Exercises: Plate tectonics

Explain what a lithospheric plate is and name three plates: one containing Europe, an American one, and one that is almost entirely oceanic.

The diagram shows the three types of margins that can exist between two lithospheric plates. State which type of plate margin (divergent, convergent or transform) corresponds to each number and explain what happens to the plates in each case.

State whether the following statement is true or false and justify your answer: "Continents move because the lithospheric plates on which they rest move."

State and briefly describe two forces that contribute to the movement of lithospheric plates.

In subduction zones, one plate slides beneath another. Describe two geological phenomena associated with these zones and give a real example of a subduction zone.

The San Andreas Fault in California is a transform margin. Explain the relative movement of the plates at this type of margin, what hazard it creates, and why volcanoes do not form there.

An oceanographic vessel records the magnetic anomalies of the seafloor along a transect perpendicular to an ocean ridge. The data reveal that the last global magnetic reversal (which occurred $780\ 000$ years ago) is found in basaltic stripes located $15,6\ \text{km}$ to the east and $15,6\ \text{km}$ to the west of the central rift.

1. a) Justify how the symmetry of the magnetic stripes originates from the cooling of magma and explain why this pattern constituted definitive proof of seafloor spreading.
2. b) Calculate the total spreading rate of this seafloor (in $\text{cm/year}$).
3. c) If at a different ridge the same anomaly from $780\ 000$ years ago is located $62,4\ \text{km}$ from the rift, interpret what differences exist between both margins regarding the rate of crust generation and tectonic dynamism.

Palaeomagnetic and satellite measurements confirm that the Mid-Atlantic Ridge generates new oceanic lithosphere at a total spreading rate of about $2,5\ \text{cm/year}$, which has enabled the widening of the Atlantic Ocean since the breakup of Pangaea.

1. a) A student proposes the following deduction: "Since ocean ridges worldwide continuously generate crust, the volume and surface area of the Earth must have grown steadily over the last $200$ million years". Refute their hypothesis by explaining what process compensates for the creation of lithosphere and at what type of plate boundaries it takes place.
2. b) If the current distance between the coasts of West Africa and North America at that latitude is about $4\ 500\ \text{km}$, estimate how many million years ago the Atlantic Ocean began to open in that region, assuming that spreading rate remained constant.
3. c) Predict and justify how the age of the rocks and the thickness of marine sediments will vary as you move from the ridge axis towards the African continental margin.

At the highest elevations of Mount Everest (above $8\ 000\ \text{m}$ in altitude), there are limestone strata containing abundant Mesozoic marine fossils. At the same time, despite the violence of earthquakes in the Himalayas, no active surface volcanism is recorded, unlike what happens in the Andes mountain range.

1. a) Justify in terms of density and buoyancy why the subduction of continental lithosphere does not occur in the collision between the Indian and Eurasian plates, explaining what crustal deformation phenomenon replaces subduction.
2. b) Explain the sequence of tectonic events that allows ancient marine sediments from the Tethys Ocean to be found today at the highest peak on the planet.
3. c) Argue why there are dozens of active volcanoes in the Andes and none in the Himalayas, relating the presence or absence of arc magmatism to the thermal and melting fate of the colliding plate.