This was originally going to be just a post, then I decided to make it into an article.
Not so OT… if you can extend your thinking…
“Spall are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ball bearing). Spalling and spallation both describe the process of surface failure in which spall is shed.”
Essentially, those flakes that pop off of a surface due to stress is spalling. Here is spalling at a joint on a highway concrete joint.
I point this out because where I grew up there was an issue with slabs of concrete along the highway literally popping up/exploding off of the road bed. Highway 80 and Highway 49 were both made out of concrete slabs. At each joint between slabs, you would find tar impregnated felt that was used as an expansion joint. Over time, as the tar and felt wore away, it was replaced by road sediment/sand. Eventually, this locked the slabs in place and there was no mechanism to relieve the stress. Let the pavement get pretty hot… as the deep south tends to do, and you have a ready made, not so pleasant issue to deal with.
When a material heats or cools, it expands or contracts. How much it does depends on it’s linear coefficient of thermal expansion. (applies to solids) Once you get to liquids, it becomes a volume related coefficient, but the reason for it happening are the same. Thermally excited molecules.
In this plot, I calculated the volume that one cubic meter of material would have at a given temperature, reference to a 20°C starting point. The actual curves are most likely not straight linear functions, but this is a simplification in order to illustrate the point. As you can see, concrete is pretty high on the scale, though it’s composed of materials that individually, don’t expand as much (limestone is about the same as marble in its expansion coefficient)
Now… why the yammering about thermal expansion? Increased heat flow can (and does) expand the overlying material. This is best illustrated with the Hawaiian islands and the Hawaiian hot-spot swell structure. This (and possibly in conjunction with underplating) is a popular theory for the swells existence.
The swell extends further east than Hawaii, and starts to decline in intensity the further west that you go. (following the seamount chain). You can find other locations across the planet where this effects may be in force… the Canaries, Mid Ocean Ridges, etc.
Many of you have already seen my Hawaii Hotspot Animated Earthquake plot… here is is again in case you have missed it.
If you will note from the earlier thermal plot, not all material expands at the same rate. This will introduce additional stresses as one material expands faster than another, and natch, earthquakes can result from the added stress/strain.
Here is the swell around the Galapagos hotspot and the nearby associated spreading center. Image is from Canales et al overlaying the seafloor using Google Earth.
In order to get an idea of just how much uplift a region can have from a change in the geothermal gradient, I ran a really simplified model using the linear expansion coefficients for the materials that commonly make up a section of the crust. This was done in 500 meter layers using a combination of Gabbro, Sandstone, Shale, Slate, Mudrock. It is not specific to any particular location but should be in the ballpark for oceanic crust. It is a bit arbitrary in that is assumes a 20km thick crust, and that generally only occurs under volcanic islands. (If I remember correctly, the MOHO under El Hierro is about 17km down). Usually oceanic crust is in the 5km thick range. This was just a quick model to see what sort of effect the gradient could impart. Think of it as a happy medium between oceanic and continental crust.
It is important to realize that this is NOT expansion from magma intrusion, this is expansion from the change in the geothermal gradient. Heat flows pretty slowly in rock, much slower than in metal. Over periods of several thousands of years, the changes in the gradient can cause significant changes in the elevation of the surface.
So, the exploding chunks of highway on a hot summers day in central Mississippi could share a common physical cause to some earthquakes…. Thermal expansion.
Yeah, it’s a short article, but mainly I just wanted to point at this physical feature and provide food for thought.
“Hawaiian hot-spot swell structure from seafloor MT sounding” Constable et al (2004)
“Mantle P-wave velocity structure beneath the Hawaiian hotspot” Wolfe et al (2011)