Conceptual Models of the Earth - 19th and Early 20th Century

Prior to the early to middle 19th century, most scientists were clerics and theologians who felt compelled to relate their observations to the Bible. This led to catastrophist theories explaining the separation of the continents.

By the middle to late 19th and early 20th century, geologists worked on the continents, mostly in mountain belts in an apparently intellectually mature discpline governed by the theories of organic evolution, uniformitarianism, and stratigraphy (Fundamental principles of geology). Continents were believed to be stationary. Deformation in mountain belts was thought to be due to vertical movements of the crust. Explanatory mechanisms for mountain building were lacking.

Catastrophist Theories

Once maps of the Americas became available, a number of workers noted the fit of the Atlantic coastlines of South America and Africa.

• F. Bacon (1620) - Recognized similarity of South America - Africa coastlines. Speculated that the were remnants of Atlantis.
• F. Placet (1668) - Earth undivided before the flood. Postulated Americas formed by emergence of floating island during flood, or separation from Europe by destruction of intervening landmass Atlantis.
• Von Humboldt (1801) - Proposed that a former continent Atlantis, had subsided beneath ocean or that enormous valley had somehow been excavated due to water flow.
• A. Pellegrini-Snider (1858) - postulated drift due to outpouring of materail from within Earth at time of great flood that caused rifting and pushed continents apart to form the Atlantic. Made first pre-drift reconstruction
• Problems: Isostasy prevents continents from sinking into the ocean.

Fixists Views

Contracting Earth Theory

This 19th century theory held that contraction due to cooling of the Earth by thermal conduction and the consequent heat loss since formation was an important contributor to deformation at the Earth's surface.

• Inner portion of Earth was cooling most rapidly, leading to tensional stresses.
• Outer portion of Earth (lithosphere) subject to compression to accomodate collapse of internal portion. Lithosphere deformed by thrust faulting rather than buckling. Earth's crust basically wrinkling like a dried out apple, due to the contraction.
• Stems from Kelvin's estimate of theage of the Earth (20 - 80 Ma) based on cooling rates of iron spheres. Implied that circumference of the Earth had decreased by 200-600 km.
• Suess (1831-1914) Invoked contraction as driving force for deformation
  • During solidfication, lighter materials move to the surface - sial. Underlying sial was denser material - sima (mafic to ultramafic rocks)
  • Shrinkage caused overall arching pressures which caused certain areas to collapse and subside to form oceans. Left continents standing high. Minor additional movements accounted for periodic inundation of seawater on continents.
• Problems:
  • Discovery of radioactivity at end of 19th century showed that the assumptions behind Kelvin's calculation were incorrect.
  • Modern estimates of cooling rates imply total contractions of 10s of km, inconsistent with 1000s km of shortening as observed in some mountain belts.
  • Normal faults, ocean ridges, rift valleys suggest that lithosphere cannot be in compression everywhere.

Isostasy and Geosynclinal Theory

At the turn of the century, the geosynclinal theory as proposed by James Hall (1857) supplanted the contracting Earth theory as the dominant explanation for mountain building. Orogeny occurred as follows:

• A fixed linear trough of sediments with deep water sediments and volcanic rocks (eugeosyncline) at the center and shallow water sedimentary rocks on the flanks develops.
• When deformation occurs, it is symmetrical, with thrusting occurring away from the center onto flanking continental platforms. Eugeosynclinal rocks are the most highly metamorphosed and deformed. Miogeosynclinal rocks - more mildly deformed.
• Dana (1873)
  • Stage 1: Crust initially solidified as granitic plateaus and basaltic basins
  • Compression due to contraction causes basaltic crust to act as lever against granitic crust.
  • Stage 2: Lateral pressure from basaltic crust produces bending and flexing of granitic crust which leads to erosion of highs and deposition in lows.
  • Stage 3: Process continues through combination of isostatic adjustment (causing uplift and subsidence) and lateral pressure (causing folding and mountain belt formation). Folding, melting occurred as sediment was depressed to great depth and heated.
• Problems:
  • No adequate explanation for what caused geosynclines to develop and then deform. Airy isostasy (observed topography due to crustal root) was mechanism invoked.
    • Airy Isostasy -
      • Layer below crust acts as fluid in the long term.
      • Where crust is thick (mountains), it sinks into the fluid layer until bouyancy provides support. Where crust is thin (oceans), fluid wells up underneath.
      • Why? - Stress Balance (fig. 2.29 in K&V):
      • Thinning of crust by erosion or thickening by deposition, leads to adjustment in the form of uplift or subsidence due to removal or addition of loads, respectively.
  • No explanation for ophiolites in mountain belts (allow geosyncline formation in deep water? not just on continental shelves?)
  • No explanation of large amounts of shortening observed in Alps (down-sucking of European platform beneath Alpine geosyncline?)

Isostasy is undoubtedly an important driving mechanism for deformation on the Earth today, but it is not the only one.

Continental Drift

Although a few early thinkers pointed out that continents may have moved laterally, the first true direct challenge to the fixist view of the Earth came in 1915 with Wegener's proposal of continental drift.

Early Ideas

• F. B. Taylor (1908) - Creeping Crustal Sheets
  • Pre-Tertiary Earth consisted of large continents occupying polar regions
  • A deforming force set continents creeping equator-ward, thrusting continental material over oceanic crust. Leading edge of sheets formed mountain belts.
  • Deforming force was thought to be tidal, including possiblity of additional tidal forces due to moon capture in the Cretaceous. Gave explanation for non-random occurrence of mountain belts (wrinkle model implies random).

Alfred Wegener (1915)

Proposed that all the continents were originally joined in a single continent (Pangea) that have since drifted apart to their present position. Evidence amassed by Wegener to support this hypothesis included:

• Geometrical Fit of the Continents - fit continental shelf edges. Positions of India and Madagascar controversial with this approach.
• Matching Geologic features -
  • Fold Belts - e. g. Applachians - Caledonides
  • Age Provinces - Matching rocks of Paleozoic age, Precambrian as well
  • Igneous Provinces- distinctive igneous provinces correlate across continental boundaries, e. g. Precambrian anorthosites and Mesozoic dolerites
  • Sratigraphic Sections - tillites, coals, seds containing glossopteris that can be correlated across Gondwanaland Metallogenic Provinces - regions containing maganese, iron ore, gold and tin can be correlated across continents.
• Distribution of late Paleozoic to early Mesozoic Fossils on Land - Many land species found across all the continents (Glossopteris, Mesosaurus), not true later. Isolated landmasses give rise to genetically diverse species.
• Varied Paleoclimates - Occurrence of sediments from different paleoclimates on same continent suggests that continents have move latitudinally through time.
• Long Term Fluid-like Behavior of the Mantle - if mantle behaves as a fluid, as vertical movements documented by isostasy suggests, then continents should be able to move through it.

Detractors

• H. Jeffreys - a reknown seismologist, knew that mantle was strong (behaved as an elastic solid for earthquake waves, could hold up mountain ranges). Argued that Wegener's proposed driving mechanism, tidal forces, was a million times too small to deform and displace mantle as continents moved through it.
• AAPG Symposium (1926) - rejects continental drift in favor of geosyncline theory. Geophysical objections remain strongest.

Supporters

• Southern hemisphere geologists such as DuToit, L. King, and S. W. Carey remained advocates of continental drift between the world wars, presumably because the geologic evidence (especially fossil fauna) for a single landmass of southern continents (Gondwana) was so strong.
• A. Holmes (1931) - observed that the amount of heat emitted by volcanoes, could not account for the amount of heat discharged by radioactivity. Suggested that heat loss was better explained by mantle convection. Also suggested that convection currents could move continents. Jeffreys strongly disagreed with this hypothesis.

Study Questions

1. What is wrong with the Suess's proposal that parts of the ocean basins are underlain by lost continents (Atlantis)?
2. We know now that there are small portions of the ocean basins underlain by continent-like crust, termed oceanic plateaus. How can this be?
3. In your opinion, which of Wegener's lines of evidence for continental drift were the strongest? The weakest? Why?
4. What evidence did seismologists have that the mantle was not truly a fluid (hint: how do we know that the outer core is liquid?)?
5. What do we now know about the rheological (deformational) properties of mantle that would have resolved some of the controversies of the 1920s - 1930s?