Questions: Rock Rheology and Elastic-Plastic Deformation

Question 1

As depth increases in the continental crust, how do brittle strength and ductile strength change, and what does their intersection represent?

• A. Both decrease with depth; their intersection marks where the crust becomes too weak to support mountains
• B. Both increase with depth; their intersection marks where earthquakes become most frequent
• C. Brittle strength increases (due to confining pressure) while ductile strength decreases (due to rising temperature); their intersection is the brittle-ductile transition
• D. Brittle strength decreases while ductile strength increases; their intersection marks the base of the lithosphere

Answer: (C)

These two opposing trends are the key to understanding the seismogenic zone. Confining pressure from overlying rock clamps fractures shut at depth, requiring more force to overcome friction — so brittle strength increases. Meanwhile, rising temperature mobilizes atoms within mineral crystals, making dislocation creep easier and dramatically lowering ductile strength. The brittle-ductile transition is where these two curves cross, typically at 300–400°C for quartz-rich continental crust.

Question 2

A geophysicist finds abundant seismicity at 8 km depth in a continental region but essentially no seismicity below 18 km. The most likely explanation is:

• A. Rocks below 18 km are too porous to store elastic strain energy
• B. Tectonic stress decreases with depth, so there is insufficient force to cause earthquakes below 18 km
• C. Below ~18 km, temperatures are high enough that rocks deform ductilely, releasing stress through continuous flow rather than sudden fracture
• D. Seismic waves cannot propagate through the warm, plastic rock below 18 km

Answer: (C)

Earthquakes require rocks that can accumulate elastic strain and then fail suddenly — brittle behavior. Below the brittle-ductile transition, rocks flow continuously via dislocation creep under the same tectonic stresses, releasing stress gradually rather than in sudden ruptures. This defines the base of the seismogenic zone. Seismic waves do propagate through ductile rock (that's how we know it's there), and tectonic stress doesn't simply disappear at depth.

Question 3

Near the Earth's surface, rocks in the brittle regime are stronger than rocks just above the brittle-ductile transition because the surface lacks confining pressure.

• T. True
• F. False

Answer: False

This is backwards. In the brittle regime, strength *increases* with depth because confining pressure clamps fractures shut and increases friction on potential fault surfaces. Rocks at shallow depths are *weaker* in the brittle regime, not stronger. The lithosphere actually reaches its *maximum* strength just above the brittle-ductile transition, where confining pressure is high but temperatures have not yet risen enough to trigger ductile flow.

Question 4

Ductile deformation in the deep crust and mantle occurs through distributed, continuous flow mechanisms like dislocation creep rather than through discrete fractures or fault planes.

• T. True
• F. False

Answer: True

Ductile deformation involves atomic-scale processes — migration of crystal defects (dislocations) through mineral lattices, diffusion of atoms, and grain boundary sliding. These produce continuous, distributed strain without discrete rupture surfaces. This is why the ductile lower crust and mantle are seismically quiet: stress is released gradually rather than catastrophically. The contrast with brittle fracture (which creates the discrete fault planes of earthquakes) is fundamental to understanding the depth distribution of seismicity.

Question 5

Why does the lithosphere have a strength maximum at intermediate depth rather than being uniformly strong or progressively weaker with depth?

Model answer: Two competing mechanisms control strength at different depths. In the shallow, brittle regime, strength increases with depth because increasing confining pressure suppresses fracture and raises frictional resistance on faults. In the deeper, ductile regime, strength decreases with depth because rising temperature exponentially reduces resistance to dislocation creep. The strength maximum occurs at the brittle-ductile transition depth, where confining pressure is high but temperatures have not yet risen enough to dramatically soften the rock. Below this point, the exponential temperature dependence of ductile strength dominates, and rock becomes progressively weaker with depth.

This 'strength envelope' concept explains a key observation: lithospheric plates act as rigid bodies despite being surrounded by weaker mantle. The strong layer near the brittle-ductile transition is what gives the plate its mechanical coherence. It also explains why large thrust faults (like subduction zones) can accumulate decades of elastic strain — that elastic storage happens in the brittle layer above the transition.