biogeo lec 6 the changing earth

THE CHANGING EARTHTHE CHANGING EARTH

The Geological Time Scale:The Geological Time Scale:

• Index or Guide Fossils - unique assemblages of fossils characteristic of a single time span

Phanerozoic eonPhanerozoic eon - -

• Past 600 million years

• Divisions mark transitions among geological strata and embedded fossil assemblages


• The largest defined unit of time is the Eon.

• Eons are divided into Eras, which are in turn divided into Periods, Epochs and Stages

Phanerozoic EonPhanerozoic Eon

• Eon (sometimes spelled aeon) • A period of time arbitrarily designated by

humans. • Geologists refer to an eon as the largest

subdivision of time on the geologic time scale.• For example, the Phanerozoic Eon, which is

about 550 million years long, covers the period of time during which animals with hard shells that fossilize would have been abundant

• Eons - Phanerozoic

• Eras - Paleozoic, Mesozoic & Cenozoic

• Periods - Cambrian to Permian

• - Triassic, Jurassic, Cretaceous

• Epochs - Pleistocene, Holocene, etc.


Millions of Years

The Holocene (present epoch) is too small to be shown clearly on this timeline.

The Theory of Continental Drift:The Theory of Continental Drift:

• Antonio Snider-Pelligrini (1858) - demonstrated the geometric fit of the coasts of continents on opposite sides of the Atlantic Ocean


• F.B. Taylor (1908) - hypotheisized that as continents moved, distortion of crusts created mountain ranges and island chains

• --> postulated that moving continents form mountains at their forward edge and leave oceans behind them


• Wegener’s Theory - Alfred L. Wegener (1910) conceived and championed the theory of continental drift

• --> Four shortcomings



• --> Four shortcomings

• 1) Challenged long-held paradigms -> too many assumptions that remained unsubstantiated by available geological evidence



• --> Four shortcomings• 1) Challenged long-held paradigms -> too many

assumptions that remained unsubstantiated byavailable geological evidence

• 2) Factual errors - proposed continents moved at a rate of 36 meters per year (based on the assumption the earth was much younger) (actual rate is 2 to 12 cm per year)




assumptions that remained unsubstantiated by available geological evidence


• 3) Needed to gather more evidence to test their model




assumptions that remained unsubstantiated by available geological evidence


• 3) Needed to gather more evidence to test their model• 4) Lacked a plausible mechanism (most critical)

Evidence for Continental DriftEvidence for Continental Drift • Bullard et al. (1965) - used computer

mapping and statistical analysis to test

• --> continents fit together if you use the submarine contours of the continental shelves to delineate the margins of the continental plates

• Marine Geology - post WWII

• --> Seafloor spreading - midoceanic ridges -> ocean expand

• --> Trenches - crust pulled downward and reincorporated into the mantle

• Paleomagnetism - evidence of seafloor spreading

• --> orientation of magnetic crystals

• --> rocks containing iron and titanium become magnetized

Current Model of Plate TectonicsCurrent Model of Plate Tectonics

• Current Model of Plate Tectonics - interaction of the earth’s crust, mantle and core.

• –> Lithosphere - outer layer of the mantle

• –> Asthenosphere - deeper , more fluid layer


• –> Three forces:

• 1) Ridge push

• 2) Mantle drag

• 3) Slab pull


• 1) Ridge push - occurs at midoceanic ridges where magma upwells from the asthenosphere to the surface

• -> causes outer rocks to spread

• -> seafloor spreading


• 2) Mantle drag - lateral flow and friction between the mantle and the overlying plate create a dragging force

• -> most of the force is created at the Subduction zones


• 3) Slab pull - subduction zones are where dense oceanic plates sink deep into the magma


• –> Three types of plate boundaries:

• 1) Spreading zones

• 2) Collision zones

• 3) Transform zones


• Three types of plate boundaries:

• 1) Spreading zones - midoceanic ridges resulting seafloor spreading

• –> Rift zones- on continents

• Ex. - Red sea, deep lakes of the Baikal rift zone, East African Rift Valley


• 2) Collision zones - where plates collide far from spreading zones– Mountain Ranges– Subduction zones


• 2) Collision zones - where plates collide far from spreading zones

• –> Mountain ranges form where plates are of roughly equal density


• –> Subduction zones and deep oceanic trenches form where dense oceanic plates sink beneath lighter continental plates

• –> Subduction zones are marked by parallel bands of


• –> Subduction zones and deep oceanic trenches form where dense oceanic plates sink beneath lighter continental plates

• –> Subduction zones are marked by parallel bands of

• 1) earthquakes and volcanism• 2) mountain ranges (Coastal range)• 3) Accumulations of marine sediments or

Terranes


• 3) Transform zones - plates of roughly equal density slide and grind against each other without either subducting

• –> characterized by high seismic activity

biogeo lec 6 the changing earth

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