Isostasy - Airy & Pratt

Isostasy

• describes the state of equilibrium of a lithospheric plate floating on the asthenosphere
• the weight of columns of rock, at some depth called the depth of compensation, is everywhere equal

 Airy Isostasy

mountains have a crustal root that compensates for the additional relief (variation in thickness)

Pratt Isostasy

density varies laterally, so that mountains have a lower density than higher density, thinner portions of crust

Possible Mechanisms of Crustal Subsidence

Mechanisms that can generate sufficient subsidence to create basins:

Crustal thinning    extensional stretching, erosion during uplift, and magmatic withdrawal
Mantle-lithospheric thickening    cooling of lithosphere following either cessation of stretching or heating due to adiabatic melting or rise of asthenospheric melts
Sedimentary and volcanic loading    local isostatic compensation of crust and regional lithospheric flexure, dependent on flexural rigidity of lithosphere, during sedimentation and volcanism
Tectonic loading    local isostatic compensation of crust and regional lithospheric flexure, dependent on flexural rigidity of underlying lithosphere, during overthrusting and/or underpulling
Subcrustal loading    lithospheric flexure during underthrusting of dense lithosphere
Asthenospheric flow    dynamic effects of asthenospheric flow, commonly due to descent or delamination of subducted lithosphere
Crustal densification    increased density of crust due to changing pressure/temperature conditions and/or emplacement of higher-density melts into lower-density crust



Principles of Sedimentology and Stratigraphy (5th ed.), Sam Boggs, Jr.

Seismic Waves

Body Waves

• penetrate the body of the Earth
• travel faster in more elastic rocks
• body wave velocities increase with depth in the interior of the Earth
• subject to refraction and reflection
• increased rock temperature = decreased velocity
• increased confining pressure = increased velocity

Figure 1. A) P-wave motion. B) S-wave motion.

P-Waves

• primary waves, compressional waves
• fastest seismic waves
• wave motion: energy moves as a succession of compressions and expansions in the direction of wave travel - an accordion-like push-pull movement
• each square in the figure changes from square to rectangle to square again as the waves move through the rock
• travel through solids, liquids, & gases

S-Waves

• secondary waves, shear waves
• slower than P-waves
• wave motion: rock segments vibrate perpendicularly (at right angles, up-and-down or side-to-side) to the direction of wave travel - this more complex motion causes S-waves to travel more slowly
• travel through solids only

Surface Waves

• large-motion waves that travel through the outer crust of the Earth
• wave pattern resembles ripples caused when a pebble is dropped in a pond
• slowest seismic waves
• cause of destruction during an earthquake since they are channeled through the thin crust and their energy is less rapidly dissipated than body waves

Figure 2. a) Rayleigh wave motion. b) Love wave motion. 

c) Surface expression of wave motion of Rayleigh & Love waves

Rayleigh Waves

• wave motion is similar to waves in an ocean (see figure)

Love Waves

• wave motion is a shear which moves the surface from side to side

Metamorphic Index Minerals

Metamorphic index minerals form under specific temperature and pressure conditions.

This diagram shows shale being metamorphosed.

Chlorite and muscovite form at relatively low temperatures. Garnet forms at higher temperatures and pressures. Sillimanite indicates the highest level of temperatures and pressures.

Shale is metamorphosed to slate and then to phyllite. Schist is next to form, then gneiss when high-grade metamorphism is reached. Beyond 800°C, the rock may completely deform by melting.

Another version showing kyanite.

Bowen's Reaction Series

Bowen's Reaction Series is the order of mineral crystallization as a magma slowly cools.

The right branch of the chart is the continuous series of crystallization because the plagioclase minerals maintain the same basic crystal structure but change continuously in calcium and sodium content away from calcium-rich plagioclase towards a sodium-rich variety.

The left branch of the chart is the discontinuous series of crystallization because the reactions result in minerals of distinctly different structure.

Potassium feldspar, muscovite mica, and quartz do not react with the melt. By the time they crystallize, there is little liquid left.

Bowen's Reaction Series allows geologists to recognize why mineral variations exist in igneous rocks. A volcanic rock from an early eruption may be rich in iron, magnesium, and calcium and thus produce basalt. Therefore, later eruptions might be depleted in iron, magnesium, and calcium but enriched in potassium, sodium, and silica. These rocks would be less basaltic and more andesitic in composition.

Source: The Earth Through Time by Harold Levin