Exercises: From continental drift to plate tectonics

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.

What was Pangaea and when did it begin to break up? State which ocean opened up as the Americas separated from Europe and Africa.

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.

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.

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.

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 folded slates and gneisses with granitic intrusions dated to about 305 Ma.

2. 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.

3. Thrust structures and thrust nappes active 25 Ma ago that stack and deform the Mesozoic limestones and Tertiary sediments, originating a mountainous relief.

1. 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.
2. 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.
3. 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?

Analyze the following table with kinematic and geometric data for four lithospheric plates:

1. a) Justify, based on the correlation of these data, which mechanism ("slab pull" versus "ridge push") exerts the dominant control on plate velocity.
2. 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.
3. 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.

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."

1. 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.
2. 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.
3. 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.