Competing Models c. 1840
Two basic models based on same data but with radically different interpretations of the tempo, mode and pattern of Earth History
Known facts and inferences by 1840
- Stratigraphy and mapping
- Formations based on lithology
- Lithologies related to changing environments
- Correlation based on fossils within formations
- Unconformities implying either
- Long time break (Lyell)
- Abrupt change (Cuvier, Elie de Beaumont)
- Folds and faults
- Geological systems: bounded by unconformities
- Defined by similar faunal content (Murchinson, Lyell)
- Defined by structural similarity with boundaries at horizons of crustal deformation and uplift (Elie de Beaumont, Sedgwick)
- Biologic aspects: fossil record
- Faunal succession
- Successive appearance of invertebrates, fish, amphibians/reptiles, mammals, humans
- Floral succession
- Ferns and kin, conifers, flowering plants
- Direct evidence of environmental changes locally and through time
- Many extinctions implies changing environments as causal mechanism
- Horizons with almost complete extinctions (top of Secondary = top Chalk = top Cretaceous) interpreted as:
- Faunal revolutions due to rapid environmental change (Curvier)
- Long gaps in record that do not record long interval of change (Lyell)
- Gradual faunal changes (recall the Devonian System)
- Faunal succession
- Tectonics, earth interior
- Heat gradient (Cordier) implies hot interior and gradual cooling (Fournier)
- Linear mountain chains
- Idea of Uplift (since even Tertiary rocks at considerable elevations)
- Igneous origin of some crystalline rocks
- Idea of metamorphism (Lyell)
- Ag of earth indeterminate; presumably long
Framing assumptions about geological/scientific reasoning (Rudwick 1971)
- Theological: Naturalistic versus Supernatural
- Universally assumption that science is naturalistic
- Methodological: Actualistic versus Non-Actualistic
- Question was really whether modern processes are sufficient sample
- Rate: Gradual versus Saltatory
- Can processes act episodically (again question about whether modern is sufficient to demonstrate full range of process rates)
- Pattern: Steady-State versus Directional
- Was overall history in flux around a mean or was there a change through time?
Theoretical frameworks
Two contrasting models to account for range of observations and inferences. Based on different assumptions about how nature works. The group analyses in classroom illustrate some of this. Here are some of the basics. See Rudwick (1971) for more information on most of these points!
- Steady-State (Lyell; Herschal-Babbage also fits here)
- Crustal movements driven by local magma bodies modifying the geography of the surface.
- Constant flux of uplifted areas (land) and ocean basins
- Gradual changes reflected the history of life
- Local geography -> local extinctions
- Large-scale and very long time changes -> global climate
- Unconformities represent prolonged breaks
- Restrict to modern processes (actualism) at similar (gradual) rates
- Directional Synthesis (Elie de Beaumont; Curvier on fauna)
- Earth starts off hot and has cooled through time
- Cooling causes contraction that leads to pressure build-up in near-surface (crust)
- Crust episodically collapses, leading to zone of crushed, uplifted rocks (mountain chain)
- Unconformities represent the deformation and uplift of individual ranges
- Long-term cooling leads to changing environment -> fossil progression
- Abrupt uplift -> catastrophic flooding -> widespread extinctions
- Mixture of gradual processes (cooling) and saltatory events (uplifts, extinctions)
- Modern, actualistic processes for most things, but episodic non-actualistic events
Some critical points of difference
- Heat gradient and implication for earth’s interior
- Fossil progression
- Mountain geometry
- Unconformities
- Validity of using non-actualistic processes
- Steady-state versus directional changes