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Why can Earth’s mantle rocks act like solids during earthquakes but flow over millions of years?

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Figure 1: The position vector x and the velocity vector v of a material point, the body force fdV acting on an element dV of volume, and the surface force TdS acting on an element dS of surface in a Cartesian coordinate system 1, 2, 3 (see text).
mechanics of solids Figure 1: The position vector x and the velocity vector v of a material point, the body force fdV acting on an element dV of volume, and the surface force TdS acting on an element dS of surface in a Cartesian coordinate system 1, 2, 3 (see text).

Earth’s mantle rocks exhibit behavior characteristic of viscoelasticity, a property where materials display both solid-like and fluid-like responses depending on the timescale of the applied stress.

During an earthquake, the stress is applied rapidly. In this short timeframe, the mantle rocks behave as elastic solids, capable of transmitting seismic waves. However, over geological timescales of millions of years, these same materials undergo creep—a slow, time-dependent deformation. This allows the mantle to churn in convective motions, where hotter material rises and cooler material sinks.

Essentially, the mantle is not a simple solid or liquid but a complex material that responds to sudden forces with immediate elastic strain, while responding to sustained, long-term forces through gradual, fluid-like flow.12