GEOL 102 Historical Geology

Spring Semester 2018
Introduction: It's About Time

Paleozoic and Neoproterozoic strata at the Grand Canyon, Arizona

The ruins of an older world are visible in the present structure of our planet, and the strata which now compose our continents have been once beneath the sea, and were formed out of the waste of pre-existing continents. The same forces are still destroying, by chemical decomposition or mechanical violence, even the hardest rocks, and transporting these materials to the sea, where they are spread out, and form strata analogous to those of more ancient date. -- The Theory of the Earth (1788) James Hutton

Gazing on the strata of the earth, he reads the fate of his species. In the undulations of the mountains is revealed to him the history of the past; and in the strength of rivers and the powers of the air he discovers the fortunes of the future. To him, indeed, that future, as well as the past and the present, are alike matters for meditation: for the geologist is the most satisfactory of antiquarians, the most interesting of philosophers, and the most inspired of prophets; demonstrating that which has past by discovery, that which is occurring by observation, and that which is to come by induction. -- Vivian Grey (1826) Benjamin Disraeli

Review course policies, schedule.

A Review of the Course, in 60 Seconds:

Historical Geology:
Use of geological principles to reconstruct the history of Earth, its environment, and its life. Combines many subdisciplines of geology

Examines geologic history as a pattern (the particular changes in sequence throughout time) and as process (rules or principles that govern these changes).

Some of the topics covered:

The most key concepts of this course:

  1. The environments on the surface of the Earth and its inhabitants have changed greatly over the course of geologic time
  2. Rocks and their contents are the record of this change

Some of the core truths of Earth history (based on Robert Hazen's The Story of the Earth):

  1. Earth is made of recycled and recycling atoms.
  2. Earth is immensely old when compared with human time frames.
  3. Earth is three-dimensional, and most of the action is hidden from view.
  4. Rocks are the record keepers of Earth history.
  5. Earth systems--rocks, oceans, atmospheres, and life--are complexly interconnected.
  6. Earth history encompasses long periods of stasis punctuated by sometimes sudden, irreversible events.
  7. Life has changed and continues to change Earth's surface.

Discovery of Earth History
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.

(Alternatively, some cultures suggested an eternally-old Earth, perhaps with cycles of destruction and renewal.)

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?

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

Two different aspects of time to consider:

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

A key idea in discovering the history of the Earth was "actualism": the method of reasoning that asserts the processes observably active at the present day are the most reliable key for interpreting the traces of the ancient past. (A related term is "uniformitarianism": sometimes people use this to mean the same thing as "actualism", but it has sometimes been used for a more extreme version that considers the rate of processes as invariant over time, and additionally that the Earth exists in a "steady-state" (and possibly eternal) form.)

The actualistic view means that the natural laws themselves do not change, but rates can vary. Furthermore, rare and exceptional events (like asteroid impacts or greatly increased levels of volcanic activity) are possible.

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Last modified: 18 January 2018

Carboniferous Rockwell and Purslane Formations at Sideling Hill, western Maryland