Rock Mechanics

Constituents of Rocks

Rocks are made up of minerals, and the mineral constituents define the type of rock and its properties.
The different minerals in the rock may have different physical properties such as hardness, density and chemical resistance.
The arrangement and size of the mineral grains within a rock can significantly affect its strength and how it responds to pressure.
Igneous rocks, such as granite and basalt, are hard, strong rocks that formed from cooled magma or lava.
Sedimentary rocks, such as sandstone and limestone, are usually softer and may be porous.
Metamorphic rocks, like marble and slate, have been altered by heat, pressure, or chemical processes.

Stress refers to a force per unit area and may cause deformation or fracture in rocks.
Stress can be caused by factors such as tectonic forces, overburden weight, changes in temperature, and erosion.
Strain refers to the change in shape or volume of a rock due to applied stress.
Rocks respond differently to stress, depending on factors such as the rate of stress application, time, temperature, pressure and rock composition.

Rocks can exhibit elastic deformation, where they return to their original size and shape once the stress is removed. This happens when the stress is within the elastic limit of the rock.
When stress surpasses the elastic limit, rocks show plastic deformation where they don’t return to their original shape after the stress is removed.
If the stress exceeds the rock’s strength, it will cause a fracture.
Under confining pressure, such as deep under the Earth’s surface, rocks are more likely to deform plastically rather than fracture.

High temperatures can weaken rocks, making them more susceptible to deformation and fracture.
At high temperatures and pressures, some minerals can recrystallise, creating new minerals and altering the rock’s properties. This forms part of the process of metamorphism.

Under intense pressure, minerals may align themselves in one direction to form foliation.
This preferred orientation of minerals can significantly impact the rock’s mechanical properties, making it behave anisotropically. Therefore, the rock will have different strengths and weaknesses in different directions.
Slate is a common example of a foliated, anisotropic rock.

Rock mechanics is crucial in global tectonics, as it helps to explain the behaviour of the Earth’s crust and mantle.
It offers an understanding of the creation and movement of tectonic plates, the formation of mountain ranges, and the occurrences of earthquakes and volcanic eruptions.
Studies involving rock mechanics can also help in predicting the stability of slopes, the safety of tunnels and the effectiveness of hydraulic fracturing.

This is the boundary between the Earth’s crust and mantle, named after the Croatian seismologist Andrija Mohorovičić.
It marks a significant change in rock properties, notably the seismic wave speed, indicating a transition from less dense crustal materials to denser mantle rocks.
Knowledge of this discontinuity aids in understanding the earth’s layered structure—an essential part of global tectonics.