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Difficulty: MediumIdentifying Underlying Assumptions and Premises

### Deep-Focus Earthquakes

Most earthquakes occur at depths of less than 70 km, where rocks are cold and brittle enough to fracture under stress. However, deep-focus earthquakes occur at depths between 300 km and 700 km, where high temperatures and pressures are expected to cause rocks to deform plastically (ductile flow) rather than fracture. Two scientists discuss the mechanisms responsible for these deep-focus events.

Scientist 1
Deep-focus earthquakes are caused by dehydration embrittlement. As a subducting oceanic slab sinks into the mantle, it carries hydrous minerals (such as serpentine) down with it. At depths of 300 km to 700 km, the increasing temperature and pressure cause these hydrous minerals to decompose, releasing liquid water into the surrounding rock. This released water is highly pressurized and enters pre-existing fractures, offsetting the extreme confining pressure of the mantle. This allows the rock to undergo brittle failure and slip, producing an earthquake.

Scientist 2
Deep-focus earthquakes are caused by transformational faulting, a process associated with mineral phase changes. The mantle mineral olivine normally transitions to denser phases (wadsleyite and ringwoodite) at depths greater than 410 km under thermodynamic equilibrium. However, because the core of a subducting slab is much colder than the surrounding mantle, olivine can persist in a metastable state well below its equilibrium depth. When this metastable olivine eventually transitions to the denser phases, the rapid volume reduction creates localized shear instabilities (anticracks) that propagate as a sudden brittle-like failure, triggering an earthquake.

Based on the passage, Scientist 2's explanation of deep-focus earthquakes relies on which of the following assumptions?

  1. A
    Liquid water must be present to reduce the effective stress and lubricate fault zones at depths between 300 km and 700 km.
  2. B
    The density of the subducting oceanic slab is less than the density of the surrounding mantle.
  3. The temperature of the subducting slab's core is low enough to prevent olivine from transitioning to denser phases at its normal equilibrium depth.Answer
  4. D
    Metastable olivine is structurally weaker than both wadsleyite and ringwoodite under all temperature conditions.

Answer

The temperature of the subducting slab's core is low enough to prevent olivine from transitioning to denser phases at its normal equilibrium depth.
The correct answer states that the temperature of the subducting slab's core is low enough to prevent olivine from transitioning to denser phases at its normal equilibrium depth. Scientist 2's hypothesis relies on the presence of metastable olivine at depth, which is enabled because the slab is much colder than the surrounding mantle. This temperature difference prevents olivine from undergoing the phase change at its thermodynamic equilibrium depth, allowing it to transition suddenly at greater depths to trigger earthquakes.

Step-by-Step Solution

1
Identify the core mechanism proposed by Scientist 2.
Scientist 2 suggests that deep-focus earthquakes are caused by transformational faulting during the phase transition of metastable olivine to denser phases.
Understanding the proposed physical mechanism is necessary to identify its underlying requirements.
2
Locate the cause of the olivine's metastable state from the text.
Scientist 2 states that olivine persists metastably because the core of the subducting slab is much colder than the surrounding mantle.
This establishes the physical condition (temperature) required for the metastability to occur.
3
Identify the implicit assumption behind the necessity of a cold slab core.
If the slab's core were not cold enough, olivine would transition under normal equilibrium conditions at shallower depths (around 410 km). Thus, the model assumes that the core remains cold enough to prevent this immediate transition.
This links the condition of metastability directly to the slab's temperature, revealing the underlying assumption.

Key Concept

Identifying Underlying Assumptions in Scientific Hypotheses
Estimated Time:1m 30s
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Topics

Identifying Underlying Assumptions and Premises

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