During tectonic plate movements, strain is placed on the land around us. The strained materials deform in one way or another as energy is released by earthquakes, faults and folds. These forces change the landscape.

The types of changes depend on the amount (magnitude) and type of stress (tension, compression or shear), the pressure and the temperature acting on the materials and the nature of the materials themselves.

How do materials change?

There are three responses to strain. The material can:

  • behave in an elastic manner (low stress) – elasticity is a material’s ability to return to its original position when the stress acting on it is removed
  • behave in a plastic manner (intermediate stress) – plasticity is a material’s ability to permanently change shape or position without rupturing
  • fracture (high stress)

Some materials will pass through all three stages.

Ductile materials, such as clay, can undergo a large amount of deformation without fracturing at all. Brittle materials fracture with little or no plastic deformation.

Temperature and pressure are important considerations – many rocks are brittle when they are cold but become more ductile as they heat up. They also become more ductile with pressure. This explains why most faults (rupture zones) occur in the top 10 km of the Earth’s crust where the temperature and pressure is less. Below this depth, the pressure and heat causes rock to bend, fold and flow in response to stress.

The deforming Earth

As with other countries that straddle plate boundaries, New Zealand’s position astride two tectonic plates means we are geologically very busy. Plate movements are continuously placing strain on our country, and when this strain is eased or released, the result is earth deformation. The landscape of our country is in a constant state of change.

Where materials are placed under so much strain that they suddenly rupture to release the built-up energy, as happens in an earthquake, this event forms a fault. For example, the Wellington region is riddled with fault lines where previous earthquake events have caused displacement along fault planes. But rocks can also respond to strain by folding and buckling as has happened on the Alpine Fault along the length of the South Island to form our Southern Alps.

Nature of Science

The study of plasticity has given us a greater understanding of how various landforms are created, such as vast folded mountain ranges. An understanding of the plasticity of materials has also helped us calculate the conditions where structures like steel bridges and buildings will bend and when they will fall down.

On a molecular level, plasticity occurs when selected atomic bonds are broken within the malleable or ductile material. Plastic deformation occurs by slipping, twining or a combination of the two.

  • If a single crystal from a ductile metal (such as coppersilver or gold) is strained beyond its elastic limit, the crystal becomes elongated, causing a slip or step (like a microscopic fault plane) relative to its neighbour. In the case of slipping, atoms are moved a whole number of atomic spaces within the lattice. If the stress continues to act on the metal, each crystal will slide over its neighbouring crystal like a deck of cards sliding across a table.
  • With twining, the bonds are broken but the atoms fail to move a whole number of spaces and thus wind up changing the orientation of the crystal lattice.

The willow which bends to the tempest, often escapes better than the oak which resists it; and so in great calamities, it sometimes happens that light and frivolous spirits recover their elasticity and presence of mind sooner than those of a loftier character.

Sir Walter Scott (1771–1832)

Plastic materials are everywhere

Q. What is one of the most plastic materials on Earth?
A. Wet chewing gum of course – it can be stretched to many times its original length before snapping!

    Published 21 July 2007