Notes from the book, A Short History of Almost Everything
We know surprisingly little about what lies beneath our feet. We have been building Fords longer than we have known that the continents drift. Before he died Einstein mocked the very idea.
The consensus of the scientific community was that the continents did not “drift” as some crackpots suggested. As with the suggestion that the earth revolved around the sun, adoption of such a theory wanted for explanation. Despite the growing evidence that the continents were once connected there was no theory for how they might have drifted. Until the 1950s.
Until that time scientists suggested that land bridges had once connected the continents which is why the fossil records match on opposite sides of the Atlantic.
Well into the 1970s, some of the most influential and popular textbooks strenuously insisted that plate tectonics was a physical impossibility. By 1980 some estimates suggest that still 1 in 8 geologists still didn’t believe in plate tectonics.
But they are wrong. The theory is a very strong one, explaining earthquakes, volcanos, mountain formation, island chains, carbon cycles, and several other phenomena that would otherwise be as yet unexplained.
But mysteries remain yet. Take Denver for example. The rocks beneath Denver are not fractured or deformed in the way they would be as a result of colliding plates. Denver is too far from the plate edges anyway. It appears that Denver has been rising like baking bread for millions of years. So too has much of southern Africa. A portion of it has risen nearly a mile in 100 million years. Australia meanwhile has been tilting and sinking. Nothing in the theories of plate tectonics can explain any of this.
In short, we know very little about what the earth is doing beneath our feet.
We think the crust is about 25 miles thick. We think the hard rocky mantle layer is about 225 miles thick, followed by a transition zone of 150 miles. We think the lower mantle begins about 400 miles beneath the surface and is about 1300 miles thick.
We think there is an outer core of molten metals and iron 1200 miles thick beginning around 2000 miles beneath the surface and an inner core of solid heavy stuff from starting about 3200 miles deep to the center at 4000 miles below the surface.
We use seismic data and the way the waves bounce and deflect off of the interior of the planet to infer all of this. But when the Russians decided to dig as far as humany possible – because why not – they had to stop after 7.6 miles. This is as far down into the crust as we have ever gone. Not even a third of the way through the very top layer. And it took the Russians 19 years to get that far. And what they found proved most of our seismic inferences wrong. Sedimentary rock was 50% deeper than predicted and the basalt layer was never found at all. Moreover, it was much hotter down there than expected – 180 centigrade at 10k meters.
Most surprising of all was that the rock at that depth was saturated with water – something that had not been thought possible.
We know the magnetic field has reversed itself across the entire planet several times. We see this is the rocks everywhere. But we don’t know what is happening down there to cause this. Geologists have only been studying the idea of convection currents since the 1970s despite being first proposed at the end of the 18th century. Some say convection begins 400 miles down, others say 2000. Geochemists and seismologists disagree, each discipline making inferences from irreconcilable data sets. It is not unlike the two disciplines Einstein invented (or discovered?) – general relativity and the quantum nature of the atomic world. Each theory has passed every test thrown at it but each theory makes predictions beyond our ability to measure which do not agree. It is the same with the ground beneath our feet.
It appears the magnetic field has weakened by about 6% in the last century. The reversals and changes in the earth’s magnetic field remains “the greatest unanswered question in the geologic sciences.”
There is a lot we still don’t understand or know about the ground beneath our feet.