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.

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.

Explain the difference between a continent and a lithospheric plate, and discuss, using the example of the South American plate, why the same plate can carry half a continent and half an ocean.

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.

In rocks of regions that are tropical today (India, southern Africa, South America), glacial striations from about $300$ million years ago are found. Explain what they indicate about the position those continents occupied at the time and how they fit into a single polar ice cap if they are reassembled.

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.

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.

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.

The "apparent polar wander" paths calculated for Europe and for North America are different, as if each continent had seen the pole move along a different route. Explain why this contradiction disappears if what moved was each continent on its own.

When reconstructing the fit between South America and Africa, the match is much better if the edges of the continental shelves are joined rather than the present-day coastlines. Explain why (erosion, sedimentation and changes in sea level modify the coast) and what the edge of the shelf really represents.

The Wallace Line, between the Indonesian islands of Bali and Lombok, separates Asian faunas from Australian faunas despite their closeness. Relate this biogeographical boundary to the tectonic history of the region (the convergence of the Sunda and Sahul shelves and the great depth of the strait) and explain why species have not crossed it.

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.

In a South Atlantic transect perpendicular to the axis of the Mid-Atlantic Ridge, three key points of the symmetrical basaltic crust on both sides of the rift have been dated:

• Magnetic anomaly C5 (10 Ma old) is located 200 km to the east and 200 km to the west of the ridge.

• Magnetic anomaly C34 (84 Ma old) is located 1 400 km on each side of the ridge.

• The oceanic crust-continental crust boundary (edge of the initial rifting, dated at 130 Ma) is found 3 000 km on each side.

1. a) Calculate the total opening velocity of the South Atlantic Ocean (expressed in cm/year) for the three time intervals: (i) from 130 Ma to 84 Ma, (ii) from 84 Ma to 10 Ma, and (iii) from 10 Ma to the present.
2. b) Interpret the calculated results: has the rate of seafloor spreading in the South Atlantic been constant? Explain how the analysis of magnetic anomalies reveals these accelerations or decelerations.
3. c) Two space geodetic positioning stations (GNSS/GPS) located in Fortaleza (Brazil) and Libreville (Gabon) currently record that the intercontinental distance is increasing at a rate of 3,8 cm/year. Compare this figure with the calculated recent geological rate and justify what advantage GNSS/VLBI technology provides over the paleomagnetic record in the study of plate kinematics.

Analyze the following comparative geodynamic data between two large active collisional orogens:

• Himalaya-Tibet system: Convergence rate: ~4,5 cm/year. Maximum crustal thickness: 75 km. Average elevation of the high plateau: 4 500 m. Lithology involved: frontal collision of a cold, dense, and rigid Precambrian craton (Indian plate) against Eurasia.

• Alps system: Convergence rate: ~0,5 cm/year. Maximum crustal thickness: 50 km. Average elevation of the orogen: 2 000 m. Lithology involved: oblique and complex collision of detached microplates (Apulia/Adriatic block) against the European margin.

1. a) Justify, based on the differences in convergence velocity, mass, and lithospheric inertia of the contacting blocks, why the crust beneath Tibet has reached nearly twice the thickening of that in the Alps.
2. b) Even though convergence between India and Asia is still active, measurements show that the Tibetan Plateau is currently experiencing normal faulting (east-west extension) and that the highest peaks have reached an orogenic ceiling. Explain what geodynamic and isostatic phenomenon (gravitational collapse) limits the indefinite vertical growth of a mountain range.
3. c) Reasonably predict the structural evolution of the Himalayan orogen over the next 15 million years if India continues its advance: in which direction will active thrusts migrate, and what will happen to the rock mass of the high plateau?

A geodynamic map of the Mediterranean shows three prominent features:

1. Deep seismicity planes (Wadati-Benioff zones) dipping beneath the Hellenic arc (Greece) and beneath the Calabrian arc (southern Italy), reaching seismic foci down to 400 km depth.

2. Chains of active volcanoes with explosive magmas rich in andesites and dacites in the Aegean Sea (Santorini) and in the Tyrrhenian Sea (Aeolian Islands).

3. Young marine basins (the Tyrrhenian Sea and the Aegean Sea) whose seafloor shows crustal thinning and active extensional faults, despite Africa and Eurasia globally converging at a rate of about 0,5 cm/year.

1. a) Interpret, based on seismicity and volcanism, what geodynamic process operates in the Hellenic and Calabrian arcs, deducing the origin and nature of the explosive magma in those areas.
2. b) Resolve the apparent paradox of point 3: how is it possible for extensional basins and opening to exist within an ocean that is globally in a phase of tectonic closure? (Hint: consider subduction hinge retreat or "slab rollback").
3. c) Predict what orographic structure and what type of crust will predominate in the Mediterranean region in about 40 million years if convergent motion continues, and mention two geological records that will attest in the future that a marine basin once existed there.

In 1915, scientists advocating fixism explained the presence of identical fossils of Mesosaurus (a Permian freshwater reptile) in South Africa and Brazil by postulating the existence of "intercontinental land bridges" that later sank into the ocean floor.

1. a) Apply the principle of isostasy and the compositional and density differences between continental crust (sial) and mantle/oceanic crust (sima) to argue why the fixist hypothesis of "collapsed bridges" is physically nonsensical.
2. b) Analyze the Permian glacial striation marks found in South America, southern Africa, India, and Australia: justify why their divergent orientation from modern coastlines toward the continental interior is inexplicable with fixed continents, but becomes immediately coherent if the supercontinent of Pangea is reconstructed.
3. c) Evaluate the classic phrase: "Wegener was right about the what and wrong about the how." Detail what erroneous driving mechanism he proposed for continental drift and explain what subsequent tectonic concept satisfactorily resolved that physical shortcoming.

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?