Final answer:
Strained earth materials that undergo permanent deformation through plastic flow are those which experience stress beyond their elastic limit, such as rocks subject to intense tectonic activity.
Step-by-step explanation:
The kind of strained earth materials that undergo permanent deformation by flowing plastically are those that experience stress beyond their elastic limit.
In the natural world, this commonly refers to rocks within the Earth's crust that are exposed to such conditions during tectonic activity.
When the stress on these materials exceeds the point of elasticity (point E), the materials no longer return to their original shape or size once the stress is removed, resulting in permanent deformation. Metamorphic rocks, for instance, often display this behavior when subject to intense pressure and high temperatures, which allow for plastic flow without fracturing.
Ductile materials, such as metals, tend to show a gradual decrease in stress with increasing strain, meaning that as they approach their breaking point, they become easier to deform.
However, with rocks, the scenario also depends on conditions such as depth, pressure, and temperature, where rocks at greater depths tend to exhibit more plastic behavior due to the increased temperature and pressure, which makes them more malleable.
Hence, understanding the concepts of shear stress, strain, and elasticity and plasticity is crucial to comprehending how geological structures and features such as folds and faults are formed, which in turn greatly influence the Earth's landscape.