From Continental Drift to Plate Tectonics
Baja California is moving away from the rest of Mexico at about 4 or 5 cm per year, roughly how fast your fingernails grow, and that is why the Gulf of California is a sea that is still opening. More than a hundred years ago, Alfred Wegener proposed that all the continents moved like this. Nobody believed him. Why was a correct idea rejected, and what evidence rescued it half a century later?
The Continental Drift Hypothesis
Cut out the outlines of South America and Africa on a map and you will see that they fit together almost like puzzle pieces. They also share rock types and fossils of freshwater reptiles that could not have swum across an ocean. How did the same land organisms end up on continents separated by thousands of kilometers of sea?
Wegener put together three classic lines of evidence:
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Fit of the coastlines. The eastern coast of South America and the western coast of Africa fit together; the fit is even better if you use the edge of the continental shelf instead of the shoreline.
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Shared fossils. Mesosaurus, a freshwater reptile from the Permian, appears only in South America and Africa. Neither could a seed plant like Glossopteris (an extinct seed fern) cross an ocean.
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Paleoclimates and rocks. There are glacial striations from about 300 Ma in regions that are tropical today (Africa, India, South America). And the rock sequences are the same on both sides of the Atlantic.
Who was Alfred Wegener and what did his continental drift hypothesis propose? Explain why the scientific community of his time rejected it despite the evidence he…
Who was Alfred Wegener and what did his continental drift hypothesis propose? Explain why the scientific community of his time rejected it despite the evidence he provided.
Summarise the three classic lines of evidence for continental drift — matching coastlines, shared fossils, and palaeoclimates — giving a specific example of each.
Summarise the three classic lines of evidence for continental drift — matching coastlines, shared fossils, and palaeoclimates — giving a specific example of each.
What was Pangaea and when did it begin to break up? State which ocean opened up as the Americas separated from Europe and Africa.
What was Pangaea and when did it begin to break up? State which ocean opened up as the Americas separated from Europe and Africa.
Paleomagnetism and Mid-Ocean Ridges
In the 1950s, oceanographic ships towed magnetometers over the seafloor and found a surprising pattern: alternating bands of normal and reversed magnetism, parallel and symmetric on both sides of the ridges. What process could have recorded this "barcode" in the basalt?
The diagram shows the magnetic stripes on the ocean floor on both sides of a ridge, along with the age of the rocks. a) Explain why the stripes are symmetrical and what…
The diagram shows the magnetic stripes on the ocean floor on both sides of a ridge, along with the age of the rocks. a) Explain why the stripes are symmetrical and what the increasing age away from the ridge indicates. b) Explain how this piece of evidence, unknown to Wegener, revived continental drift in the form of seafloor spreading.
Seafloor Spreading
With the magnetic bands you can measure speed: if you know the age of a band and its distance from the ridge, speed is distance divided by time. This is what Wegener could not have: the ocean floor is created at the ridges and moves away.
The expansion of the Atlantic proceeds at an average rate of $2.5\ \text{cm}$ per year. Calculate how many kilometers the ocean has widened in the last $80$ million…
The expansion of the Atlantic proceeds at an average rate of $2.5\ \text{cm}$ per year. Calculate how many kilometers the ocean has widened in the last $80$ million years and comment on why this figure is only an approximation.
What Moves the Plates?
Seafloor spreading explained that the floor moves, but the question that sank Wegener remains: what force pushes it? Today it is believed that the mantle, although it is solid rock, flows very slowly by convection: hot material rises and cold material sinks. Two main forces act on the plates over this flow:
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Slab pull. The cold, dense edge of an oceanic plate sinks into the mantle and drags the rest of the plate along with it.
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Ridge push. The ridge is higher than the surrounding ocean floor, and the lithosphere slides down that slope, moving away from the axis.
Analyze the following table with kinematic and geometric data for four lithospheric plates: | Plate | % of perimeter in subduction zones | % of perimeter at ridges |…
Analyze the following table with kinematic and geometric data for four lithospheric plates:
| Plate | % of perimeter in subduction zones | % of perimeter at ridges | Continental surface area (%) | Average displacement velocity (cm/year) |
|---|---|---|---|---|
| Pacific | 45 % | 20 % | 0 % | 8.5 |
| Nazca | 40 % | 35 % | 0 % | 7.2 |
| African | 5 % | 55 % | 40 % | 2.1 |
| Antarctic | 0 % | 80 % | 50 % | 1.2 |
- a) Justify, based on the correlation of these data, which mechanism ("slab pull" versus "ridge push") exerts the dominant control on plate velocity.
- b) If ridge push were the main driver, what relative velocities would you expect to find for the Antarctic plate compared to the Pacific plate? Explain the physical reason why ridge push is not sufficient to move plates at high speed.
- c) Explain why the African plate moves slowly despite being largely surrounded by ridges and upwelling mantle convection, analyzing the effect of basal drag and thick continental roots.
The Synthesis: From Drift to Plate Tectonics
In Mexico you can see it clearly: the Cocos plate sinks beneath the North American plate (hence the earthquakes and the Trans-Mexican Volcanic Belt), and the San Andreas Fault is the boundary where the Pacific plate slides northwest relative to the North American plate. Baja California travels with the Pacific plate, and that is why it is separating from the mainland. This closes the hook: what Wegener called "drift" is the motion of rigid plates, driven by slab pull and convection.
Explain the difference between Wegener's continental drift and current plate tectonics theory: what actually moves in each model and on which layer it does so.
Explain the difference between Wegener's continental drift and current plate tectonics theory: what actually moves in each model and on which layer it does so.
In 1960, before plate tectonics was accepted, a proponent of geological fixism stated: "The oceanic crust is a permanent and passive basin where sediments have been…
In 1960, before plate tectonics was accepted, a proponent of geological fixism stated:
"The oceanic crust is a permanent and passive basin where sediments have been accumulating uninterruptedly since the Precambrian; the idea that continents drift is a fantasy devoid of physical basis."
- a) If the fixist's statement were true, what relative thickness and what age should the sediments and rocks of the ocean floor have? Justify why the actual data obtained through ocean drillings and cores (absence of marine crust older than 180 Ma and remarkably thin sediments near ridges) completely falsified the fixist model.
- b) Explain how the bathymetric maps produced by Marie Tharp and the discovery of the axial rift valley in the Mid-Atlantic Ridge forced a reinterpretation of the role of the ocean floor in terrestrial dynamics.
- c) Based on this historical case, evaluate why Wegener's 1915 hypothesis could not be consolidated as a scientific theory at the time and what decisive role the development of new technology (sonar, marine magnetometry) played in the paradigm shift.
Oceans Are Born and Die: The Wilson Cycle
To interpret a column of rocks, events are ordered by position: the oldest at the bottom, unless there is deformation. Folded rocks with granites correspond to a collision (closing of an ocean). Red continental layers and evaporites indicate an initial rift. Marine limestones indicate an open sea, and the most recent thrusts indicate a new collision.
A geology team analyzes a stratigraphic and structural column in the Iberian Peninsula and documents the following sequence of events: 1. A basal basement of intensely…
A geology team analyzes a stratigraphic and structural column in the Iberian Peninsula and documents the following sequence of events:
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A basal basement of intensely folded slates and gneisses with granitic intrusions dated to about 305 Ma.
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Unconformably overlying the basement, continental layers of red sandstones and evaporites from the Triassic (230 Ma), on which rest thick sequences of marine limestones and dolostones containing warm-water fauna from the Jurassic and Cretaceous.
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Thrust structures and thrust nappes active 25 Ma ago that stack and deform the Mesozoic limestones and Tertiary sediments, originating a mountainous relief.
- a) Deduce which geodynamic context and which orogeny the basal basement (1) corresponds to, justifying whether it represents the formation or the breakup of Pangaea.
- b) Explain the geodynamic significance of the transition between the Triassic sediments and the Jurassic-Cretaceous limestones (2), relating it to the opening of the Tethys Ocean and the Atlantic.
- c) Interpret the tectonic origin of the Tertiary deformation (3): what orogeny does it represent, which continental masses collide, and what is its relationship with the genesis and configuration of the present-day Mediterranean?