Lecture 2 Flashcards
What did Alfred Wegener propose in his Continental Drift Hypothesis?
He proposed that all continents were once joined in a supercontinent called Pangaea, and then gradually drifted to their current positions
What were the main lines of evidence Wegener used for Continental Drift?
- Fit of the continents (e.g., Africa and South America fitting like puzzle pieces)
- Matching rock units and mountain belts across oceans
- Matching fossils of land animals and plants (e.g., Mesosaurus, Glossopteris)
- Matching glaciations and associated striations on different continents
- Matching paleo-climate belts (coal swamps, deserts, etc.) in unexpected places today.
Why didn’t scientists initially accept Wegener’s hypothesis?
He didn’t have a mechanism to explain how continents could move. At the time, there was no technology or data to demonstrate continental motion or seafloor spreading
What key development led to the paradigm shift confirming Continental Drift?
The discovery of seafloor spreading during and after WWII, made possible by increased seafloor exploration using sonar technology
How did WWII sonar exploration contribute to plate tectonics research?
By mapping ocean depths, scientists found:
* Mid-ocean ridges (long underwater mountain chains)
* Trenches (deep, narrow troughs)
* Changes in seafloor depth near continents
What is paleomagnetism, and why is it important in proving seafloor spreading?
Paleomagnetism is the record of Earth’s magnetic field preserved in rocks. It showed symmetrical patterns of magnetic reversals on either side of mid-ocean ridges, confirming new crust is formed at the ridge and then pushed away.
How do rocks become magnetized according to Earth’s magnetic field?
When hot magma cools and solidifies, any free iron minerals within it align with the Earth’s current magnetic field. As the magma solidifies, this orientation is “frozen” in place, creating a snapshot of the field at that time.
What is a magnetic reversal, and how is it recorded on the seafloor?
A magnetic reversal is when Earth’s magnetic field flips (north and south poles switch). Newly formed seafloor rocks capture the orientation of the magnetic field at that time, producing alternating stripes of normal and reversed polarity on the ocean floor
What does it mean that the pattern of magnetic “stripes” is symmetrical relative to mid-ocean ridges?
It means that on either side of the ridge, the polarity stripes match in age and width, indicating crust forms at the ridge and is pushed outward equally in both directions over time.
What is the lithosphere, and how does it relate to the asthenosphere?
- Lithosphere: Crust + uppermost mantle, ~100–250 km thick, relatively rigid.
- Asthenosphere: Part of the upper mantle beneath the lithosphere, ductile and capable of flow.
The lithosphere (plates) “floats” on the asthenosphere
- Asthenosphere: Part of the upper mantle beneath the lithosphere, ductile and capable of flow.
What are the differences between continental crust and oceanic crust?
- Continental crust: ~35 km thick on average, lower density (~2.65 g/cm³).
- Oceanic crust: ~10 km thick, higher density (~3.2 g/cm³).
How does seafloor spreading create new oceanic crust?
Magma rises at mid-ocean ridges, cools to form new oceanic crust, which then moves away from the ridge as more new material emerges
Why is oceanic crust eventually destroyed, but continental crust is not?
Oceanic crust is denser and subducts (sinks) at convergent plate boundaries into the mantle, where it’s recycled. Continental crust is lighter and cannot easily be forced deep enough to melt
Name and describe the three types of plate boundaries.
- Divergent – Plates move apart; new crust forms (e.g., Mid-Atlantic Ridge, East African Rift).
- Convergent – Plates collide; one plate often subducts (ocean-ocean, ocean-continent, or continent-continent).
- Transform – Plates slide horizontally past each other (e.g., San Andreas Fault).
What surface features or events typically form at ocean-ocean or ocean-continent convergent boundaries?
- Subduction zones with trenches
- Volcanic arcs on the overriding plate
- Accretionary prisms where sediments are scraped off the subducting plate
- Deep earthquakes along the descending slab.
What happens at continent-continent convergent boundaries?
Neither plate can easily subduct because both are less dense; they crumple and form large mountain ranges (e.g., Himalayas).
What are transform boundaries, and can you give an example?
Transform boundaries are where plates slide laterally past each other, causing shear stress. Example: The San Andreas Fault in California
What mechanism drives the movement of tectonic plates?
Mantle convection: Hot material from the deeper mantle rises, cools, then sinks back down. Plate motion is also aided by ridge “push” at spreading centers and slab “pull” at subduction zones.
How can we measure plate velocities today?
Using:
1. Seafloor magnetic anomalies, comparing distance from ridge to known age of crust
2. GPS measurements over time to detect plate motion and direction directly.
Summarize the “plate tectonics” theory.
Earth’s lithosphere is broken into plates that move atop the asthenosphere, create and destroy oceanic crust, shape continents, generate earthquakes and volcanoes, and are driven by mantle convection and related forces.
