The theory of plate tectonics

What is a lithospheric plate and over which more ductile layer does it move? Explain the difference between the lithosphere and the asthenosphere.

What is the Earth's crust and how does it differ from the mantle? Which discontinuity separates them and how was it detected?

Compare oceanic and continental crust in terms of dominant composition, density, thickness and maximum age of their rocks.

Name the three types of lithospheric plate boundaries and describe the relative movement of the plates at each of them.

Why are earthquakes and volcanoes not randomly distributed across Earth's surface, but concentrated in narrow, elongated belts?

What is an oceanic trench and at what type of plate boundary does it form? Give a present-day example.

The diagram shows three plate boundaries (a, b and c). Identify the type of boundary in each panel, describe the plate movement taking place, and give a real-world example of each.

At a convergent subduction boundary, earthquakes become progressively deeper away from the trench towards the continent. Explain the cause of this distribution (Benioff zone).

Transform boundaries have abundant earthquakes but hardly any volcanism. Explain why, relating it to the type of movement and to the fact that lithosphere is neither created nor destroyed there.

At a mid-ocean ridge, each flank moves away from the axis at a rate of $2,5\ \text{cm/year}$. Calculate how many kilometres the ocean will have widened after $10\ \text{Ma}$, taking into account that total widening is the sum of both flanks.

At the East Pacific Rise each flank moves away from the axis at $8\ \text{cm/year}$. Calculate how much that ocean will have widened in $5\ \text{Ma}$ and compare that rate with that of the Atlantic ridge, $2,5\ \text{cm/year}$ per flank.

The magnetic anomalies of the ocean floor form symmetrical bands on both sides of the ridge. Explain what these bands record and how they demonstrate that the ocean floor is spreading.

Two GPS stations located on either side of a ridge move apart at a total rate of $4\ \text{cm/year}$. Estimate the age of the oceanic crust located $240\ \text{km}$ from the ridge axis and state the assumption you make about the constancy of the spreading rate.

Along a section of the San Andreas Fault, two GNSS (geodetic GPS) stations located in the crust $15\ \text{km}$ on either side of the main trace have recorded no relative displacement between them over the last $160\ \text{years}$. However, regional stations located more than $100\ \text{km}$ on both sides of the fault record a continuous and steady relative motion of $35\ \text{mm/year}$. Furthermore, paleoseismic studies show that a $40.000\text{-year-old}$ Quaternary alluvial fan is laterally offset by a total of $1,4\ \text{km}$ along the fault.

1. a) Interpret the apparent discrepancy between the lack of local movement over the last $160\ \text{years}$ and the regional velocity measured by GNSS by applying the elastic rebound theory.
2. b) Verify with calculations whether the long-term geological displacement rate (deduced from the alluvial fan) is consistent with the current instantaneous velocity obtained by GNSS.
3. c) Calculate the slip deficit accumulated on the locked fault plane during those $160\ \text{years}$ and predict the expected coseismic displacement if all that elastic strain were released at once in a single earthquake.

Name at least two forces that contribute to the movement of the lithospheric plates, such as ridge push, mantle drag or the slab pull of the subducting plate, and briefly describe each one.

What is a hot spot? Give an example and explain why its volcanism is not associated with any plate boundary.

In volcanic arcs magma is generated some $100\ \text{km}$ above the subducting plate. Explain the role played by the water released by the plate as it dehydrates in the melting of the overlying mantle.

The Hawaiian-Emperor seamount chain displays a systematic morphology: the southeastern islands (such as Hawaii) are huge, subaerial shield volcanoes with elevations up to $4.200\ \text{m}$ and ages younger than $1\ \text{Ma}$. Toward the northwest, the islands gradually transition into coral atolls and, further north, into guyots (flat-topped seamounts submerged at depths greater than $1.000\ \text{m}$ with ages up to $80\ \text{Ma}$). Located $3.500\ \text{km}$ northwest of Hawaii is Daikakuji guyot, dated at $47\ \text{Ma}$, at the exact point where the alignment abruptly changes orientation (from NW-SE to N-S).

1. a) Explain the sequence of geological processes (thermal, isostatic, and morphosedimentary) that transform an active volcanic island into a submerged guyot as the lithosphere moves away from the hotspot.
2. b) Calculate the average rate of motion of the Pacific plate between the present day and the formation of the bend $47\ \text{Ma}$ ago, expressing the result in $\text{cm/year}$.
3. c) It is debated whether the Emperor "bend" reflects an abrupt change in the kinematic trajectory of the Pacific plate or the motion of the mantle plume itself. What testable geological evidence (by analyzing other contemporaneous intraplate chains in the Pacific or through paleomagnetism) would allow confirming whether the direction of the plate changed?

Consider the following average geophysical data for the lithosphere and asthenosphere:

• Oceanic lithosphere freshly created at the ridge: thickness $\approx 12\ \text{km}$; mean density $\approx 3,05\ \text{g/cm}^3$.

• Mature oceanic lithosphere ($>60\ \text{Ma}$): thickness $\approx 100\ \text{km}$; mean density $\approx 3,35\ \text{g/cm}^3$.

• Typical continental lithosphere: thickness $\approx 150\ \text{km}$; $35\ \text{km}$ crust (density $\approx 2,75\ \text{g/cm}^3$) and $115\ \text{km}$ lithospheric mantle (density $\approx 3,30\ \text{g/cm}^3$); overall mean density $\approx 3,17\ \text{g/cm}^3$.

• Underlying upper asthenospheric mantle: density $\approx 3,25\ \text{g/cm}^3$.

1. a) Based on the principle of buoyancy and the data above, justify why an oceanic ridge can never subduct spontaneously, whereas old oceanic lithosphere can collapse and initiate subduction.
2. b) Explain why continental lithosphere cannot undergo deep and continuous subduction into the asthenospheric mantle, and predict what mechanical response it experiences when a passive continental margin is dragged into a subduction trench.
3. c) During subduction, the basalts and gabbros of the oceanic crust undergo high-pressure, low-temperature metamorphism, transforming into eclogite (density $\approx 3,50\ \text{g/cm}^3$). Justify how this phase transformation affects the slab pull force.