Message #175715

[Carolyn Silver]

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
          o During solidfication, lighter materials move to the surface - sial. Underlying sial was denser material - sima (mafic to ultramafic rocks)
          o 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:
          o Discovery of radioactivity at end of 19th century showed that the assumptions behind Kelvin's calculation were incorrect.
          o Modern estimates of cooling rates imply total contractions of 10s of km, inconsistent with 1000s km of shortening as observed in some mountain belts.
          o Normal faults, ocean ridges, rift valleys suggest that lithosphere cannot be in compression everywhere.