Study Guide for Exam 4, Spring 2002

GENERAL GEOLOGY (GEOL 1113)

STUDY GUIDE FOR EXAM IV

The following study guide is provided as an aid to help you identify the major concepts you should have learned concerning GEOLOGIC STRUCTURES (Ch. 15). Key vocabulary with which you should be familiar is highlighted in GREEN UPPER CASE throughout this document and on future documents of this type for the course. Additionally, some key words are highlighted in blue and underlined which means they contain hyperlinks to additional information of interest. Clicking on the blue words will transport you to various Internet locations or on-line images to enhance your studying. You should, at the very least, be able to define the highlighted terms in order to complete the exam. Ideally, however, I hope you will be able to do more than simply respond to definitions. I would like you to learn to be able to apply the definitions and the concepts they represent to a fuller understanding of the Principles of Geology and Earth as a planet by visiting the added hyperlinks.

GEOLOGIC STRUCTURES - GENERAL BACKGROUND

We have examined some basic elements of the Plate Tectonic Theory and have some knowledge regarding the common plate interactions (CONVERGENT, DIVERGENT, TRANSFORM) and the geological phenomena associated with these interactions. This background prepares us now to investigate the details of GEOLOGIC STRUCTURES which result from plate interactions and the basic methods geologists use to unravel Earth's GEOLOGIC RECORD.

In order to truly appreciate the GEOLOGIC RECORD OF EARTH'S HISTORY, geologists have had to learn the "language" in which that record is preserved. We need not only the skill to recognize specific geologic structures, but also need to appreciate elements of GEOLOGIC TIME and the manner in which this time is represented by rocks on the Earth.

Over the next few weeks, our task will be to develop a basic understanding of the rudiments of EARTH HISTORY. Our starting point will be a survey of GEOLOGIC STRUCTURES and their meaning with respect to Earth's history. Then, we will spend a good deal of our remaining time during the semester contemplating the nature of GEOLOGIC TIME. Together, an understanding of these elements of earth history permit geologists to interpret that history.

MECHANICAL BEHAVIOR OF ROCKS

We have already discussed the MECHANICAL BEHAVIOR OF MATERIALS on Earth and the subdivisions of Earth's interior based on the MECHANICAL PROPERTIES (as opposed to COMPOSITION OF MATERIALS).

In general, the MECHANICAL BEHAVIOR of materials can be considered to be their response when extreme forces are applied to them. On Earth, materials behave as BRITTLE SOLIDS, PLASTIC SOLIDS or FLUIDS.

BRITTLE DEFORMATION refers to the process of FRACTURING AND FAULTING when extreme forces are applied to a brittle solid.

PLASTIC DEFORMATION refers to the process of FOLDING when extreme forces are applied to a plastic solid.

Recall that rocks within the Earth behave as BRITTLE SOLIDS from the surface to depths of about 100 km (i.e. All rocks in the crust and those of the uppermost mantle exhibit brittle behavior). This region of Earth is called the LITHOSPHERE. The LITHOSPHERE includes all of the CRUST and a portion of the UPPER MANTLE (remember, crust and mantle are defined according the composition of rocks).

Rocks below 100 km behave as PLASTIC SOLIDS. This region is called the ASTHENOSPHERE. The transformation from brittle to plastic solids occurs because of the high temperatures and high pressures below 100 km. These conditions cause rocks to behave differently than at the surface.

In addition, we have discussed the concept of TECTONIC METAMORPHISM - that is, metamorphism associated primarily with CONVERGENT PLATE BOUNDARIES. At these boundaries, tremendous DIRECTED STRESSES can generate extraordinary PRESSURE along the zone of convergence, and this dramatic increase in pressure will cause rocks to behave as plastic solids, EVEN AT RELATIVELY SHALLOW DEPTHS in the lithosphere, perhaps as shallow as 10 KM!

remember, we ordinarily don't consider rocks to behave as plastic solids until depths of about 100 km which marks the transition from the lithosphere to the asthenosphere.

also recall that TEMPERATURE during TECTONIC METAMORPHISM is relatively low - the changes we associate with metamorphism are caused mainly by high pressure.

The geologic effect of these forces is to induce DEFORMATION of rocks which results in a variety of large-scale GEOLOGIC STRUCTURES.

GEOLOGIC STRUCTURES: Chapter 15

(Click button for Geologic Structures slide set)

In order to describe the GEOMETRY OF GEOLOGIC STRUCTURES, geologists have developed a variety of measures. The two most important measures are STRIKE and DIP. Let's examine these two measures to see how they help us interpret geologic structures.

STRIKE is the LINE OF INTERSECTION OF A ROCK LAYER OR GEOLOGIC STRUCTURE AND EARTH'S SURFACE.

DIP is the ANGLE OF INCLINATION of a rock layer and is measured FROM THE HORIZONTAL SURFACE OF EARTH.

Imagine a single layer of rock tilted as you can see above. This layer will intercept Earth's surface along a line. The red line shows the trend of this LINE OF INTERSECTION which geologists call STRIKE. Notice also that the layer is inclined at an angle relative to Earth's HORIZONTAL surface. This ANGLE OF TILT (marked by the yellow arrow) is called DIP.

STRIKE and DIP measurements permit geologists to determine the type of geologic structure, even if the subsurface orientation of layers is not visible!

GEOLOGIC STRUCTURES are classified as either FOLDS or FRACTURES. Geologic structures are three-dimensional - they often cover many kilometers of Earth's surface, and can extend to some depth beneath Earth's surface. The 3-dimensional nature of the structures is often difficult for beginning students of Geology to grasp, but can be mastered with practice.

FOLDS result when rocks under great stress are DEFORMED PLASTICALLY. There are two principal classes of folds: SYNCLINES and ANTICLINES.

SYNCLINES are DOWNWARPED FOLDS. Synclines have CHARACTERISTIC GEOMETRY which is a consequence of the fact the it is folded downward:

ALL LAYERS OF THE SYNCLINE DIP TOWARD THE AXIS OF THE FOLD.

THE YOUNGEST ROCK UNIT IS EXPOSED IN THE AXIS OF THE FOLD.

THE OLDEST ROCK UNIT IS EXPOSED ON THE FLANKS OF THE FOLD.

ANTICLINES are UPWARPED FOLDS. Anticlines also have CHARACTERISTIC GEOMETRY related to the upwarped nature of folding.

ALL LAYERS OF THE ANTICLINE DIP AWAY FROM THE AXIS OF THE FOLD.

THE OLDEST ROCK UNIT IS EXPOSED IN THE AXIS OF THE FOLD.

THE YOUNGEST ROCK UNIT IS EXPOSED ON THE FLANKS OF THE FOLD.

FOLDS are not always as simple as those illustrated above. Many folds are intensely deformed during their formation, and the AXIS of the fold may be tilted as a result of this deformation. When the AXIS OF A FOLD IS TILTED, the fold is said to be PLUNGING. PLUNGING SYNCLINES and PLUNGING ANTICLINES also have characteristic geometries which can be described using strike and dip.

PLUNGING SYNCLINES are downwarped folds whose axis is tilted. However, they have all of the other characteristics of simple synclines in that

all layers dip toward the center of the structure

the youngest rock unit is exposed in the axis of the fold.

the oldest rock unit is exposed on the flanks of the fold.

The SURFACE OUTCROP PATTERN of a plunging syncline is different from that of a simple syncline. The rock layers of a plunging syncline form a V-SHAPED OUTCROP PATTERN on the earth's surface with the "V" OPENING in the direction of plunge.

PLUNGING ANTICLINES are upwarped folds whose axis is tilted. However, they have all of the other characteristics of simple anticlines in that

all layers dip away from the center of the structure

the oldest rock unit is exposed in the axis of the fold.

the youngest rock unit is exposed on the flanks of the fold.

The SURFACE OUTCROP PATTERN of a plunging anticline is different from that of a simple anticline. The rock layers of a plunging anticline form a V-SHAPED OUTCROP PATTERN on the earth's surface with the "V" CLOSING in the direction of plunge.

FRACTURES are simply CRACKS IN ROCKS. Fractures result from the BRITTLE BEHAVIOR of rocks exposed to STRESS. A special class of fractures are known as FAULTS.

FAULTS are fractures in rock across which there has been some DISPLACEMENT. That is, the two blocks of rock separated by the fracture have moved relative to each other. Again, there are different types of FAULTS which can be described according to the GEOMETRY OF DISPLACEMENT observed on the fault. That is, by examining the nature of displacement of the two rock blocks along the FAULT PLANE.

The types of displacement which can occur on a fault are VERTICAL, HORIZONTAL or a combination of vertical and horizontal movement called OBLIQUE.

Faults whose displacement is dominantly VERTICAL are called DIP SLIP FAULTS because the SLIP (displacement) occurs along the DIP of the fault plane.

If the displacement on a dip slip fault is such that the HANGING WALL IS DISPLACED DOWNWARD relative to the FOOTWALL (or the FOOTWALL IS DISPLACED UPWARD), that fault is said to be a NORMAL FAULT.

If the displacement on a dip slip fault is such that the HANGING WALL IS DISPLACED UPWARD relative to the FOOTWALL (or the FOOTWALL IS DISPLACED DOWNWARD), the fault is said to be a REVERSE FAULT.

Faults whose displacement is dominantly HORIZONTAL are called STRIKE SLIP FAULTS because the SLIP (displacement) occurs along the STRIKE of the fault plane.

If the displacement on a strike slip fault is such that the SENSE OF DISPLACEMENT ACROSS THE FAULT is to the RIGHT, then the fault is termed a RIGHT LATERAL STRIKE SLIP FAULT (or a RIGHT SLIP STRIKE SLIP FAULT).

If the displacement on a strike slip fault is such that the SENSE OF DISPLACEMENT ACROSS THE FAULT is to the LEFT, then the fault is termed a LEFT LATERAL STRIKE SLIP FAULT (or a LEFT SLIP STRIKE SLIP FAULT).

Faults whose displacement is a combination of both VERTICAL AND HORIZONTAL movements are called OBLIQUE FAULTS.

INTERPRETING GEOLOGIC STRUCTURES

You have seen some of the variety of geologic structures which can be found on Earth. Where are these structures usually found? How do they form? Do they all form in the same locations? Do they all form as a result of the same processes? If you want to see a fold, where on Earth would be a good place to look? How about a normal fault? A reverse fault?

As you might expect, there is a tendency for these structures to form in different TECTONIC SETTINGS.

FOLDS will be most common in those geologic settings where COMPRESSION is likely to occur. COMPRESSION is a dominant directed stress at CONVERGENT PLATE BOUNDARIES. Thus, much folding of rock layers will occur associated with plate convergence, subduction, etc.

The Ouachita and Appalachian Mountains are classic FOLDED MOUNTAIN BELTS, formed during the convergence (or collision) of two or more continents.

FAULTS occur in many geologic settings. Some faults will form as a result of PLATE DIVERGENCE (or RIFTING). As plates move apart, a type of directed stress called TENSION develops. TENSION is the force induced when you pull on something. If you pull hard enough, it is possible for rocks to FRACTURE. If the tensional force continues after the rock FRACTURES, blocks on opposite sides of the fracture may be displaced (that is, FAULTING WILL OCCUR).

Point to ponder: What type of fault do you think forms under TENSION? .

The north-south trending mountain ranges of the western United States were formed by TENSION which is literally tearing the western portion of North America apart!

TENSIONAL FAULTS ARE COMMON ACROSS NORTHWEST ARKANSAS!

STRIKE SLIP FAULTS are those associated with HORIZONTAL DISPLACEMENTS.

Point to ponder: Which TECTONIC SETTING is dominated by lateral movement of plates?

The SAN ANDREAS FAULT in California is a STRIKE SLIP FAULT. If I told you that Los Angeles is moving northward on the west side of the fault, and San Francisco is moving southward on the east side of the fault, what type of STRIKE SLIP FAULT is the San Andreas Fault? Sketch yourself a picture and see if you can get the right ( hint) answer!

Strike slip faults are most common at TRANSFORM PLATE BOUNDARIES.

This is a shaded relief image of a portion of California showing the San Francisco Bay Region. On the left is a raw image on which you can see the trace of the San Andreas Fault (a RIGHT LATERAL STRIKE SLIP FAULT). On the right is an image with the fault indicated by the red line. Also note the locations of San Francisco and Point Reyes. At the present time, these two landmarks are on different tectonic plates. The North American Plate is moving southward relative to the Pacific Plate. Thus, Point Reyes and San Fancisco are getting farther apart!

Finally, some faults form as a result of PLATE CONVERGENCE. As we already know, when plates collide folding occurs at some depth beneath Earth's surface (where rocks behave as plastic solids). However, at depths above the point where plastic behavior occurs, rocks are brittle. When subjected to great COMPRESSION, these rocks will FRACTURE. If compression continues after the rocks have fractured, large blocks of rock may be displaced along those fractures (which, by definition are now faults because there is displacement on them!).

Point to ponder: What type of fault would result from the process above?

REVERSE FAULTS are commonly associated with CONVERGENT PLATE BOUNDARIES.

Across the State of Arkansas, one may observe virtually all of the geologic structures that we have discussed in class. Consider how this can be. In this one state, we can observe geologic structures which formed at DIVERGENT PLATE BOUNDARIES and CONVERGENT PLATE BOUNDARIES.

Of course, the geologic stuctures did not all form at the same time - Arkansas's geologic history is long and varied. Early in the geologic history of Arkansas, our state was part of a DIVERGENT PLATE BOUNDARY. Later, the state became the site of a CONVERGENT PLATE BOUNDARY (this is the time during which the Ouachita Mountains formed and the area surrounding Fayetteville was uplifted above sea-level). Finally, the most "recent" geologic history of our state is that of a DIVERGENT PLATE BOUNDARY once more!

This concludes the study guide for Exam IV. Learning to visualize geologic structures is no easy task. You need to train your mind to visualize the rock layers in three dimensions - and most people are not accustomed to this. Indeed, for some it is very difficult (I hope this is not the case for you!) - but it can be done with lots of practice. I hope you will practice and learn the basic methods to recognize different structures on Earth. Many of you will drive past these structures on your journeys around Arkansas - it's always fun to see a fold in a road cut and be able to identify its features! If you can master the information on this study guide, I know you will do well on the exam. HAPPY STUDYING and GOOD LUCK! - Dr. Boss :-)

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