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. Continue Reading…