CPSP118G Fall Semester: Earth, Life & Time Colloquium

The Restless Earth: an Introduction to Geology

Thomas R. Holtz, Jr.

Traditional (i.e., pre-scientific) View of Earth: the origin of the Earth and its structures (mountains, oceans, seas, rivers, etc.) are totally distinct from modern disasters (earthquakes, floods, landslides, volcanoes, etc.). The former was thought to be a singular event at the Dawn of Time, and completed; the latter were ongoing processes due to divine, diabolic, astrologic, etc., influences. (Indeed, "disaster" means "bad star"). Earthquakes, volcanoes, floods, and the like were thought to have no more relation to the formation of the Earth and its structures than diseases have to do with the birth of a child.

Various philosophers proposed "Theories of the Earth": several models beginning in the 17th Century questioned a single Creation week as the entirety of the formation of the Earth and its structure. Did the Earth itself have a history?

Two main competing philosophies among the "Theories of the Earth":

Neptunism was a leading (but not the only) catastrophist model in the 18th and early 19th Centuries. Postulated that all rocks precipitated out of the primordial seas. As the seas evaporated to certain depths, the type of rock deposited would change. Thus, under neptunism, rock type (lithology) equals rock age. Under neptunism, rocks could be catagorized as:

Many promoters of neptunism worked primarily in the lab rather than in the field. The switch to modern interpretations of Geology came from a combination of field and lab observations. Some of the main contributions leading to modern geology came from Nicolaus Steno (1638-1686) and James Hutton (1726-1797). Steno demonstrated that "Secondary" and "Tertiary" rocks were bits of previously existing rocks that were stuck together (solidly stuck in "Secondary", poorly stuck in "Tertiary"): what we now call sedimentary rocks. Fossils were just the remains of dead animals and plants that got buried with the sediment and preserved in the rock.

Hutton proposed a uniformitarian model termed plutonism, which is the basis of modern geology. He had observed that you could trace out huge masses of crystalline rock ("Primary" rock under the neptunist model) grading directly into veins of volcanic rock (which under Neptunism were from an entirely different period of time!). Hutton instead suggested that these were variations of the same process, and the reason the rocks produced looked different was that they formed in different environments.

The plutonist model:

  • Heat at depth creates molten masses of magma
  • Magma, because it is hot and relatively bouyant, rises
  • Uplifted rocks are subject to erosion, transportation, and redeposition as layers of sediment: strata So under plutonism, rock type does NOT equal rock age, but rather equals rock environment. Or, to use the official ELT Geology Mantra:

    Every Rock is a Record of the Environment in Which it Formed

    Hutton perceive the word as an "Earth Engine", where the processes around us today are responsible for the formation of its structures. These include both slow gradual processes (streams, wind, rain, etc.) and dramatic "disasters" (floods, earthquakes, volcanoes, etc.). Indeed, the latter were consequences of the normal operation of the Earth!

    Hutton's arguments were good, but he was not a good writer. His 1798 Theory of the Earth was not as well read as it might have been, due to the style of the prose. In the next generation, geologist Sir Charles Lyell (1797-1875) would be more successful in promoting uniformitarianism and plutonism with his well-written three-volume Prinicples of Geology.

    A major point of modern geoloy: the Rock Cycle:

    Rocks (naturally occurring cohesive solids comprised of one or more minerals or mineraloids) are generated in one of three primary manners (basis of rock classification):

    The modern view of geology shows that environments have changed dramatically over the face of the Earth, but that we can use the clues in the rocks to interpret those changes.

    The Stenonian-Huttonian-Lyellian view actually gave geologists a way of understanding the history of the Earth, and in particular the fact that it changed through time.

    "Deep Time": analogy to "deep space"; the vast expanse of time in the (geologically ancient) past.

    Many attempts at calculating age of the Earth:

    Two different aspects of time to consider:

    In the history of geology and paleontology, relative time was determined LONG before absolute time.

    Sedimentary rocks, because they are deposited, naturally form horizontal layers (strata, singular stratum). Because of their layered form, strata allow geologists to determine relative time (that is, sequence of deposition of each layer, and thus the relative age of the fossils in each layer). These form the basic Principles of Stratigraphy. The first three principles were developed by Niels Steinsen (better known as Nicholas Steno):

    As Steno and others mapped out strata, they found that sometimes there were types of breaks (discontinuities) in the layers. These are called unconformities, and represent gaps in the rock record (periods of erosion and/or non-deposition). No one region has a continuous sequence of time. Any given location has likely had periods of non-deposition or erosion, which would leave gaps in the geological and fossil record. Hutton, of Uniformitarianism fame, studied these and recognized that they represented aspects of relative time.

    From unconformities, Hutton added additional Principles of Stratigraphy:

    Use these principles to figure out time sequence in any particular section of rock. Using these principles, early geologists were able to figure out the sequence of events of deposition, the changing local environments, and the folding, faulting, igneous intrusions, etc. for any particular section of rock. BUT, how to extrapolate the sequence at one section with the sequence at another? And how could they tell numerical time?

    We'll see those questions answered in the next few weeks.

    Last modified: 10 August 2007