By this time, you have probably figured out the study of geology is far more than just looking at pocket-sized rocks and minerals in a box and that the various geologic landforms you see outside are often the result of millions if not hundreds of millions of years of twisting, eroding and melting of a constantly evolving planet. These processes, in turn, lead to the current position of the world's continents and oceans, its resources and even life on earth.
Structural geology is the study of the planet's landforms and the processes that make them. It is an incredibly diverse field which we will only scratch the surface of but should greatly assist you in developing an understanding of the world through a geologist's eyes. For example, how does a geologist make observations about not only what type of environment is present now but also what types of environments were present millions of years ago? What do the presence of folds and faults tell about the forces acting on a given location on earth? The answers to these questions and others like them all lie in an understanding of structural geology.
In this particular unit, we will focus on a variety of topics and lab investigations that feature some of the larger principles of physical geology and then apply them to a series of geologic problems. Let's take a look.
Siccar Point, on the east coast of Scotland, is a classic example of how structural geology can reveal the secrets of the Earth's distant past. At this site in 1788, James Hutton, nicknamed the Father of Modern Geology, challenged the thinking of the day that largely assumed the earth was a static, ever-constant planet. Hutton, however, looked at these vastly different rock layers that showed clear signs of being severely tilted and deformed and suggested that the earth was actually a dynamic place with immense forces acting upon it.
Early Unifying Principles
In all honesty, our understanding of the Earth is rather young as a scientific discipline. Faced with persecution from religious entities and an inability to accurately describe what they were actually seeing in front of their eyes, early geoscientists had to do the best they could with what they had. However, there were several moments of great discovery which helped to revolutionize how we thought of the earth and the principles theorized by these pioneers still play a key role in the field of structural geology.
James Ussher (1581-1656)
Every timeline has to start somewhere and one could argue that Geology's started with the Irish Archbishoip James Ussher. Ussher, like many of his day, sought to explain most of the earth's processes and features as a result of biblical events that all occurred in a relatively short period of time by a divine being. Ussher published his work "Chronology" in 1650 and indicated the earth was precisely 4004 years old from his literal interpretation of events in the Bible. He also deduced that the major landforms of the planet were all formed in a series of catastrophes including the Great Flood, etc. This idea, referred to as "catastrophism" was widely accepted in Ussher's day and was used to explain many otherwise unexplainable phenomena. For example, fossils of creatures unlike any other animals found on the planet in Ussher's time were generally explained away as evil and were destroyed in the Great Flood. Quite literally, they missed the boat (Noah's ark). This general idea of a world without change as presented by Ussher and others was eventually called the "static earth" theory.
Nicolaus Steno (1638-1686)
Danish geologist/anatomist Nicolaus Steno (right) perhaps represents the closest thing to a "missing link" in geologic thought the field has. Although he was a pious Lutheran, Steno nonetheless spent much of his career observing his natural surroundings and developed several key ideas that are fundamental to understanding structural geology today. In 1669, Steno postulated what is now known as the Law of Superposition which states that in underlying sedimentary rock, the lower the layer, the older the rock/event. Keep in mind that Steno still readily accepted that Biblical events may have caused the various landforms he could observe but he also correctly recognized they did not all occur at once and that, as long as the rocks he was observing were undisturbed, there was a recognizable pattern of old to young in consecutive layers. This recognition and Steno's theories eventually gave rise to the field of stratigraphy, the study of rock layers and their layering.
Second, Steno also correctly suggested that layers of sediment were laid down horizontally and would remain that way unless disturbed by some other force. This Principle of Original Horizontality is also a fundamental physical geology concept and has a key role to play when a geologist attempts to piece together the history of a region. Along with the Principle of Lateral Continuity which states that sedimentary deposits will spread out in all directions from their source until that environment changes, Steno was among the first to put geology on the correct scientific path.
James Hutton (1726-1797)
Ussher's notion of a static earth was widely accepted especially since and to suggest otherwise would bring a scientist in direction confrontation with Church doctrine. However, the 17th and 18th centuries were a time of great scientific enlightenment especially in Scotland where religion and science coexisted with much less fanfare than in many other European nations. A physician by trade, Edinburgh-born Hutton (right) also had an intense interest in the geology in and around Edinburgh and over time began to question the catastrophist philosophy in light of various rock formations he observed near his home. These observations led to a key principle of physical geology known as the Principle of Cross-Cutting Relationships builds upon Steno's ideas and was used to explain unusual geologic features like igneous intrusions that did not fit Steno's Original Horizontality model. Here, features that cut through another feature must be younger than the feature they cut regardless of which one rests above the other.
In 1785, Hutton also published his now famous work Theory of the Earth in which he theorized that the earth was much older than previous estimates and that the planet's landforms were the result of extremely slow yet constant forces that gradually built and
destroyed the geologic features he observed even to the present day. This idea, called uniformitarianism, became the basis for most future theories about the history of the earth. Since Hutton's ideas suggested the planet was actually changing over time rather than locked in a final form, his general ideas were eventually called the "dynamic earth" theory.
Charles Lyell (1797-1875)
Working in Hutton's footsteps, Charles Lyell (a fellow Scot, lawyer and geologist) took Hutton's ideas of uniformitarianism and garnered ever increasing support for the theory through his 1830 work Principles of Geology. A close friend of Charles Darwin, Lyell's first volume was actually among Darwin's library when he began his famous voyage on the HMS Beagle in 1831. Like Hutton, Lyell spent much of his field work in the diverse rock formations of the British Isles and found evidence of volcanoes, deserts, shallow seas and other environments very different from what was there at the time. In his work, Lyell theorized that the processes that worked upon the surface of the planet took eons to create and destroy what he was seeing and the evidence of fossils creatures vastly different from those found today was not the result of a Biblical flood but rather inexorable change over millions of years.
So, if the concepts presented by these pioneering scientists represented the pathway to truly understanding the earth's ever-changing surface, how can they be applied by the typical field geologist. Read on to find out.
Faults and Folds, Strike and Dip
Although the Principal of Original Horizontality presumes that sedimentary rock layers will be deposited horizontally, it should come as no surprise that they do not stay that way. Over time, various forces like tectonism and uplifting can bend and even break rock layers so that they look little like they did when first laid down. This section begins to look at the various structures that are produced after horizontal rock layers are changed and how they are quantified by geologists.
When rocks are deformed, a number of specific features can result. These features can, in turn, say a lot about the level of deformation, the geographic direction the pressure came from and can even be used to identify likely places where natural resources may be found. Below are a few of the most commonly found features.
Synclines, anticlines and monoclines
Synclines and anticlines are basic geologic structures that result when rocks are deformed to the point they begin to fold but do not actually break or fracture. These features are usually formed under lighter amounts of stress. They can be very small or massive in size depending upon the nature of the forces causing the deformation. Note in particular how, although both features obey the Law of Superposition, they are opposites of each other when it comes to the ages of the rock contained in them due to the nature of the fold. In a syncline, the youngest rock layer is located in the center of the fold whereas the anticline contains the youngest rocks at the edges. If only one side of the feature is present, it is referred to as a monocline. The sides of these features are referred to as limbs and the center is called a hinge.
When rocks reach their breaking point due to excessive deformation, they crack and form faults. Understand that faults can occur horizontally or vertically depending on the types of forces acting on the rock. For instance, if compression or tension is applied to the rocks (like along divergent or convergent tectonic boundaries) the rocks will typically fracture vertically as the rock is either pushed up or down. However, shearing forces (side to side) like those encountered along transform boundaries will reveal horizontal fault lines.
In the case of vertical faults (generically known as "dip slip" faults) the fault is identified by the hanging wall (the side of the fault that represents the upper block) and the footwall which represents the lower block. Where two adjacent blocks of rock are pulling away from each other by tensional forces, the hanging wall slips below the footwall and is called a normal fault. Where compressional forces are at work, the opposite occurs and the hanging wall is thrust up and over the footwall. This dip slip fault is known as a reverse fault. If this slip is at a very low angle, it is sometimes referred to as a thrust fault. If this same fault does not show itself at the surface, it is referred to as a blind thrust fault. California, for example, is riddled with compressional faults that are the cause of many of the region's earthquakes as North America continues to move westward against the Pacific Plate. In faults where shearing occurs horizontally, the faults are known as "strike slip" or lateral faults and are either identified as left or right depending on the direction of movement. The San Andreas Fault is perhaps the most famous strike slip fault.
Strike and Dip
As in most fields, one of the primary goals of research, etc. is for making money. Geology, as you have already seen, is no exception. The search for mineral and rock resources is not a random one. Prices to drill for ores and other valuable commodities is very expensive and must be conducted systematically and only after a full understanding of the underlying geology is gained. Part of this understanding must come from recognition of certain basic geologic structures whose presence is often the determining factor between striking it rich and finding financial ruin. Add to this reality that the rock layers may be severely folded and faulted like the examples provided above and the process of finding valuable resources is made especially difficult.
For geologists, the difficulty of finding resources is made even more complex due to the fact that they must work in three dimensions. In other words, they must be able to evaluate not only what is at the surface but also what is below it. Two specific measurements are used to assist geologists in evaluating subsurface rock layers. The first, strike (trend), is the compass direction of the line produced by the intersection on an inclined rock layer or fault with an imaginary horizontal plane at the surface. The strike of the rock layers is expressed as an angle relative to north. For example, a strike of "north 10° east" (N10°E) means the strike is ten degrees to the east of north. In the diagram at right, the strike of the rock layers is approximately north 60° east (N60°E).
Dip, the second measurement, is the angle of inclination of the surface of the rock unit from the horizontal plane. Dip includes both an angle of inclination and a direction toward which the layers are inclined. The direction of the dip will always be at a 90o angle to the strike.
On a typical geologic map, strike and dip are shown with a long line drawn in the direction of the strike and a short line extending from the center of the long one to show the direction of the dip (Letter A). At the end of the short line will be an angle measurement that shows the severity of the dip.
Geologic maps like the one of Pennsylvania shown at right are a fundamental tool of the geologist. Showing the geographic distribution of geologic layers exposed at the Earth’s surface as well as subsurface features, geologic maps can tell the astute geologist immense amounts of information about a region's history, its current geologic makeup and even the potential for certain resources. Because of erosion and deformation, it is often quite difficult to piece together the geologic layout of the Earth as these processes bend, fold, fault and erase many of the keys to understanding Earth’s geologic past. Geologic maps are therefore very useful in delineating the existence of exposed structural features like folds and faults, the locations and links of seemingly unrelated strata, and the presence of important igneous and metamorphic features. Most often, the basic
geologic map is created from data obtained by field geologists conducting detailed mapping projects of a local area using a standard topographic map as a reference. However, with today’s modern technology, aerial photos and even satellite imagery are now used to map much of the Earth’s surface and have served to greatly enhance the data collecting ability of geologists. Geologic maps come in many shapes and sizes depending on their intended use but most include three basic types of features all of which are very useful to a field geologist. These three features are a surface landforms map, stratigraphic columns, and geologic cross sections.
The surface landform portion of a geologic map (the colored part of the PA map above) is very familiar to you and consists of a top-down view just like a standard road map. However, instead of focusing on roads and towns, a geologic map will feature special annotations regarding surface rock type(s), rock age, surface faults and folds and any other features of geologic significance.
Stratigraphic columns (like this one of Big Bend National Park) help to decipher the surface landforms of the map and also provide a 3D view of what is underneath the surface features. Like you've already read, a working knowledge of such features as faults and folds can save considerable time and money when searching for economically important rock and mineral resources and not all of them are visible at the surface so an understanding of what is under your feet (sometimes to a depth of thousands of feet), can be very important. This knowledge can also reveal hidden subsurface features that could represent a risk to human populations like blind thrust faults or highly porous limestone (which often leads to sinkholes). Normally, drilling operations like the one pictured below are required to construct stratigraphic columns for a particular region but, since they are so commonplace already in urbanized areas from past construction, mining operations, etc., most of the subsurface geology of the United States has been mapped in significant detail. In a typical drilling operation, a series of pipes is connected at a drilling machine (left) and then drilled into the ground often with a diamond studded drill bit especially if the geologic setting of the area promises hard rock. The cores (right) fill the hollow pipe of the corer and are brought back to the surface for analysis.
Geologic Cross Sections
Geologic cross sections are the end result of groups of stratigraphic columns linked together to create an uninterrupted 3D view of a region's geology. This means that the information derived from sets of different drill cores are often compared to one another in order to find patterns that geologists can use to link the cores together even in areas where they might not have drilled. When geologists find a pattern or similar rock layer in multiple core samples, a correlation between the drill cores is observed (see the upcoming section Correlations for details). In other words, if the same rock layer is found in two different core samples separated by one mile, it is usually assumed that the rock layer connects the two points below the surface even though the geologist has no direct proof it is there. On a geologic map, there are usually one or more cross sections that cut through the main map (like this one from Sichuan Basin of China) that shows some of the interesting subsurface geology on a regional scale. This can be very useful when judging the scope and size of such features as faults, igneous features and large scale tectonic effects.
Rocks and Rock Units
It is important to remember that geologic maps usually illustrate subsurface geologic features as much as surface ones through the use of core samples. These core samples are then turned into stratigraphic columns which are then connected into geologic cross sections. And, just like in biology, systems of classification exist for the identification of various types of related rock units.
As you begin to evaluate stratigraphic columns and cross sections from geologic maps, it will become useful to identify certain features by their names. In geology, scientists typically name distinct rock layers according to the names of towns, geographic features, etc. found in that area. The Wissahickon schist is an example of a metamorphic rock you have already been introduced to at Valley Green. These named rock layers are known as formations. If there are multiple formations that seem to be related (but are still distinct), they are grouped into a group. Formations can also occasionally be subdivided into members while they are in turn divided into beds. A bed is the smallest stratigraphic unit used in geology and may be only a few inches thick if it is significant
enough to note. The ashfall from a large volcanic eruption is a good example of a bed. However, it is important to note here that rock units are not determined based upon a minimum thickness. In fact, thickness plays no direct role in identifying unique rock units at all. A single bed may be hundreds of feet thick or no more than a few inches. What is important are the physical characteristics of the rock more so than the thickness. A great example of how this all fits together can be found in this cross section of the Grand Canyon. Here, formations, groups and supergroups are all identified.
The Grand Scale
While most geologic maps tend to concentrate on regional geology, they can easily be spliced together into one, grand view of the United States making it possible to evaluate various geologic events that have impacted the country over billions of years (check out this version by USGS). In general, the continent and all of its landforms are a direct function of its different rock and mineral deposits and the ongoing modification of these deposits by major weathering and tectonic processes. Currently, the United States is divided into 8 regions and 25 major geologic provinces (or terranes) where the rock types, structure and ages are roughly similar (or at least related). Since most geologic maps are regional in scope, it is not unusual to see several of these zones on a single map. For instance, Montgomery County is part of a linear region stretching from New York state to Alabama. Consisting mainly of metamorphic rock deformed by long since extinct tectonic forces, this geologic province is known as the Piedmont Terrane. Like the Piedmont, each other province has its own distinctive set of geologic structures and other variables that set it apart from the others. The uniformity and similarities of the overall geology within a province therefore suggest that processes of geologic change have affected the area as a unit and that all parts of the region have a similar geologic history.
Believe it or not, Wikipedia is of special value here and can be used for a listing of all U.S. geological provinces (including their subdivisions). Go to the Files and Downloads section for a listing of the various U.S. geologic terranes along with an associated map.
One of the greater joys I have found as a geologist lies in the challenge of gleaning hidden information out of rock samples. A rock can speak if you know its language but it does not give up its secrets easily and since most of the answers to questions lie deep beneath the surface rather than right on top of it, a geologist often has to base theories on incomplete or cryptic information. Recall from earlier that geologic maps are generally based on a series of core samples that are drilled from time to time in an effort to determine subsurface features (especially when searching for rock or mineral resources). Using the principles set forth by early geologists like Steno and Hutton, it is possible to then link these individual core samples into one cohesive picture of exactly what lies beneath the surface without ever seeing it. When individual stratigraphic columns are linked in this manner, it is called a correlation.
Correlations are generally established based on two varieties of inferences: lithostratigraphic and biostratigraphic. Lithostratigraphy is generally the one used the most since it is based on similarities between rock units in different stratigraphic columns and can be used on any type of rock (igneous, sedimentary or metamorphic). Quite often, both physical and chemical properties of the rocks are used to determine matches between two or more columns. For instance, properties such as color, grain size, mineralogy, erosional surfaces and bedding characteristics are all used to identify related rock layers in correlations.
If the rock layers are fossiliferous, biostratigraphy correlations can also be made. These correlations are made by looking closely at fossil remains found in the rock and matching related animal and plant species from one stratigraphic column to the next. Of special importance here are index fossils. Index fossils are species that occur in high abundance but generally only exist for a short time in the stratigraphic record. Since they are common enough to be found frequently yet limited in their range of time period, the presence of such fossils is usually a clear sign the rock is of a certain age. The trilobite at right (Paradoxides) is just such an example of an organism that was very common around 435 million years ago but then disappeared from the fossil record shortly after.
Along the same lines as index fossils are non-biological events that can lead to establishing correlations between rock layers too. For instance, volcanic eruptions are geologic events that occur in the blink of an eye in geologic time but can still leave large enough deposits of ash and other debris behind that can be easily linked from one location to another. The tell tale signs of glaciation and the debris fields they usually leave behind when they retreat and melt can also be used to establish time lines from one place to another as well.
Correlations and Environmental Change
Since rock and mineral deposits are the direct result of environmental conditions, it should come as no surprise that these same rock layers can tell an astute geologist about the environmental changes that took place at a location over vast periods of time. Of special importance are series of certain rock and mineral types that are found in sequence with each other in a stratigraphic column. For instance, thick deposits of halite (rock salt) on top of limestone beds hints at the presence of an ancient shallow marine environment that eventually evaporated and left the salt. Recall that limestone beds are usually the remains of huge amounts of shelled marine organisms like clams, oysters or corals. In yet another example, consider the possible clues contained in a thick deposit of shale. Shale is composed of compacted clays and fine silts typically found underwater where the water movement is very low. Typically, these conditions are found in swamps but also in deep ocean settings. However, if a thick bed of coal is found in association with the shale, the deep ocean environment can be ruled out since coal is the result of decomposition of organic material now found in the ocean.
Correlations can also be made about environmental changes through time. In geology, a facies is a rock layer(s) that has noticeably different characteristics from those around it. Usually, this term is applied in sedimentary rock layers but it can also be used for other types of rock as well. In 1894, Johannes Walther made an important discovery in stratigraphy when he proposed the Law of Facies. Walther realized that vertical changes in rock facies could be translated as lateral changes in a depositional environment. This discovery was instrumental in understanding ideas like ancient sea level changes. Take a look at the example below.
This geologic cross section above is a view of part of the Michigan Basin, a huge bowl-shaped series of thick sedimentary rock deposits that is a very obvious feature on any geologic map of the region. As you can see, there are several thick deposits of igneous "basement" rock (red), considerable sandstones (yellow), limestone/dolomite (light blue) and even some shale (grey). The Precambrian deposits are at least 540 million years old (click here to bring up the Geologic Timescale again) while the others are younger (based on the Law of Superposition). The Principle of Original Horizontality also states that these layers (being largely sedimentary) were deposited horizontally. So what does the rock tell us about sea level changes? If you can remember the environments the sediments are deposited in, you're halfway to the answer already.
The sandstones found in the Cambrian Period are probably the work of either a desert or beach-type environment but which one is it? The limestone just above the sandstones gives it away. Since limestones are deposited in shallow marine environments, it stands to reason that the sandstones were deposited in a near shore zone. So, from this vertical change in the rock facies, it is logical to conclude that sea level changed laterally (i.e. moved landward) since what was once a beach became a shallow marine environment. This rise in sea level is known as a transgression.
Shortly thereafter (geologically speaking), another layer of sandstone suggests that the ocean once again receded (called a regression) before surging back in again to lay yet another layer of limestone. After that, the water became even deeper and the presence of the shale layer suggests that it was deposited in a deep marine environment before the ocean once again receded laterally and deposited more limestone. Since the uppermost layer of limestone is Silurian in age (~440 million years old) and the base of the Cambrian sandstones are ~540 million years old, we can identify at least two major changes in regional sea level over approximately 100 million years.
Gaps in the Rock Record
Not all stratigraphic columns offer up a continuous record of everything that has happened for millions of years at a given location. In fact, there are often gaps (sometimes referred to as a hiatus) where the given location was not a depositional environment. For instance, if a period of mountain building (known as an orogeny) occurred in a region, that area would experience erosion, not deposition so not only would new sedimentary levels not form there, older ones could be completely scoured away. However, the bright side to this missing information is that the gaps in the rock record can often be used to match up different stratigraphic columns in much the same way a deposit of volcanic ash can. Together, the various types of missing segments in the rock record are referred to as unconformities and come in several different forms. The first is perhaps the easiest to picture and is called a disconformity. A disconformity is an erosional surface between two successive rock strata. Disconformities may represent short periods of time but are usually at least tens of thousands of years (if not more) in duration. An angular unconformity is an erosional surface where the layers below the break have been tilted or folded but then buried by new horizontal layers on top. The photo on the Introduction page illustrates an angular unconformity that James Hutton used to explain his theories about a Dynamic Earth. Finally, a nonconformity is an erosional surface between igneous or metamorphic rock and younger sedimentary rock above it.
This cross section of the lower portion of the Grand Canyon (above) illustrates the three main types of unconformities. At the base of the Grand Canyon where the 1.7 billion year old Vishnu Schist lies is a distinct nonconformity where the overlying Unkar Group of rock layers meets it. Around one billion years ago, the entire region was tilted by tectonic forces and created mountain ranges that have long since disappeared but nonetheless created a hiatus of nearly 500 million years before the Tapeats Sandstone was deposited on top of the heavily eroded Unkar Group rocks around 525 million years ago creating an angular unconformity. From that time forward, no less than one dozen disconformities are recorded in the nearly horizontal layers of rock that overlay the basement rock of the canyon. Click here for a full view of the Grand Canyon strata.
Structural Geology Maps
The American Association of Petroleum Geologists is a professional organization whose mission is to "foster scientific research, to advance the science of geology, to promote technology, and to inspire high professional conduct". The AAPG is one of the leading geologic associations in America with over 40,000 members and it is often their members who are hired by the world’s major oil and gas companies to perform field surveys and resource prospecting missions. For this reason, it is their geologic maps that will form the basis of your introduction to geologic mapping in an upcoming lab. Called "geologic highway maps", AAPG combined some basic road map features with geologic ones so that any geologist driving through a region could quickly match up what they were seeing out the window with a road location on the map...quite a handy combination!
Reading the Maps
The AAPG maps follow a few basic conventions common to most USGS maps which you need to familiarize yourself with. First, they use standard rock type symbols in the stratigraphic columns. These symbols have been introduced to you already (at least in part) but will
now play a much more important role. Second, their stratigraphic columns are arranged according to the Law of Superposition with the oldest layers at the bottom. This means the geologic timescale is also in play. To help in standardize geologic maps, AAPG uses a pretty standard color scheme to show rock units of various ages on all of their maps. This color scheme is shown in the chart at right.