Lecture 15 Flashcards
Theory
A coherent group of general propositions used as principles of explanation for a class of phenomena.
Hypothesis
A proposition proposed as an explanation for the occurrence of some specified group of phenomena.
Law
Well-established propositions that are regarded as reporting matters of fact.
Plate Tectonics
A unified theory of the solid earth that explains many of the puzzles that we identified in previous lectures.
Continental Drift Hypothesis
All of the present-day continents were once part of a single super-continent called Pangaea.
Evidence of Continental Drift Hypothesis
- The close fit between the coastlines of Africa and South America.
- Glacial features from essentially the same time period appear in South America, Africa, India, and Australia.
- Fossils of identical plants and animals found on all continents.
- Large-scale geological features on different continents matched when the continents were brought together.
- Fossils found in some places do not match current climates.
How did Wegener construct Pangaea?
Used the location of specific rock types to determine the distribution of climate zones in the geologic past.
What was the major weakness of the Continental Drift Hypothesis?
Major weakness was the lack of a viable mechanism for the “drifting continents.”
Plate Tectonics replaces Continental Drift
Convection currents which bring hot magma up from the deep mantle and cooler rock downward could drive continental motion.
Sea-floor spreading
Magma comes to ocean floor at ridges; Magma erupts as sub-sea volcanoes and produces new basaltic crust.
What did the 1968 deep sea drilling project reveal?
Ocean floor is geologically young (~180 million years)
Oceanic Crust
Compose of basalt; relatively thin
Continental Crust
Composed of granite; relatively thick
Theory of plates
Sea-floor spreading and continental drift combined. The Earth’s outer shell (lithosphere) consists of rigid plates. The plates slide on the softer asthenosphere. As the plates move they carry continents and ocean floor. When continents and ocean floor converge, a deep valley is formed and ocean floor “subducts” beneath continents.
The types of Plate Motion
Divergent, Convergent, Transform
Divergent plate boundaries
Lithospheric plates which move away from each other.
Smokers
Provide unique enviroments for microbes that are able to flourish at high temperatures. It is thought that they may have been the cite of the initial evolution of living organisms.
Convergent Plate Boundaries
The surface area of the Earth is finite. If new crustal material is being created at divergent boundaries, there must be convergent boundaries where crustal material is consumed.
Subduction
When the oceanic lithosphere arrives at the edge of a continent, it sinks into the asthenosphere. The area where this sinking occurs is called a subduction zone.
Ocean-Continent Convergent Plate Boundary
The oceanic lithospheric plate is pushed toward and beneath the continental plate. The subduction of the oceanic material can produce magma which can cause volcanic activity and new mountain formation.
Ocean-Ocean plate convergence
One plate is subducted under the other, forming a trench. Volcanic activity, produced by the partial melting of the descending plate and/or the overlying oceanic lithosphere, causes a pile up of lava on the ocean floor which eventually rises above sea level to form an island volcano.
Continent-Continent Convergent Plate Boundaries
When 2 continents meet, neither is subducted. Instead the crust buckles and is pushed upwards or sideways.
Transform Plate Boundary
Plates slide past each other.
