EES215

Lecture 4

Plate Tectonics (cont.)

Formation of rocks at boundaries: diverging boundaries have basaltic volcanism: dominated by minerals with high melting point (high Me/O ratio), volcanic rocks à oceanic crust à high density, young age

Convering boundaries typically have andesitic volcanism: dominated by minerals with low melting point (low Me/O ratio), often plutonic rocks à continental crust à low density, old age

Plate tectonic processes started early in the Earth’s history, other planets do not have similar processes

Hot spots: (almost) stationary locations of thermal anomalies, produced by upwelling of hot material from core-mantle boundary, independent of plate tectonic boundaries. Example Hawaii-Emperor trace. Hot spot produce trace due to movement of plate over stationary upwelling, leaving set of volcanic island and seamounts.

Seismology

Earthquake: Sudden release of energy along fault plane à relative movement of blocks, displacement

Three types of boundaries between blocks: extensional movement à normal fault; compressional movement à thrust fault; tangential movement à strike-slip fault (Fig. 1)

Definitions: focus (hypocenter) location of start of break between blocks; epicenter: projection of location of focus to surface. (Fig. 2)

Example of lateral displacement along the San Andreas Fault (Fig. 3)

Seismology: study of propagation of seismic waves through earth; sudden forces (stress) exerted on solid material (rock), which causes temporal deformation of rock (strain) which propagates through the earth

Relation between stress, strain and velocity of propagation; leads to definition of equations for elastic waves: waves with two types of components, one with movement of individual particles parallel to direction of propagation of wave: compressional or P-wave; one with movement perpendicular to direction of propagation: shear wave or S-wave (for more information about derivation of seismic wave equations see appendix 2 in Fowler, Solid Earth).

Wave equations define propagation of two types of seismic waves:

Compressional wave (p-waves) with velocity a=Ö (l+2m)/r = Ö (K+4/3m)/r;

Shear wave (s-waves) with velocity b= Ö m/r

Definition of elastic constants:

m shear modulus (or rigidity): N/m² = Pa (Pascal);

l Lamé constant: N/m² = Pa;

K = l + 2/3 m bulk modulus or modulus of imcompressibility N/m² = Pa

Examples:
Rock:	Density r, kg/m^3	Bulk modulus K, 10^10 N/m^2	Shear modulus m, 10^10 N/m^2
Shale	2600	1.5	1.0
Limestone	2,200	3.4	2.5
Granite	2,700	5.5	2.75
Gabbro	2,900	6.6	4.4

Note: These values are averages, the actual range for rock types is quite large.

SUMMARY

• Earthquakes - release of energy from the inner of the earth; concentrated at plate boundaries, especially converging boundaries.
• Earthquakes (and some artificial means) cause seismic waves, which propagate through the earth; mode of propagation is fx of material properties (density; rigidity; compressibility): P-waves; S-waves.

Study of seismic waves gives information of material characteristics of solid earth (main source of information for layering of earth; density, temperature distribution; element distribution); used widely for study of crust - mineral deposits; oil reservoirs; aquifers; stability for major constructions; presence of faults etc.