Nick on the Rocks - Basalt Columns 

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Separate episode

The sheer volume of ancient lava flows was huge compared to what has been seen since photography & video shown us so far. Kinda think we don’t want to ever see flows that size again.

Nope. CRBG basalt flows would not have differed significantly in temperature from the hottest flows in Iceland or Hawaii. What made the difference was the rate of eruption and thus the sheer volume erupted. As a lava flow gets thicker the time it takes to fully cool increases extremely quickly. A 10 m thick lava flow will take vastly more than 10 times longer to cool than a 1 m thick flow. The heat conduction of this type of rock is very low. It's a solid with no free electrons and thus there is no easy mechanism to pass heat through the material. At the microscopic level all temperature reduces to is how fast particles and molecules move around, how much they vibrate and how much they rotate. Due to their symmetry single particles can only translate and cannot rotate or vibrate. In liquids it's the molecules themselves that transmit the heat most well as they are free to move around with respect to each other and collide with each other. With a solid metal or other conductor it's the free electrons in the solid that transmit the heat most well as they are also free to move around with respect to each other and the matrix structure of the solid. So in solid igneous rock all that happens is very slow conduction and some radiative cooling (transmission of heat by the emission and absorption of photons by molecules). Hence very slow flow of heat and hence very slow cooling. That slow flow of heat makes the outer solid layer of lava a very good insulator. Effectively the lava gets a thermos flask around it allowing the centre to remain liquid. The other thing to remember is that volume does NOT increase linearly with surface area for a solid of a given shape. A 1 m side length square has a lot more surface area per unit volume than a 2 m side length square. The amount of cooling by conduction and radiation is dependent on the surface area and NOT the volume since heat can only escape from something at the surface of that something. So pretty much all basaltic lava flows get an insulating thermos flask-like effect, forming a lava tube and the flood basalt flows have a much higher effusion rate and thus form much thicker flows bringing in the ratio of surface area to volume effect. The higher rate of effusion also helps to keep the lava tubes open for greater distances as there is more pressure at the start of the lava tube pushing the lava down it.

@@wdwerker we most certainly do not want to see flows of that size again. The Laki eruption of 1783 had a volume of 14 km^3 and it is one of the largest Holocene lava flows. That produced continent-wide effects that were devastating in Europe. In Iceland it killed between a quarter and a fifth of the population through famine. A lot of the problems in Iceland were produced by fluorine poisoning which is what caused a lot of the livestock deaths. The eruption's sulfur dioxide output was roughly six times that of the VEI 6 Pinatubo eruption in 1991. Pinatubo injected the vast majority of that sulfur dioxide into the stratosphere and hence there was a small volcanic winter over the next couple of years. Laki on the other hand injected the vast majority of that sulfur dioxide into the lower atmosphere. Thus the longer term effects were smaller but the immediate effects were much worse. Sulfur dioxide comes out of the troposphere a lot faster than it does out of the stratosphere. For example death rate estimates have been used to come to the conclusion that approximately 23,000 people were killed in the UK by the eruption. Extrapolate that across Europe (although direct effects would have been less bad in places further away from the eruption) and it's easy to conclude that the death toll from Laki was north of 100,000. Then consider that where Laki was 14 km^3 and the largest Holocene eruption was 26 km^3, the typical flood basalt flows of the CRBG were 500 or 600 km^3!!! There were several hundred of those individual flows making up the overall CRBG. The fluorine release from those may well not have been as proportionately large over Laki as might be thought as the lava source for Laki was unusually fluorine-rich. However the sulfur-dioxide release WOULD have essentially been proportionately larger than the Laki flow. So if Laki released about 6 times the sulfur dioxide of Pinatubo than a CRBG flow of 550km^3's release would be about 235 times that of Pinatubo. So each CRBG flow would produce somewhere between 175 and 275 times the amount of sulfur dioxide as a low-end VEI 6 caldera-forming eruption. That's well over 2 orders of magnitude and would be absolutely devastating. The VEI of Pinatubo was pretty much exactly 6, so scaling the eruption size proportionately to get the same kind of sulfur dioxide release as that hypothetical 550 km^3 CRBG flow gets an explosive eruption with a VEI of 8.4 (2350 km^3)! Such an explosive eruption would release the sulfur dioxide over a considerably shorter time period than a CRBG flow as each flow's eruption probably lasted several years. Nevertheless it gives a flavour of just why flood basalts are so devastating in their effects and have caused most of the mass extinctions in earth's history. Remember as well that the CRBG is the smallest-known flood basalt, mainly due to the fact that it's likely not due to the same mechanism as other flood basalts. Other flood basalts are caused by the emergence of a plume head at the surface, and the emergence of the plume head for the plume that caused the CRBG was actually some 50 Ma, resulting in the formation of Siletzia. The CRBG by contrast appears to have been triggered by the plume reaching the strontium 706 line in North America that represents the suture between the old, cratonic continental crust and the oldest of the west coast exotic terranes with a build up of melt from the plume being underneath the exotic terranes for many millions of years beforehand finding the weakness that the suture of the strontium 706 line represents and pouring out onto the surface.