GEOL 102 Historical Geology

Spring Semester 2014
Ruins of An Older World: The Discovery of Earth History

Review course policies, syllabus.

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.

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":

Many early geologists recognized several main types of rocks:

In 1759 Giovanni Arduino put these into an historical context, modified by Abraham Gottlob Werner in 1787. This model came to be called neptunism (after Neptune, the Roman God of the Sea). The model is catastrophist (in that it assumes different natural laws in past than at present).

The Neptunist model of rock formation:

In one scheme, Primary (and Transitional?) rocks were pre-Noah's Flood, Secondary rocks were the record of the Flood, and Tertiary and Volcanic rocks were post-flood.

In any case, under neptunism, rock type and rock age were related: all rocks of a certain type (say secondary rocks) were formed during a certain period of time; similarly, all rocks of a certain time would be the same kind of rock.

Also, all mountains built of Primary rock were formed in Primary time; the Secondary mountains (built of Secondary rock) crystallized on the sides of the Primary mountains; and the Tertiary hills and volcanoes were all deposited later. Thus, under Neptunism, the conditions to form big mountains were under a different set of natural laws than those of the current world.

Others developed a uniformitarianist alternative to neptunism. Most influential was James Hutton, and his work of 1785. Because it recognized crystalline rocks as being formed underground rather than at sea, called plutonism (after Pluto, Roman God of the Underworld) instead of neptunism.

Thus, under plutonism, rock type and rock age are NOT related: there can be extremely ancient sedimentary rocks (Secondary in the neptunist scheme) which are much older than some crystalline rocks (Primary in the neptunist scheme). Futhermore, mountain building is still going on today.

Work of Hutton was developed upon and further popularized by Charles Lyell & his Principles of Geology (1830). Lyell was famous for an extreme form of uniformitarianism, which assumed the same rates of operation in the past, rather than merely the same natural laws. (Lyell also predicted that organisms now extinct would reappear in the future!)

Most geologists operate under the uniformitarian philosophy of actualism: that the same natural laws now recognized operated in the past, but at different rates according to particular conditions.

Although the plutonists showed that rock age and rock type are not related, their recognition that the forces that produce rocks in the past are the same as those today allowed geologists to use modern observations to establish ancient environments. By discovering this, they were able to unravel the sequence of events in the past: historical geology.

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Last modified: 19 January 2012