Page 61 - Laboratory Manual in Physical Geology

a lateral or strike-slip fault ( FIGURE 2.4 ). Plate tectonic level of internal energy ( temperature ) and/or amount
forces can be understood by how the lithosphere is of stress (force, pressure) acting upon them over a given
strained and faulted. amount of time . To understand how temperature affects
viscoelastc materials, think about steel. Red hot steel
has a great amount of thermal energy, so it has more of
ACTIVITY a plastic-ductile behavior (can easily be hammered into
new shapes) than cold steel. Cold steel is less plastic-
2.3 Lava Lamp Model of Earth ductile and more brittle than red hot steel. The effects
of stress and time are more variable. Using lots of stress
THINK | like hard hammer strikes, you can hammer a small ball
of red hot steel into a flat disk in seconds. Alternatively,
About It What causes plate tectonics?
you can press on the steel with giant cold steel rolling
pins over minutes of time to roll the steel into a flat disk.

OBJECTIVE Investigate viscoelasticity and rheidity, The effect is the same; the difference is the amount of
then evaluate a lava lamp model of mantle convection.
stress and the amount of time it is applied. One study
PROCEDURES of these effects has been carried out at the University of
Queensland since 1927.
1. Before you begin , read about viscoelasticity,
rheids, mantle convection, and seismic Physicist Thomas Parnell has studied the properties
tomography below. Also, this is what you will of pitch, a black solid material derived from naturally
need to do the activity: occurring asphalt. Unless heated, pitch normally
behaves as a brittle solid with little elasticity, so it
____ blue and red colored pencils or pens shatters when struck with a hammer. But at the same
____ plastic ruler or popsicle stick temperature, under the stress of gravity, Parnell found
____ Activity 2.3 Worksheets (pp. 59 – 60 ) and that Pitch also behaves as a viscous fluid. In 1927,
pencil he heated some pitch to make it a liquid, and then
____ Silly Putty™ and Lava lamp (provided in lab)
poured it into a funnel with a sealed stem at its base.
2. Answer every question on the worksheet in a There, it solidified into its rigid solid form. He waited
way that makes sense to you and be prepared to 3 years, and then cut one the stem of the funnel to
compare your work and inferences with others. see what would happen. After several years it began
to flow like a viscous fluid, and 8 years later a drop
of the pitch fell from the stem of the funnel. Since
then, a drop of pitch drops from the funnel about
What Are Viscoelasticity and every 9 years. The experiment is now maintained by
physicist John Mainstone and can be viewed at the
Rheids? University of Queensland via webcam at http://smp.
Viscoelasticity is a combination of two words: viscos- uq.edu.au/content/pitch-drop-experiment . Although,
ity and elasticity. Both terms describe the mechanical temperature and the funneling of the pitch towards
behavior of materials and whether their deformation the stem of the funnel are factors in the flow, pitch
(change shape under stress) is permanent or reversible. clearly has two behaviors based relative to time and the
Viscosity describes a fluid (like liquids and gases) behav- stress (force) of gravity. When acted upon by gravity
ior that leads to irreversible plastic-ductile deformation on a short timescale (days or less), pitch acts like a
(like shaping a lump of clay into a new form). The solid solid. When acted up by gravity on a longer timescale
cannot recover from its plastic-ductile deformation. (years), pitch acts like a viscous fluid. The main
Elasticity describes a behavior of solids that bend or difference is time.
stretch (like a rubber band) but return to their original Rheidity is the time it takes for a solid material un-
shape (recover from their deformation) when you stop der stress to lose its elastic-brittle behavior, entirely, and
bending or stretching them. The deformation of elastic just permanently deform (flow) like a viscous fluid. The
materials is not permanent, unless you exceed their abil- rheidty of pitch is several years. Ice in a glacier has a rhe-
ity to bend (their elastic limit) and they break (a brittle idity of several weeks. At the time when a solid material
behavior, like stretching a rubber band until it breaks). exceeds its rheidity it is no longer viscoelastic—it is just

So viscoelasticity describes materials that can behave, to viscous. A solid material that has changed from visco-
some degree, as both elastic-brittle solids and very viscous elastic behavior to just viscous behavior is called a rheid
liquids (plastic-ductile solids). (pronounced RAY-id).
Many materials that you may regard as rigid solids Rocks can become rheids. For example, inside Earth
are actually viscoelastic materials. The degree to which it gets hotter with depth. So as you go deeper into the
they behave more or less as solids is determined by their Earth, the rocks stop acting like elastic to brittle solids

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