Red Mountain Open Space: Fold it in EarthCache

Geologic Structures

In a nearby EarthCache, you should have learned about the Anticline at this location. However there is still more to learn. There are two other types of folds which you will learn about here.

Sedimentary rocks are important for deciphering the geologic history of a region because they follow certain rules. First, sedimentary rocks are formed with the oldest layers on the bottom and the youngest on top. Second, sediments are deposited horizontally, so sedimentary rock layers are originally horizontal, as are some volcanic rocks, such as ash falls. Finally, sedimentary rock layers that are not horizontal are deformed in some manner. Often times looking like they are tiling into the earth.

You can trace the deformation a rock has experienced by seeing how it differs from its original horizontal, oldest-on-bottom position. This deformation produces geologic structures such as folds, joints, and faults that are caused by stresses.

Folds

Rocks deforming plastically under compressive stresses crumple into folds. They do not return to their original shape. If the rocks experience more stress, they may undergo more folding or even fracture. There are three major types of rock folding: monoclines, synclines, and anticlines. A monocline is a simple bend in the rock layers so that they are no longer horizontal. Anticlines are folded rocks that arch upward and dip away from the center of the fold. The oldest rocks are at the center of an anticline and the youngest are draped over them. When rocks arch upward to form a circular structure, that structure is called an adome. A syncline is a fold that bends downward, causing the youngest rocks are to be at the center and the oldest are on the outside. When rocks bend downward in a circular structure, that structure is called abasin. If the rocks are exposed at the surface, where are the oldest rocks located?

How did this happen?

The formation of an orogen can be accomplished by the tectonic processes such as oceanic subduction (where a continent rides forcefully over an oceanic plate for accretionary orogeny) or continental subduction convergence of two or more continents for collisional orogeny).

Orogeny usually produces long arcuate (from the Latin arcuare, "to bend like a bow") structures, known as orogenic belts. Generally, orogenic belts consist of long parallel strips of rock exhibiting similar characteristics along the length of the belt. Although orogenic belts are associated with subduction zones, subduction tectonism may be ongoing or past processes. The subducting tectonism would consume crust, thicken lithosphere, produce earthquake and volcanoes, and build island arcs in many cases. Geologists attribute the arcuate structure to the rigidity of the descending plate, and island arc cusps relate to tears in the descending lithosphere. These island arcs may be added to a continental margin during an accretionary orogeny. On the other hand, subduction zones may be reworked at a later time due to lithospheric rifting, leading to amphibolite to granulite facies metamorphism of the thinned orogenic crust.

The processes of orogeny can take tens of millions of years and build mountains from plains or from the seabed. The topographic height of orogenic mountains is related to the principle of isostasy, that is, a balance of the downward gravitational force upon an upthrust mountain range (composed of light, continental crust material) and the buoyant upward forces exerted by the dense underlying mantle.

Frequently, rock formations that undergo orogeny are severely deformed and undergo metamorphism. Orogenic processes may push deeply buried rocks to the surface. Sea-bottom and near-shore material may cover some or all of the orogenic area. If the orogeny is due to two continents colliding, very high mountains can result (see Himalayas).

An orogenic event may be studied: (a) as a tectonic structural event, (b) as a geographical event, and (c) as a chronological event.

Orogenic events:

• cause distinctive structural phenomena related to tectonic activity
• affect rocks and crust in particular regions, and
• happen within a specific period

Based on what you read above and what you see here answer the following questions:

1. You should be able to see a fold to the side of the Anticline, are you looking at a monocline or a syncline?
2. What factors helped you to make this determination?
3. How was this geologic formation produced?
4. Take a photo of yourself or a proxy at or near this location to demonstrate that you visited the site. (Required)

1. "EarthCache Logging Tasks". Geocaching.com. Retreived 2020-04-12
2. Frank Press (2003). Understanding Earth (4th ed.). Macmillan. pp. 468–69. ISBN 978-0-7167-9617-6.
3. ^ Yuan, S.; Pan, G.; Wang, L.; Jiang, X.; Yin, F.; Zhang, W.; Zhuo, J. (2009). "Accretionary Orogenesis in the Active Continental Margins". Earth Science Frontiers. 16 (3): 31–48. Bibcode:2009ESF....16...31Y. doi:10.1016/S1872-5791(08)60095-0.
4. ^ Gerald Schubert; Donald Lawson Turcotte; Peter Olson (2001). "§2.5.4 Why are island arcs arcs?". Mantle Convection in the Earth and Planets. Cambridge University Press. pp. 35–36. ISBN 978-0-521-79836-5.
5. ^ PA Allen (1997). "Isostasy in zones of convergence". Earth Surface Processes. Wiley-Blackwell. pp. 36 ff. ISBN 978-0-632-03507-6.
6. ^ Gerard V. Middleton; Peter R. Wilcock (1994). "§5.5 Isostasy". Mechanics in the Earth and Environmental Sciences (2nd ed.). Cambridge University Press. p. 170. ISBN 978-0-521-44669-3.
7. "Geologic Structures" https://courses.lumenlearning.com/geophysical/chapter/geologic-structures/ 2020. Retreived 2020-04-14