Basement rocks, the ones far below the Cambrian sediments, don’t get much love. Where’s the poem? Kipling wrote one featuring the less interesting ocean abyssal plains. They are of little interest to palaeontologists, as there are no fossils (baked bacteria are pretty invisible), and none to oil and gas drillers. They are vital to us simply because the continents rest on them.
The Grand Canyon, unusually, has a tidy horizontal stratigraphy mapping geological history. You would think, wouldn’t you, that the basement igneous rock at the bottom of the gorge was simply a boring sheet of granite, all the way down. But no. Wikipedia on the Vishnu Schist, which is what it’s romantically named:
Specific names have been assigned to individual plutons and dike swarms because the plutons and swarms differ greatly in their age, origin, and tectonic significance. The oldest of these plutonic complexes, Elves Chasm Gneiss, likely represent a small fragment of basement upon which the metavolcanic rocks that comprise the Granite Gorge Metamorphic Suite accumulated. The remainder of the Early Paleoproterozoic granites, granitic pegmatites, aplites, and granodiorites – are parts of either younger plutons or dike swarms, that have intruded the Granite Gorge Metamorphic Suite, either contemporaneously with, or after they were metamorphosed. Etc etc.
If this sounds more like the result of an explosion in a connoisseur jam factory, it’s because the actual processes were similarly violent though much slower.
The oldest rocks at the bottom of the gorge are about 1.75 billion years old (Gya). The basement rock below England is considerably younger, about 700 million years old (Mya). The Earth has been around for 4.6 billion years, and the oldest dated rocks are from 4.0 Gya. Just when the tectonic plate system started is Controversial but those arguing for a late start in the Archaean say that rock life was just too violent:
small protocontinents were common, prevented from coalescing into larger units by the high rate of geologic activity.
Do the Priorplater and Posteriorplater factions speak to each other, or anathematise from their rival journals? Either way, the exploding jam factory was at work for several billion years, moving large lumps of crustal rock about, colliding and subducting them, and metamorphosing early sediments with ferocious volcanic heat. That’s why very old rocks (over 3 Gya) are rare, and the basement is typically complex.
The basement rock may lie on the surface, as in much of Scotland, or be covered by ten kilometres of sediments. What deep rock is, is hot. A heat map of Britain at 5 km and 7 km depths.
This study is the source of the maps and also gives estimates of the British geothermal resource. Tl;dr: it depends. The heat in place in the rocks is mind-blowing: to 9.5km, 357,000 exajoules.
If it were possible to develop just 2% of this resource (7144 EJ), this would be equivalent to 1242 times the final UK energy consumption in 2015.
So now deep igneous rocks have found a fan club: geothermal engineers. They prefer basement rock, especially granite, because it cracks when exposed to hot water. The problem with engineered geothermal (as
opposed to conventional hydrothermal, which consists simply in tapping an existing underground reservoir of superheated water) is not heat but permeability. The EGS scheme is to drill at least two holes into a deep block of hot granite: one to push water down under pressure, the others to bring it back up heated by its passage through the cracked rock. The water generates the cracks by itself. This works in principle, and has been demonstrated in several pilots, notably at Soultz-sous-Forêts in Alsace. The problem, on top of the high cost of drilling into very hard rock, is to get a sufficient flow of superheated water for commercial scale operation. An Australian commercial pilot failed – party because the location was outlandishly remote, and it was not worth an expensive grid connection. There is just one operating pilot well in the USA, at the Newberry volcano in Oregon. Its future is uncertain, as it failed to secure the nomination as the DoE’s main EGS research centre (FORGE), which went to Sandia in Nevada. The continued funding of such long-range programmes is in doubt under the Trump administration.
As a result of this globally slow-walked and underfunded research effort, estimates of EGS potential are too uncertain to be useful. The same British study estimates the “total technical potential power” under Britain to 6.5 km depth and above 150 C as 222 GW, over four times current peak electric demand of 53 GW. But if you restrict the depth to 5 km and the temperature to >200 C, the potential drops to a very modest 2.3 GW. A 100:1 range is the same as saying “we don’t know”.
Alternative forms of renewable energy must now struggle with the very low cost of wind and solar. In Mark Jacobson’s latest all-renewable energy scenarios for 139 countries, wind and pv solar would account for 85% of all delivered electricity, and an only slightly lower percentage of all energy in a basically electric economy – the difference is accounted for by geothermal district heating. The 15% gap could easily be met with fossil gas from plants already in place, but that’s cheating and the gas has to go to zero by 2050 for the world to reach net zero emissions.
EGS geothermal is competing, along with CSP solar, P2G, biomass and grid storage, for a slice of that 15% renewable gap. The winner needs to be reliable, despatchable and affordable. Affordable does not mean “as cheap as wind or solar”, it won’t be. The conditions do rule out nuclear and wave.
Advantages of EGS: completely reliable (99% if you are ready to pay for it); safe; despatchable in minutes; low land footprint; typical size of 100 MW matches much geographic demand, cutting transmission costs and increasing grid reliability; widely available – universally so if you are prepared to drill down to 10 km, as oilmen routinely do .
Disadvantage of EGS: doesn’t work yet.
So as a long shot it’s utterly dependent on continued research support, which IMHO it deserves much more than small “and completely safe” nuclear reactors, fusion, wave, power kites, and concentrating pv, not to mention drones to deliver parcels, green ketchup, credibly scary orcs in video games and similar problems of absorbing commercial interest.
If you are going to be purist, a single EGS plant is not 100.00% renewable, as the lump of granite beneath it cools down over time and you have to move the plant to a new site every 50 years or so. The rock heats up again over the following century, so in the longer run it’s renewable. In the very, very long run the Earth’s core cools down and our hypothetical descendants freeze. I’m not sure though if this happens before the Sun bloats up into a red giant and swallows the planet.
Some say the world will end in fire
And some in ice …
But ice is nice
And will suffice.
Though the correct spelling is “gneiss”.