Earth Science Concepts: Driving Forces, Resistivity, and Rotation

Posted on Apr 1, 2025 in Geology

Driving Forces in Plate Tectonics

  1. Slab Pull: Descending slab pulls the slab downwards into the mantle.
  2. Mantle Drag: Can be either a driving force or a restricting force, depending on the direction of the mantle movement with respect to the overlying plate.
  3. Ridge Push: Sum of two effects:
    1. Upwelling magma pushes the plates away from the ridge.
    2. Elevated ridges encourage gravitational sliding towards the trenches.
  4. Trench Suction on the Upper Plate: Vertical pull on the descending plate causes a pulling of the upper plate toward the trench.
  5. Hotspot Force

Electrical Properties of Rocks

Voltage: Electric potential difference, the difference in electric potential energy between two points per unit electric charge. The voltage between two points is equal to the work done per unit of charge against a static electric field to move the test charge between two points.

Current: The flow of electric charge (moving electrons or ions in an electrolyte, plasma = both electrons and ions).

Permittivity: The ability of a substance to store electrical energy in an electric field / the measure of resistance that is encountered when forming an electric field in a particular medium. The amount of charge needed to generate one unit of electric flux in a particular medium – the measure of a material’s ability to resist an electric field.

Resistivity: Intrinsic property that quantifies how strongly a given material opposes the flow of electric current.

Conductivity: Reciprocal of electrical resistivity, i.e., measures a material’s ability to conduct an electric current.

Isochron: A line on a diagram or map connecting points relating to the same time or equal times. If all data points lie on a straight line, this line is called an isochron. The better the fit of the data points to a line, the more reliable the resulting age estimate. Since the ratio of the daughter and non-radiogenic isotopes is proportional to the ratio of the parent and non-radiogenic isotopes, the slope of the isochron gets steeper with time.

Factors Influencing Resistivity/Conductivity in Rocks:

  1. Porosity (connected/effective – fractures or pores)
  2. Pore saturation (% air or gas)
  3. Hydrocarbon fluid saturation
  4. Water content
  5. Water salinity
  6. Clay content
  7. Metallic sulfide mineral content
  8. Fluid temperature
  9. Rock matrix intrinsic resistivity

Causes of Changes in the Earth’s Rotation

  1. Effect of lunar tidal friction on the length of the day (unelastic Earth creates a decelerating torque).
  2. Slow changes in the Earth’s rotation axis location and speed due to changes in mass distribution of the Earth (hot, lighter mantle plumes; cold, denser subducting slabs; plate configurations – continental crust lighter). These cause:
    1. Changes in moment of inertia (slowing down or speeding up of the Earth’s rotation speed).
    2. Rotation axis seeking to align with the center of the mass.
  3. Short-term fluctuations due to changes in angular momentum within the Earth’s atmosphere and core (global wind speed, angular momentum of core, i.e., fluid speed).
  4. Increase of the Earth – Moon distance (another effect of lunar tidal friction).
  5. The Chandler wobble (displaced rotational axis executes a circular motion about its mean position ~Euler nutation or free nutation).
  6. Precession and nutation of the rotation axis (gravity-induced changes due to Moon and Sun; precession has a 26,000-year periodicity).
  7. Milankovitch climatic cycles (gravitationally driven changes in the orientation axis direction and shape and orientation of Earth’s orbit affect the amount of radiation Earth receives from the Sun, leading to periodic changes in Earth’s climate).

Bouguer Anomaly

∆g: Bouguer anomaly

g: Measured value of g (obtained with the gravimeter in the field)

∆gFA: Free air correction – compensates for gravity’s decrease with distance from the Earth’s surface.

−∆gB: Bouguer correction – corrects for a rock layer of thickness h between the measurement elevation and the reference level.

∆gT: Terrain correction – correction for nearby topography which affects the gravity measurement upward due to excess mass or too little mass (hills and valleys).

∆gD&T: Drift and tidal correction – the elastic properties of the spring change and the solid Earth moves with the tides about 1 m with every tidal cycle.

−gN: Normal gravity – needed to subtract from the measurements in order to remove the reference ellipsoid and end up with only the gravity anomaly.