Core
• Inner Core

Origin of Earth Internal Heat

• Primordial heat: heat retained from formation.
• 1) Accretion energy: growth of matter → heat from impacts.
• 2) Core formation: differentiation sinks dense material; heat released.
• 3) Adiabatic compression: high pressure traps heat; minimal exchange with surroundings.
• 4) Decay of short-lived radioisotopes: provides early heat.
  • Examples: $^{26}\mathrm{Al} \rightarrow ^{26}\mathrm{Mg}$ (half-life ≈ $7.17\times 10^{5}$ years); other short-lived isotopes listed include $^{236}\mathrm{U}$ and $^{205}\mathrm{Pb}$ (as per source).
• 5) Decay of long-lived radioisotopes: sustains heat over geological time.
  • Examples: $^{40}\mathrm{K}$, $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, $^{232}\mathrm{Th}$.
  • Example: $^{238}\mathrm{U} \rightarrow ^{206}\mathrm{Pb}$ with a half-life of about $4.5\times 10^{9}$ years.

Magma Formation

1. Decompression melting

• Pressure decreases with ascent in mantle; bonds weaken.
• Common at divergent plate boundaries.

1. Flux melting

• Volatiles added to hot, dry rocks; lowers melting point.
• Common in subduction zones.

1. Heat-transfer melting

• Heat from rising magma transfers to surrounding rocks, causing melting.
• Viscosity considerations.
• Magma classifications: $\text{Basaltic}, \text{Andesitic}, \text{Rhyolitic}.$

Factors Affecting Magma

• Crystal fractionation: denser materials settle; composition varies with depth (basaltic bottom, rhyolitic top).
• Partial melting: melting of surrounding rocks changes magma composition.
• Magma mixing: convection currents blend magmas from different chambers.

Crustal Deformation

• Definition: any change in shape, position, or volume of a rock layer.
• Stress causes strain in rocks.

Stages of Deformation

• Elastic deformation
• Ductile deformation
• Fracture

Types of Stress

• 1) Compressional: squeezing → shortening and thickening.
• 2) Tensional: pulling apart → extension.
• 3) Shearing: sliding in opposite directions.

Figure caption (as described in source)

• The three principal types of stress: A. compression; B. tension; C. shear.