Geology at 70 mph! EarthCache

To log this EarthCache you must fulfill the following requirements:

You will probably want to drive by this EarthCache once, maybe twice, and take a good look. I suggest slowing down to about 55 mph. There is no need to stop your car or get out at the coordinates.

E-mail to me, via the profile link above, the answers to the following questions:

• What type of rock is seen in this road cut: igneous, metamorphic, or sedimentary? How do you know?
• Is this an anticline or a syncline?
• What stress – tension, compression, or shear – is causing this feature? How do you know?

Any claimed finds without proper documentation within 48 hours will be deleted without notice.

Crustal Deformation

Rocks, whether igneous, sedimentary, or metamorphic, are subject to powerful stress by tectonic forces, gravity, and the weight of overlying rocks. The three types of stress are tension (stretching), compression (shortening), and shear (tearing and twisting). Strain is how rocks respond to these stresses, as expressed in folding (bending) and faulting (breaking). Whether a rock bends or breaks depends on several factors, including composition and how much pressure is on the rock. An important quality is whether the rock is brittle or ductile. The patterns created by these processes are evident in the landforms we see today. (Christopherson)

Folding and Warping

Syncline and anticline are terms used to describe folds based on the relative ages of folded rock layers. A syncline is a fold in which the youngest rocks occur in the core of a fold (i.e. closest to the fold axis), whereas the oldest rocks occur in the core of an anticline. (Source: enotes)

It is important to note that syncline and anticline do not necessarily relate to the shape or orientation of folded layers, although the origin of the words implies this. The term originates from the Greek word sun (xun), meaning together, and the Greek word klei, meaning to lean, so syncline implies leaning together or leaning towards. Ant is the Greek prefix meaning opposite or opposing, so the word anticline implies oppositely leaning. Beds dip towards the fold axis in a syncline and away from the fold axis in an anticline only when the folded layers were upright before folding (i.e., where younger layers overlaid older layers). Before describing folds, it is therefore necessary to establish the primary order in which layers were deposited. To do this, facing, younging, or way-up criteria are used. These are synonymous terms for primary sedimentary structures (e.g. graded or cross-bedding) or igneous structures (e.g. vesicles, pillows) preserved in the folded layers. Where the relative ages of rocks are not known (as is often the case in metamorphic rocks), the term synform and not syncline should be used to describe folds where layers are bent downwards so that they dip towards the fold axis, and antiform and not anticline should be used where beds are arched upwards so that layers dip away from the fold axis. (Source: enotes)

Where rock layers have been inverted prior to folding, such as by folding about a larger fold with a shallowly inclined axial surface, the oldest rocks now occur in the core of folds where layers dip towards the fold axis. Such folds are called synformal anticlines; synformal because of their shape and anticline because of the relative ages of folded layers. The youngest layers in an overturned sequence occur in the core of folds called antiformal synclines where layers dip away from the fold axis. (Source: enotes)

Application and Usefulness

Anticlines and synclines are useful to geologists for a variety of reasons. They give information regarding geologic history. By determining the age of a fold, geologists can determine when the earth’s crust in a given area was stressed. Furthermore, the orientation of the fold provides information regarding the direction of forces within the crust. (Source: encarta)

Sources:

Christopherson, Robert W. Elemental Geosystems (5th ed.). New Jersey: Pearson-Prentice Hall, 2007.

http://www.enotes.com/earth-science/syncline-anticline

http://facweb.bhc.edu