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

Principle of Superposition

The principle of superposition states that in a sequence of undisturbed sedimentary layers, the oldest layers are on the bottom and the youngest layers are on the top.

This principle was formulated by Nicolas Steno in the 17th century.

Thrust faults can alter the understanding of the principle of superposition because they occur parallel to bedding and can be hard to detect. They can create situations where older layers overly younger layers. Scientists of Nicolas Steno's time were unaware of thrust faults.

Anderson's Theory of Faulting

Assuming that there is no shear stress at the Earth's surface (shear stress cannot occur in fluids), one of the principal stress components must be vertical and thus the other two must be horizontal.

σv = σ1 normal fault

σv = σ2 reverse fault

σv = σ3 strike-slip fault

Anderson's Theory of Faulting

Normal, Reverse, and Strike-Slip Faults

Top 10 Signs You Might be a Geologist

10. You've responded "yes" to the question, "What have you got in there, rocks?"
9. You've taken a 15-passenger van over "roads" that were really intended only for cattle.
8. You've found yourself trying to explain to airport security that a rock hammer isn't really a weapon.
7. Your rock garden is located inside your house.
6. You've hung a picture using a Brunton as a level.
5. Your collection of beer cans and/or bottles rivals the size of your rock collection.
4. You consider a "recent event" to be anything that has happened in  the last hundred thousand years.
3. Your photos include people only for scale and you have more pictures of your rock hammer and lens cap than of your family.
2. You've been on a field trip that included scheduled stops at a gravel pit and/or a liquor store.
1. You have uttered the phrase "Have you tried licking it?" with no sexual connotation involved.

Ramsay Fold Classifications

John Ramsay's Fold Classification

John Ramsay proposed a classification scheme for folds that is used to describe folds in profile based upon curvature of the inner and outer lines of a fold, and the behavior of dip isogons.

dip isogon: a line that connects points of equal inclination or dip on the outer and inner bounding surfaces of a folded layer

Class 1 - Dip isogons converge downward towards axial surface, signifying that the curvature of the outer arc is less than that of the inner arc

Class 1A - Limbs thicker than hinges

Class 1B - Layer thickness constant; parallel fold

Class 1C - Limbs thinner than hinges

Class 2 - Dip isogons are parallel, signifying that the curvature of the outer arc exactly matches the curvature of the inner arc; similar fold

Class 3 - Dip isogons diverge downward towards axial surface, signifying that the curvature of the outer arc is greater than that of the inner arc

Class 1B (parallel) and Class 2 (similar) folds are the most common folds seen in the field. Concentric folds are a special case of Class 1B (parallel) folds where the outer and inner bounding surfaces define arcs with a common center of curvature. These types of folds are common in upper crustal tectonic settings, where most deformation occurs by processes that only permit limited ductile flow of rock. Most of the deformation is accommodated by slip on bedding or layer boundaries (flexural slip folding). Class 2 (similar) folds have relative thinning of the limbs and thicking of the hinges. These types of folds are common in metamorphic terranes, where most deformation occurs by processes that permit extensive ductile flow of rock.


Sources:
Folding and Fracturing of Rocks, John G. Ramsay, 1967
Structural Geology of Rocks, 2nd Edition, George H. Davis & Stephen J. Reynolds, 1996
http://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-113-structural-geology-fall-2005/lecture-notes/part6_dctl_fldfb.pdf