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

Near passerine? How can you be near passerine?

Near passerine? How can you be near passerine? Doesn't that just make you non-passerine?

I know very little about bird lineages, and I recently read an article about the replacement of non-passerine birds in Europe with passerine birds. I was trying to determine what passerine versus non-passerine birds were, generally, but when I looked up woodpeckers I found that they, and several other lineages, were known also as "near passerines." My search led me to citations that stated that near passerines are a group that are believed to be related to true passerines due primarily to ecological similarities. It is thus far undetermined whether or not all near passerines are related to true passerines, but new molecular data apparently makes it appear unlikely. So, that is, briefly, a near passerine.

Subduction zones and earthquakes

I am still waiting to hear from a couple friends in Japan, and I hope to hear that they and their families are safe. There are also extensive fears about nuclear radiation, as well as problems with evacuations and shelter for those displaced, throughout the Tohoku region. There are many good resources for information on these factors, so I will not focus on them.

Instead I will focus on the geological effects of this earthquake, since I understand these a bit better.

The 8.9 magnitude earthquake has shifted the Earth of its access by about 16.5 cm. This will cause the planet to rotate faster and shorten the length of the Earth's day by 1.8 millionths of a second or so. It has also moved the coastline of Japan in the Tohoku region by about 4 m to the east in some regions.
The cause of this earthquake is due to the subduction zone which lies to the east of Japan (the thick line to the east of the epicenter on the image below). The Pacific Plate is being subducted, or pulled under, the North American Plate, on which Japan lies. The motion of the scraping of the two plates together builds up strain energy which then must be released at some indeterminable period. The release of the strain results in an earthquake, in this case a very large earthquake.

When the movement of the Earth occurred underwater, it displaced a large bit sea water and thus created the large tsunami which hit the coastal regions of the Tohoku region. The Pacific Plate's maximum westward movement was about 20 m, with the movement along the fault reducing as you go away from the epicenter. This is also why the amount of shaking and damage decreased away from the epicenter and fault.





BBC - How the quake has moved Japan

8.9 Magnitude Earthquake Offshore Japan

An 8.9 magnitude earthquake occurred offshore of Honshu, Japan, on Friday, March 11, 2011, at 14:26 local time. This is the largest earthquake the country has ever experienced, and the seventh largest ever recorded. It occurred 382 km northeast of Tokyo.

The towns along a 2,100-km range of eastern coastline of Honshu were affected by the earthquake and its 19 aftershocks, most of which were greater than 6.0 magnitude. A 7-metre tsunami was launched which carried cars, boats, and even planes inland. At least 60 people have been killed.

Tsunami warnings have been issued for the west coast of South America, the west coast of the U.S., Hawaii, New Zealand, and other areas in the Pacific: Pacific Tsunami Warning Center.

More News:
Earthquake: Japan Hit by 8.9 Earthquake/Japan Tsunami Warning (Sydney Morning Herald)