Plate Tectonics
On September 19, 1985, and on the same date in 2017, Mexico City shook violently. Those earthquakes were no accident: they occur because the Cocos Plate sinks beneath the North American Plate off the Pacific coast. Why is geologic activity concentrated at the edges of enormous blocks of rock?
From continental drift to plate tectonics
If you mark the epicenters of earthquakes and active volcanoes from the last decade on a world map, you will see that they are not scattered at random: they trace continuous lines that surround huge, nearly quiet zones. In Mexico we see this in the Pacific Ring of Fire, home to Popocatépetl, the Colima Volcano, and Pico de Orizaba (which are part of the Trans-Mexican Volcanic Belt).
A lithospheric plate may include only oceanic crust (like the Pacific Plate, almost entirely oceanic), only continental crust, or both at once (like the North American Plate, which spans Mexico, the United States, and Canada plus part of the Atlantic; the Eurasian Plate, where Europe is located; or the South American Plate). Mexico sits on several plates: North America, Cocos, Pacific, and Caribbean.
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.
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."
The forces that move the plates
High-precision GPS stations measure that, for example, the distance between Mexico City and Tokyo changes by a few centimeters each year, on the order of $2.5\ \text{cm}$. What internal mechanism pushes masses of rock covering millions of square kilometers?
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.
A mid-ocean ridge separates two plates at a rate of $2.5\ \text{cm/year}$ on each flank. Calculate how many kilometers the ocean will have widened after $10$ million…
A mid-ocean ridge separates two plates at a rate of $2.5\ \text{cm/year}$ on each flank. Calculate how many kilometers the ocean will have widened after $10$ million years. Remember that total widening takes both flanks into account.
The three types of plate boundaries
In Iceland the ground splits open in cracks from which lava pours out; in the Mexican Pacific a plate sinks and feeds volcanoes like Popocatépetl; in California two blocks grind sideways past each other with no volcanoes. Why does contact between plates produce such different landscapes and hazards?
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.
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.
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…
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.
- 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.
- 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.
- 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.
Evidence for seafloor spreading
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.
Back to the beginning: the 1985 and 2017 earthquakes in Mexico City originate at the convergent boundary between the Cocos Plate and the North American Plate, where a dense oceanic plate sinks beneath another.