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.

Mesosaurus, a freshwater reptile, appears fossilised only in eastern South America and in southern Africa. Discuss why it could not have swum across the Atlantic and what conclusion Wegener drew from that distribution.

Explain what palaeomagnetism is and how igneous rocks, as they cool, "photograph" the direction of the Earth's magnetic field at the moment they formed.

What is an oceanic ridge? Describe which process occurs along its axis, what type of rock is generated there and what relationship it has with the expansion of the ocean floor.

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 kilometres the ocean has widened in the last $80$ million years and comment on why this figure is only an approximation.

Iceland is being split in two at a rate of about $2\ \text{cm}$ per year along the ridge that crosses it. a) Calculate how many metres the island will widen in $1\,000$ years and how many kilometres in one million years. b) Explain why Iceland concentrates active volcanism precisely along that fracture.

The thickness of the sediments on the ocean floor increases away from the ridges, and no ocean floor is older than about $200$ million years, in contrast to continents with rocks almost $4\,000$ million years old. Discuss how both pieces of data fit with oceanic expansion and with the existence of zones where lithosphere is destroyed.

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 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.

A field geology team describes four terrestrial tectonic basins (A, B, C, and D) with the following lithological and structural records:

• Basin A: Sunken continental valley with stepped normal faults, crustal thinning, bimodal volcanism (rhyolites and continental basalts), and elongated lakes with fluvial and evaporite sediments.

• Basin B: Narrow arm of the sea with parallel continental margins; in its central axis, the incipient formation of basaltic oceanic floor and hydrothermal vents is already recorded.

• Basin C: Wide ocean whose continental margins exhibit neither seismicity nor volcanism (passive margins), accumulating thick sedimentary wedges over the continental shelf and slope.

• Basin D: Collisional orogen without active volcanism, with a Moho located deeper than 65 km, massive thrusts, and the presence of ophiolites (an association of peridotites, gabbros, and pillow lavas) embedded within deformed sediments.

1. a) Deduce which stage of the Wilson cycle corresponds to each basin (A, B, C, and D), justifying your choice based on two observable geological criteria in each case.
2. b) Assign an actual modern geographic example to each of the four described stages.
3. c) Based on the presence of ophiolites in basin D and the dynamics of the Wilson cycle, answer: why is there no in situ oceanic crust older than 180 Ma in modern oceans, whereas on continental landmasses we find rocks older than 3 800 Ma?