Of course thousands of things happened in geology in the 20th century, but two stand out as changing the whole approach to geology not just one particular detail. These two are plate tectonics and space geodesy. The first is a new and more comprehensive view of the Earth and its changes, based on the idea that whole continents move about on an underlying sea of convecting rock. The second is the vision and then the use of a new set of tools, specifically satellites that would look at the earth from above and, using camera and radio, send whole new images of everything we have ever seen or known or tried to map.
What makes mountains? The effort to answer this question had gone through a lot of changes by the twentieth century. Obviously some mountains are volcanic – but equally obviously to the thoughtful observer, others are not. Mount Monadnock – and all other monadnocks – are just fragments of high land, leftover after the erosion of everything else. They are not and never were volcanic.
Why didn’t they erode with everything else? Maybe there was an especially resistant stone formation at the top of a monadnock, such as the vast slab of pure quartz on top of South Pack Monadnock. But that’s not a common form. Other questions persist.
Why do mountains march in lines?
So many of our mountains are in lines and chains. Why is that? A line of mountains demands that the geologist look for the reason for the line, not just the reason for the mountain. That’s much more demanding. Even if it’s volcanic, why would a bunch of volcanoes be lined up? On the Moon and on Mars, they’re just random pock-marks. Why is Earth so different?
Kudos if you know the answer, but then ask yourself at what point in history might you have guessed it?
Could continents drift?
The concept that whole continents might be on the move, the concept of continental drift, is generally thought of as a 20th century idea, but it was first proposed in 1596 by Abraham Ortelius. Isn’t that a surprise! He was a Flemish cartographer, just 100 years after Columbus, and so he was basically one of the first men to compare the coastlines of the Americas with the matching coastlines of Europe and Africa. The match leaps to the eye, and he suggested that volcanism and floods had separated these vast lands. At a certain simple level, this is not far from the truth: continental rifting involves volcanism and can open interior lands to the nearby ocean; but notice that now we are talking about an extraordinarily extended line of volcanism – the mid-Atlantic rift. Look at it! Thousands of miles long! How does that happen?
Here is the center of the Atlantic Ocean, assumed to be a plain until discovered to be quite otherwise!
The failure to find an explanation for such massive rifting prevented the acceptance and development of Vesalius’ idea for the next 300 years. Not for lack of effort! It was proposed that the interior of the earth was more dense than the outer crust (which is true) and that periodically the interior simply burst through original super-continent, breaking it up into the continents as we now know them.
Alternatively, it was proposed that the Earth gradually enlarged by consuming the ether, and this enlargement stretched the original continent and then pushed its pieces apart.
Wrong; almost funny. Note that such growth would change the gravity of the Earth and its relationship with the Moon.
The idea of a broken super-continent would have been dropped completely except that new evidences for a past closeness between the continents kept surfacing in things like similar fossils or similar stone and soil chemistry in precisely the locations that would once have been close if the matching coastlines were attached in ancient times.
From continental drift to plate tectonics
In 1912, Alfred Wegener marshaled all the evidence for continental drift and argued forcefully for it. The idea never went away again, but there was still no credible mechanism to drive the motion of entire continents. He even pointed to the Mid-Atlantic Ridge zone as a spreading zone, but there seemed to be no way for the continents to ride over the wide oceans, so he dropped it. He wasn’t seeing the continual generation of new ocean floor as part of the picture.
In 1928, Arthur Holmes suggested that convective motions in the mantle might drive the motions of the continents.
If you go to make oatmeal, jelly, or soup from soup bones – any cooking that involves an episode in which scum rises to the surface, — you have a chance to see how convection cells from the bottom of the pan drive little mats of surface material from side to side, occasionally joining them, occasionally breaking up a large mat. In the case of cooking, the scum is soon driven to the sides of the pan, but on the Earth, the bottom of the pan is the inside of the planet, and there are no sides. The mats of scum, the islands and continents, go from side to side and are occasionally driven all together, and later driven all apart.
So at least Holmes had a mechanism, but it seemed too far-fetched. The pressures on the mantle are very powerful and it is very rigid. And if ever the continents were driven together, why not stay that way?
Well, for one thing, there would be a build-up of heat below such a large continent, and that might soften it and start volcanism… Maybe.
But even aside from heat buildup, the convection cells which are carrying the continents change in size and orientation – convection cells are more or less random things – and when two cells are pressed together such that their upwelling centers draw near, the stress on the overlying land is such as to pull it apart. This is happening right now in Nevada, and is the cause of its extensive hot springs.
Still, continental drift was not accepted until the 1960’s. It was hard to imagine the convection of solid rock, no matter how hot. It was only when all the ocean floors were surveyed in detail in the 1950’s that deep-ocean ridges were found in every ocean and a new piece of evidence needed a home. Now the question changed from: could the Mid-Atlantic ridge be a spreading zone? to the question: why does every ocean have these ridges?
It was in answer to that question that Arthur Holmes’ suggestion finally took its proper place in geology and the slow, slow convection of the mantle, solid as it seems, was accepted.
We know, then, why mountains often form in lines and ranges. Mountain ranges are formed by the subduction of one tectonic plate below another and the resulting turmoil (long story for another day) along the line of contact.
Mars and the Moon don’t convect, and thereby hangs a tale.
The second great shift in 20th century geological understanding was a matter of new tools, and was as fundamental as the invention of the telescope.
The basic difficulty about mapping the whole Earth so that you can really see these mountain ranges and their rivers, lakes, and other geology, is that it is so hard to measure great distances. You can walk across your garden with a tape measure; fine. You can, with a good watch and a good companion and good weather, time the difference between the moment that stars disappear behind the Moon in different locations. That can be translated into miles. And, once you have the instruments, you can even send mirror blinks from mountaintop to mountaintop and time them for distances, presuming that the mountaintops are not too full of vegetation to see or too glacial to climb or too full of wild animals or volcanic smokes to prevent access. No mountain will help you measure the Pacific, of course. Airplanes help, but you have to know your speed and direction, and so many things confuse that, jet streams for one.
So it’s difficult, you see.
But if you could go to the Moon and just calmly look down…?
No, the Moon is too far away to be much use.
But… If you had an artificial moon, a satellite that was just high enough to stay in orbit but just low enough to stay in good communication and take good pictures, that would really help. When John O’Keefe proposed this to the Army Map service after World War II, they thought it was ridiculous. But then they changed their minds and it happened. Satellites are totally a part of our world now, and what they have told us about everything from inaccessible mountain ranges, to magnetic anomalies is a crucial part of our comprehensive geological knowledge. Life without satellites seems like life in the stone age.
Ah, if Mungo Park could have had just one such picture of the river he was surveying!
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