Geology 111G
Lecture 2. Plate
Tectonics and Science in Geology
Plate Tectonics
Tectonics,
defined
3
Types of Plate Margins
Continental
Drift
Scientific Method and the
Practice of Geology
Rock
Cycle
Uniformitarianism
Catastrophism
I. Plate Tectonics: a unifying
paradigm or dynamic model that explains earth processes that create and modify
earth materials, and also drive the rock cycle.
A. Tectonics: the study of movement and deformation of
the EarthÕs lithosphere on a broad scale; also, the study of the broad-scale
architecture of the outer part of the earth. Plate tectonics is a special branch of
tectonics that deals with lateral movements of the outer part of the earth as
they move laterally over more interior parts of the earth.
B. Lithospheric
Plates: Large pieces of lithosphere move independently above the
asthenosphere. Although the
lithosphere is strong and behaves more or less rigidly, it is broken up into a
number of caplike pieces called plates, or more properly, lithospheric
plates. The plates move
laterally as a result of thermal energy that causes the asthenosphere to turn
over convectively. The interactions
of plates at their edges, or plate margins, create many of the most
impressive geologic phenomena on the globe, including volcanoes and
earthquakes.
1.
Plate boundaries: These are faults
that cut through the entire crust and separate the plates. Locations are indicated by linear arrays
of earthquakes and volcanoes. There
are three types of plate boundaries:
a. Divergent. Plates move apart.
1. Seafloor spreading and plate
motions. The essence of plate
tectonics is that oceanic crust is formed at mid-ocean spreading ridges by the
upwelling of magma (hot, liquid rock), whereupon it moves away from the
spreading center, and becomes part of the oceanic plate.
b. Convergent. Plates collide.
2. Plates are ultimately consumed again by
descending back into the asthenosphere at a subduction zone.
c. Transform or Transcurrent. Plates move laterally past one another.
2.
Continental drift: Lateral
movement of continents that happen to ride on the lithospheric plates. Lithosphere may be capped by either
continental or oceanic crust, or both, and the crust on the surface of the
plate rides passively along with the rest of the lithosphere.
II. Scientific Method and
the Practice of Geology
A. Conservation of matter dictates that
rocks cannot be destroyed, but are rather cycled from one form to another. This is termed the rock cycle. Insightful observations about the rock
cycle lead to some early ways of thinking about earth processes.
1.
Uniformitarianism:
hypothesis holding that the processes that shape the earth today also
shaped the earth in the past.
a. James Hutton realized in late 1700s that there is no
evidence that the rock cycle has a beginning or an end; therefore, the
operating processes may be projected indefinitely into the past.
b. The formal statement of this hypothesis was extremely
important to the history of geologic though in that it countered then-popular
claims of divine intervention into Earth history. It thus helped elevate
geology to the rank of a true natural science capable of predicting natural
phenomena.
c.
Paraphrased as Òthe
present is the key to the past.Ó
2. Catastrophism: a contrasting hypothesis that holds that
Earth is shaped by cataclysmic or catastrophic events, not necessarily, but
possibly of divine or supernatural origin.
Can also be applied to rare events of natural origin,
such as floods, earthquakes.
a.
Uniformitarianism
originally served as a counterpoint to catastrophism and at the
time, as a validation of geology as a science.
b. Return of catastrophism has accompanied the maturation of
geology as a science.
1. Example of the terminal Cretaceous
event: Thin layer of clay enriched
in Iridium found world wide, coinciding with the disappearance of 65% of
EarthÕs organisms at 66 Ma.
2. Chicxulub crater in
3. Certainly, the 26 December 2004 tsunami
in the