It is fairly
simple to demonstrate convection in water. All you need is a bowl, some warm
water, ice and food colouring (see our video). Students can relate this to oceanic circulation, a major driver of the
movement of heat and nutrients around the globe (see our post on this).
Convection in water (R Nave, Hyperphysics, accessed 17/6/2020).
Convection
in air is another everyday example of this phenomena and possibly simpler to
demonstrate. Think about winds or the
heating of a room.
Convection in air (Energy Education,
accessed 17/6/2020).
What is more
difficult to demonstrate, and relate to, is convection in the Earth’s mantle. How
can a solid convect? This puzzle leads to one of the major misconceptions
in the Earth Sciences, that the mantle is liquid. It is easy to understand
where the misconception comes from. Although students have observed convection
in gases and liquids around them they are very unlikely to understand the
convection of solids. The classic diagrams for this process are also very easy
to misinterpret. If you were already relating convection to liquids, nothing
about these diagrams would contradict that idea.
Classic diagram of mantle convection
(Surachit, licensed under CC BY-SA 3.0, accessed on 17/6/2020).
Add to this the
commonly told story of heat from the core providing the energy for mantle
convection and you can see why many students imagine the mantle as thick,
liquid rock (melted by heat from the core) that is travelling around in almost
perfect circles beneath our feet!
In reality,
approximately 50% of mantle heating occurs due to the decay of the radioactive
isotopes of uranium, thorium and potassium. About 40% is attributed to the
long-term cooling of Earth and only about 10% is from the core (Olson, P.,
2003). Leading to heat transport by subsolidus (or solid state) convection.
The
important concept to grasp in mantle convection is viscosity. It helps us to
understand that the mantle is fluid over long time scales. In fact, the
hypothesis that the mantle convects was entirely refutable until evidence that it
is viscous was identified (Bercovici, D., 2015).
So how do we
measure mantle viscosity? We measure postglacial rebound. That is the uplift of
continental masses, such as Canada, following the melting of glacial ice caps
(find out more about isostasy). These measurements defined an
average viscosity of the mantle of μ = 1021 Pa s
(Bercovici, D., 2015). To put that in perspective, water at 20°C has a
viscosity of 1.0016 mPa s.
In short, the mantle is
made up of very viscous rock that is heated by radioactive decay, long-term
cooling and the core. This causes subsolidus convection, ONE of the drivers of
plate tectonics (the others will just have to be the subject of another post…)
- For an activity to simulate mantle convection driving plate tectonics movement click here. The full activity can be found here.
References
Bercovici,
D., Treatise on Geophysics (Second Edition), 2015
Olson, P.,
Encyclopedia of Physical Science and Technology (Third Edition), 2003