Plate tectonics
If you place a map of South America next to a map of Africa, their coastlines almost fit together like the pieces of a jigsaw puzzle. This coincidence, which caught people's attention more than a century ago, is one of the clues that led scientists to understand that the continents are not stationary.
From continental drift to plate tectonics
If you examine palaeontological maps, remains of the land reptile Mesosaurus appear only in southern Africa and eastern Brazil, which are now separated by thousands of kilometres of open ocean: how could the same species have crossed an ocean when it could not swim?
Explain what a lithospheric plate is and name three plates: one containing Europe, an American one, and one that is almost entirely oceanic.
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)…
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.
The plates and what drives them
Networks of high-precision GPS stations measure that Europe and North America move apart by about $2{,}5\text{ cm}$ every year: what gigantic engine inside the Earth is able to drag rocky masses on the scale of continents?
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 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.
State and briefly describe two forces that contribute to the movement of lithospheric plates.
The three types of plate boundary
If you plot on a world map the epicentres of all the earthquakes recorded over the last century, the points are not scattered randomly, but lined up in very narrow, well-defined strips: what do these lines of fracture mark out so precisely?
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.
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…
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.
What landforms and phenomena each boundary produces
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…
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.
- 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.
- b) Calculate the total spreading rate of this seafloor (in $\text{cm/year}$).
- 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 $2,5\ \text{cm/year}$,…
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.
- 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.
- 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.
- 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.
Back to the beginning: The fit between the coastlines of South America and Africa is no coincidence: both blocks formed a single continuous mass in Pangaea until the Mid-Atlantic Ridge began to produce crust and push them apart.