The Misconception

Earth is molten except for the crust is a common misconception in the geosciences. The crust, the solid upper/outer portion of our planet, is often thought of as a thick shell containing the molten or liquid interior of our planet. Lava (molten rock on the surface of the planet) is thought to flow from the mantle through the crust or directly from the liquid core.

The interior of our planet is made of chemically distinct layers: the solid crust, the brittle upper mantle and plastic or flexible lower mantle, the liquid outer core, and the solid inner core. Magma or molten rock is supplied to the surface in a few ways. First by way of decompression melting (a way of reducing the pressure and, as a result, lowering the melting point) as occurs at mid-ocean ridges and rift valleys. Second by way of saturation melting as happens when crust saturated with sea water subducts under a lithospheric plate (made up of the crust and the brittle part of the upper mantle) in oceanic-collision zones reducing the melting point producing magma (molten rock). And third, through mantle plumes or hot spots that are intense heat plumes that locally increase temperature of the rock in a particular spot on the planet and when reach a certain depth/pressures sufficiently low enough to allow for lower melting points. These hot spots or plumes are thought to be relatively stable and aren’t liquid flows from the mantle or core, but instead are fed with intense heat energy that moves through the solid interior through convective currents.



WHERE DO THE MISCONCEPTIONS COME FROM?


Color of Diagrams of the Interior

Diagrams of the interior of the planet typically illustrate the crust as a brown or grey rock-like layer with the mantle, outer core, and inner core displayed as different degrees of reds and oranges moving toward the center of the Earth as seen in the image below. Common experience tells us that red glowing items are hot and just as with a campfire, the whiter the flames the hotter the flame.
interior-planet.png
2012 (c) Cassie Grether, free for use for public use



Convective currents

Students often are given examples of convective currents as a boiling pot of water on the stove and other sources that stem from liquids. It is hard for students to imagine on their own that solids can carry convective currents too.

Literature & Movies

Dante’s Inferno, movies, and cartoons often depict the interior of the Earth as caverns where molten lava runs in river-like streams. Some religious beliefs also portray this outlook placing hell in the center of the Earth.

Use of the Term Deep in Geoscience

Textbooks and educational movies use the term deep often causing many to think much below the crust and upper mantle and deep into the center of the Earth. When using this term, it is often not used with depth and location of the center of the Earth causing students to go deeper than intended.

Magma comes from the core

The misunderstanding that magma generates from the core and pushes itself up through the mantle and crust to produce eruptions generally is a self developed concept students derive at once they learn and understand that the crust and mantle are both solids.

Temperatures rise as depth increases

As you get deeper into the Earth the temperature rises. We know this as we see it on TV and in the news as geothermal energy resources are being explored. As real life evidence tells us, higher temperatures melt solids. We see this in ice, butter, chocolate, and many other everyday instances. Unfortunately all of our everyday instances are experienced at relatively the same pressures. In reality, higher pressures translate to higher melting points causing more energy or temperature to be added to a system to go from the solid phase to the liquid phase.


Resources


1. Geological Society of America, 2005, Plates, Plumes, And Paradigms, p. 590.
2. Herndon, J. M., 1980, The chemical composition of the interior shells of the Earth, Proc. R. Soc. Lond, p. 149–154.
3. Kirkby, K., 2008, 'Easier to Address' Earth Science Misconceptions, http://serc.carleton.edu/NAGTWorkshops/intro/misconception_list.html, University of Minnesota, accessed 4/15/2012.
4. Neild, T., 2009, Blow Up the Earth - Fact or (Science) Fiction?,http://www.geolsoc.org.uk/gsl/pid/6584;jsessionid=482191A597A9C8434953F2CDFE1E204E, The Geological Society, 4/15/2012