The Nature of the Earth’s Rock Record

From the surface of the earth to the center of its core is a distance of about 6000 km. The surface of the earth is covered in a very thin crust. The continental crust has a range of thickness from 35 to 60km. The oceanic crust rarely exceeds 5 km in thickness (Levin 1986, 9). The geologic history of the earth is preserved in these relatively thin layers of rocks (Press 28). The analysis of the rock record begins with data collection, “… which consists mostly of describing, measuring, and interpreting… rock sequences in the field” (Prothero 25). An understanding of this rock record is dependent upon understanding the nature of the earth’s rocks and what rules are used to decode them.

Types of Rock in the Rock Record

In a historical analysis of the rock record one may often begin with the origin of rocks present in the geologic record. “The initial subdivision of rocks is made on the basis of origin, and the three major categories are igneous, metamorphic, and sedimentary” (Press 75). An examination of the rocks reveals the geologic history that formed those rocks. A geologist will receive detailed training about each of these types of rocks.

Igneous rocks are formed as molten rock cools. When igneous rocks form below the surface of the earth they are called intrusive igneous rocks. When igneous rocks from on the surface of the earth they are called extrusive igneous rocks. Extrusive igneous rocks will generally have very small crystals or no crystals at all. This is because they cool very quickly. Intrusive igneous rocks cool more slowly, and will typically have larger crystals. The mineral composition of igneous rocks is a function of pressure (under which it was formed), temperature, and composition of the last stable equilibrium of the system forming the rock. The bulk chemical composition of igneous and metamorphic rocks may be used to determine the source of the material that formed the rocks (Best 18).

Sedimentary processes produce sedimentary rocks. Sedimentary processes include the breaking down of previously existing rocks into sediments, the transportation of sediments by water, wind, or glacial movement, the depositing of sediments in a new location, and the solidification of sediments into rocks. The properties of the sedimentary rocks may be used to determine which sedimentary processes produced any given sedimentary rock or sequence of sedimentary rocks. Geologists look at the overall geometry of the sedimentary rock bodies and their physical and biological structures. The porosity and permeability, acoustical features, resistance to erosion and radioactivity of sedimentary rocks also help explain how they originated. Sedimentary geologist will typically look at a number of sequences of rocks with similar features. They will then determine which features are similar to those occurring in the local sedimentary record. Based on this information a model of the possible environment that produced the local sedimentary sequence. Geologist will use these models to help them develop models for new locations and will continue to test the model by comparing it to new data discovered at other locations (Prothero 42).

The nature of metamorphic processes that form them and the composition of the parent material from which they are derived determine the characteristics of metamorphic rocks. Metamorphic rocks are classified based upon texture and composition. Looking at the texture and composition of metamorphic rocks allows geologist to determine both their history and the type of metamorphism they have undergone (Levin 1986, 161).

Understanding what rocks can tell us

Beginning about the end of the Eighteen-century geologist recognized that geologic forces reshape the earth. In 1785 James Hutton proposed that geologic process occurred in cycles. These processes, according to Hutton, occur in a uniform fashion throughout time. A study of present geologic process allows us to determine how geologic processes worked in the past. The physical and chemical laws that govern geologic activities remain uniform throughout time (Press 38). As geologists use the present to decode the past, they must be careful not to assume that geologic changes in the past occurred in the same manner and on the same scale as today. Geologist should also remember the possibility that some the earth’s past geologic processes may not still exist or be active. We can use the theoretical framework of similar processes working throughout the geologic past, but must not assume that these processes occur with the same energy or at the same rate. The geologic record forms at best an imperfect chronicle of the earth’s history (Geikie 638).

Hutton believed in explaining the past history of the world by what is observed in nature today. William Whenwell coined the term uniformitarianism. Charles Lyell later popularized this term. Geikie coined the catchy phrase, “the present is the key to the past” (Levin 1988, 12). The limited application of uniformitarianism may be see by reading Geikie’s article in the Eleventh Edition of the The Encyclopedia Britannica (see the references for this article). Within its historical context, uniformitarianism was introduced as a philosophical outgrowth of the work of men such as Isaac Newton. Newton and others that shared his mechanical view of the world saw the universe as governed by natural laws, rather than a sovereign God. Many of these scientists were Deist and preferred to think of God dealing with his creation through natural laws rather than supernatural intervention. The movement toward naturalism was a resurrection of Greek and Roman ideas. Hutton’s uniformitarianism contradicted the traditional view of catastrophism. Traditional catastrophism viewed the earth as the product of supernatural creation and Noachian deluge. For the supporters of traditional catastrophism most of the earth’s geologic record resulted from a universal flood. Hutton’s uniformitarianism contradicted the prevailing interpretation of scripture at the time (Prothero 9). A typical statement of the view of catastrophism of the time may be found in the 1691 edition of Thomas Burnet’s The Sacred Theory of the Earth.

“The universality of the Deluge is also attested by profane [i.e. secular] History…. And we cannot without offering violence to all Records of Authority, Divine and Humane, deny that there hath been an universal Deluge upon the Earth; and that if their was an universal Deluge, no question it was that of Noah’s, and that which Moses described…” (38 – 39).

Rules for Interpreting the Geologic Record

In 1669 Nicolus Steno, while observing tilted rocks in Tuscany, derived three laws of sedimentary rocks, based on his conclusion that sedimentary rocks were at one time soft sediments. “The principle of superposition states that in any sequence of undisturbed strata, the oldest layer is at the bottom, and successively higher layers are younger” (Levin 1988, 9). For the most part, the principle of superposition governs the depositing of sedimentary rocks. In a zone where two plates of oceanic crust meet, one (the older one) will be subducted (pulled down) below the other plate. During subduction, pelagic sediments are scraped off the subducted plate onto the overriding plate. An accretionary wedge forms along the arc as these sediments begin to accumulate. In this process new material is constantly added to the bottom of the pile, a reversal of the principle of superposition. The rocks produced by underplating are typically a mass of chaotically mixed, brecciated blocks in highly sheared matrix known as a melange (Prothero, 332).

Most strata of sedimentary rocks are tilted or folded. Steno concluded that since most sediments settle from a fluid under the influence of gravity. For this reason, sediments must have been deposited nearly horizontal and parallel to the surface on which they were accumulating. This became known as the principle of original horizontality. When a geologist observes sediments that are tilted, this indicates a time of crustal disturbance after deposition (Levin 1988, 10). Steno’s third principle states that sedimentary layers from in continuous sheets that either covered the entire earth or were bounded by solid substances. Erosion can act as an agent to separate rocks with a valley from their original continuous state. This is known as the principle of original continuity (Prothero 6).

The Use of Fossils in Stratigraphy

The Englishmen William Smith became one of the first people to carefully observe and record in which rock bed fossils occurred. Smith soon noticed that similar fossils in rock units could be used to identify the rock units. At first Smith used this to correlate rocks over short distances. The correlation of these rocks units allowed him to match rocks of the same relative age, but of differing lithologies. Soon Smith used this principle to correlate rocks over great distances. Eventually the principle developed into the idea of biological succession. Biological succession holds that the life of each age of the earth’s history was unique for a given period time. Fossils were used to correlate rock units based on similar fossil content and assumed to be of the same age (Levin 1988, 15).

Today, biologists recognize that organisms exist in specific etiological settings. Entire continents or large portions of a continent separated by major geographical barriers make up biogeogrpahical realms. Each biogeogrpahical realm is distinguished with a specific assembly of plants and animals. Each biogeogrpahical realm is made up of distinct communities called biomes (Davis 778). Many of the changes observed in fossil assemblages are due to changes in the physical environment. Biological successions brought about by changes in the environment occur in tens to hundreds of years (Stearn 395). When environments suddenly change, a species that was once successful in that environment could become extinct. At this time, surviving organisms can freely move into the recently vacated niches (Davis 700). What the fossil record shows is periods with no new species appearing, and then suddenly new species appear, first as peripheral members of other niches, and then as dominating niches vacated through extinction. “Because transition takes place rapidly, intermediate forms are unlikely to be preserved anywhere in the fossil record” (Stearn 349).

The Frenchmen, Cuvier and Brongniart validated Smith’s findings of fossil succession. They also found that this sucession was almost the same wherever they found it. Cuvier and Brongniart believed that the fossil record resulted from a series of catastrophes that caused complot extinction. And that after each catastrophe new species arose. They considered that lst of these catastrophic episodes to be the Noachian Deluge. The main flaw with the theories of Cuvier and Brongniart was that they did not explain how new species arose on earth (Levin 1988, 15 & 16).

In 1830 Charles Lyell expanded upon the work of Steno by publishing his Principles of Geology. Lyell explained that geologic features that cut across another body of rock must be younger than the rocks, which they cut across. This is the principle known as crosscutting relationship, and applies not only to rocks but also to geologic features such as faults and unconformaties. Furthermore, Lyell concluded that rock fragments contained in larger rock masses are older than the rock masses in which they are enclosed. “Thus whenever two rock masses are in contact, the one containing pieces of the other will be the younger of the two (Levin 1988, 16 – 17). With the work of Lyell in place geologists had most of the basic tools in hand to study and begin to understand the earth’s geologic history as preserved in the pages of the rock layers of the crust.

Over the next several decades geologist from all over the world studied rock layers and correlated them. Fossils were the key element used to accomplish this correlation. The earth’s geologic record can be divided into series of strata, characterized by distinctive organic remains. Steno’s laws are used to correlate rocks on a regional and global scale. The geologic record is nowhere complete, but generally follows the same sequence (Geikie 668). “In most areas, the stratigraphic record must be patched together from many short local section (Prothero 185).

The geologic column only gives us the relative ages of rocks. Rocks simply do not tell us how old they are. A geologist must seek further evidence to determine the age of rocks. When geologist look at all the evidence they may begin to assign absolute dates to a sequence of rocks for which they only knew relative dates by looking at the sequence of deposition. Establishment of an absolute age of rocks is subjective and dependant upon the geologist interpretation of certain physical evidence. From the geologic record we can tell that a certain event occurred before or after another event. The various divisions of the earth’s rock record may be viewed as stages in earth’s geologic history. The work of people like William Smith shows that the correlation of rocks over even large distance may have practical application. Presented below is a representation of the stratigraphic record with information about the origins of subdivion names added. The information on subdivision names was obtained from Prothero pages 5 – 23 and Levin 1988 pages 135 – 156. The oldest stages of the development of earth's history are listed at the bottom of the chart, and the newest stages are toward the top. The charts are not drawn to scale.