Lecture 5: Continental Drift and Plate Tectonics Flashcards
At what time did scientists begin considering the possibility of drifting continents?
Since the 17th century
What were the two earliest evidences for continental movement?
The coastlines of the continent (or continental rises) fit together like pieces of a jigsaw
Continents in the southern hemisphere have similar geological histories for the period 350 – 150 Ma
What are tillites?
sedimentary rocks deposited when glaciers erode or melt
What are striations?
scratch marks left on bedrock the glacier moved over
What did tillites and striations indicate?
that massive glaciers covered large parts of the southern hemisphere continents approx. 200 Ma ago
What was strange about the striations found onland?
The scratch marks suggested the ice was moving inland, which didn’t make sense with what we know about glaciers
How did we explain striations moving inland?
if all the continents were one big continent, then the scratch marks make sense, radiating out from the central, coldest area
What do we call the past large landmass in the Southern Hemisphere?
the Gondwanaland continent
How did plants and animal fossils give evidence for the continental drift theory?
Certain plant and animal fossils are found across the Gondwanaland continent
These species could not have crossed the present-day oceans
What did palaeomagnetism show to back to continental drift theory and how was this explained?
Paleomagnetism of ancient lavas show that the magnetic poles appear to have moved over time
We know that the poles weren’t moving however, the continents moved
As they moved, the alignment of the magnetite within the rocks changed
What did Wegener deduce?
All continents once grouped together as Pangea – have since drifted apart like ice floes breaking up
What does Pangea mean?
All lands
Who created the continental drift theory?
Alfred Wegener
How did we think continental drift was driven in 1929?
Subcrustal convection currents
Who strongly refuted the Continental drift theory?
Harold Jeffreys
When was the Benioff zone discovered?
1954
When was the first detailed seafloor maps made and what did they show?
1959 - shows MORs as global system
What did Hess propose in 1960?
volcanic origin of seafloor, and seafloor spreading at MORs, driven by mantle convection
What proof of seafloor spreading was discovered in 1962?
magnetic stripes as evidence of seafloor spreading
When was the sea floor drilled and what did it discover?
1968 - found basalt everywhere
When was the plate tectonics theory widely accepted?
Late 1960s
What did we confirm related to the tectonic plate theory in the 1990s?
that subduction drives mantle convection – irrelevant of size of the plate
