Structure and composition of the geosphere

Cite two direct methods of studying the geosphere and explain why, even with the deepest boreholes, we directly know only a tiny fraction of the Earth's radius.

Classify these methods of studying the Earth's interior as direct or indirect and justify your answer: a deep borehole, the recording of seismic waves, the analysis of xenoliths brought up by a volcano and the study of meteorites.

The Kola borehole (Russia) reached a depth of about $12\ \text{km}$, the drilling record. Calculate what percentage of the Earth's radius ($6\,370\ \text{km}$) this depth represents and conclude why knowledge of the planet's interior depends on indirect methods.

Compare continental crust and oceanic crust in terms of their thickness, composition, and density, and relate these differences to their behaviour when two plates collide.

Explain the principle of isostasy using the iceberg analogy, and discuss why high mountain ranges have deep crustal "roots" and what happens when erosion thins them.

Explain how P and S seismic waves differ (type of movement and media they pass through) and how their shadow zones allowed the existence and state of Earth's core to be deduced.

Distinguish between crust, lithosphere and asthenosphere: indicate which criterion defines each boundary (composition versus mechanical behaviour) and which layer contains which.

What is a seismic discontinuity? State which layers are separated by the Mohorovičić and Gutenberg discontinuities and at roughly what depths they are located.

In a debate on internal geodynamics, a student states: "The Mohorovičić discontinuity is the plane where the lithosphere detaches and slides over the asthenosphere. Furthermore, since oceanic crust is silicate and less dense than mantle peridotites, it is impossible for a lithospheric plate to sink spontaneously into the mantle during subduction."

Identify the geological errors in this statement and refute them by answering the following:

1. a) Precisely differentiate the nature of the crust-mantle boundary versus the lithosphere-asthenosphere boundary, indicating which compositional layers make up a mature oceanic lithospheric plate.
2. b) Resolve the buoyancy paradox: justify why an oceanic lithospheric plate as a whole does become denser than the underlying asthenosphere and ends up subducting spontaneously despite including the oceanic crust.
3. c) Justify why the peridotites of the lithospheric mantle behave as a brittle, rigid solid while the peridotites of the asthenosphere, chemically identical, behave in a ductile manner, analyzing the relationship between the geothermal gradient and the solidus curve.

The outer core is liquid and the inner core, although even hotter, is solid. Explain this apparent paradox by resorting to pressure, and discuss which essential planetary phenomenon (the magnetic field) is generated in the moving outer core.

A geophysical exploration probe deploys a network of seismometers on a newly discovered rocky planet. Following a major meteorite impact at its north pole ($0^\circ$), the seismographs record:

• Direct P-waves between $0^\circ$ and $105^\circ$ of angular distance.

• A total shadow zone (without direct P- or S-waves) between $105^\circ$ and $142^\circ$.

• Between $142^\circ$ and the antipodes ($180^\circ$), P-waves are recorded again, but they arrive with a noticeable delay, and no direct S-waves are recorded.

1. a) Interpret these records by applying the laws of seismic wave propagation and refraction to deduce the existence of the core, its physical state, and its relative propagation velocity compared to the mantle.
2. b) Predict which new direct seismic phases you would expect to record in the antipodal zone ($142^\circ$-$180^\circ$) if, with the progressive cooling of the planet, a solid inner core were to begin nucleating and growing.

A team of astrobiologists investigates the habitability of a rocky extrasolar planet. Their measurements reveal that it currently lacks a discernible global magnetic field, but analysis of its oldest basalts shows that $3\,500$ million years ago it possessed an intense dipolar field that experienced periodic polarity reversals.

1. a) Justify why the remanent magnetism in the basaltic lavas (acquired below the Curie temperature) proves that the field was generated by a dynamo mechanism in a convective fluid core and rules out the hypothesis of a permanent magnet of magnetized rock in the deep interior.
2. b) Propose a plausible geophysical hypothesis regarding what happened to that planet's interior for the geodynamo to cease, and predict the sequence of impacts that this magnetic collapse caused on its atmosphere, its surface liquid water bodies, and its biological potential, using the geological evolution of Mars as a reference.