This is the seventh in a series. The first one can be found at http://sigma5.blogspot.com/2012/07/50-years-of-science-part-1.html. Part 2 can be found in the August 2012 section of this blog. Parts 3 and 4 can be found in the September 2012 section. Parts 5 and 6 can be found in the March 2016 section. I take the Isaac Asimov book "The Intelligent Man's Guide to the Physical Sciences" as my baseline for the state of science as it was when he wrote the book (1959 - 1960). More than 50 years have now passed but I am going to stick with the original title anyhow even though it is now slightly inaccurate. In these posts I am reviewing what he reported and examining what has changed since. For this post I am starting with the chapter Asimov titled "The Layers of the Planet" and then moving to "The Ocean" and finishing with "The Ice Caps". These three chapters finish his "The Earth" section.
Asimov starts with the great Lisbon earthquake of 1755. The actual quake was followed by a "tidal wave", what we now call a Tsunami. But at the time Asimov was writing the connection between earthquakes and Tsunamis was poorly understood. Recent history has given us the giant Tsunamis following giant earthquakes in Indonesia and Japan. The Indonesian event was an especial wakeup call because the Tsunami crossed thousands of miles of ocean to wreck devastation on far flung coastlines.
Prior to the Indonesia event computer models of Tsunamis had been developed but they were pretty primitive. They have since been substantially improved. But the biggest change is seen with respect to warning systems. An urgent need was finally recognized and acted on to develop international warning systems. There is now at least the beginnings of a network covering the Pacific and Indian oceans.
The earthquake and Tsunami that struck Japan is now remembered (at least outside Japan) mostly for the Fukushima nuclear disaster. The fact that billions of dollars worth of damage was sustained and tens of thousands of lives were lost in the part of the disaster that did not involve the nuclear plants is now pretty much forgotten.
And on a side note, there was a large earthquake off the coast of Washington State in 1700. We even know the day it happened. How? Because it spawned a Tsunami that traveled several thousand miles across the Pacific ocean and was still large enough to cause a noteworthy amount of damage when it struck Japan. So the Japanese made a record of it down to the exact date and time it came ashore. And this record was recently matched back to the earthquake off the Washington coast. Now back to Asimov.
The Lisbon earthquake kicked off the serious study of earthquakes in the western world. (The Chinese and Japanese, among others, had already been studying the subject for millennia.) The seismograph, then an assortment of pens, springs, and weights, was developed in 1855. Only modest improvements had been introduced in time for Asimov's book. Currently, seismographs are constructed from the same kinds of electronic components used in computers and cell phones.
The new designs are ruggeder, more accurate, and have a larger "dynamic range". The old designs used to peg out during a large close earthquake. This meant that only devices located a goodly distance away and, therefore, only able to record a weak and distorted signal, could provide data on the strongest part of the earthquake. The newer devices are able to make accurate and detailed readings of even the largest earthquakes even if they are close to the epicenter.
In 1890, Asimov writes, Milne determined that some of the waves from an earthquake traveled through the earth. This allowed earthquakes to be used as a diagnostic to study the inside of the earth. The earthquake is like a flashbulb going off. Various recording stations around the world act like photocells. A large amount of analysis allows some of the characteristics of the earth the signal travels through to be determined.
Earthquakes literally shake the earth. These waves radiate out and are hopefully captured by a seismometer. And the actual situation is more complex than you would think. Two kinds of waves are emitted: surface waves and body waves. The body waves are subdivided into P- primary waves and S - secondary waves. I am going to skip the details and just note that geologists could tell them apart and use the different characteristics of each to tease out information about the rock the wave passed through. Asimov goes into some detail on this but here's the main discovery.
The earth has layers. There is a thin layer called the crust. That's the only part we can directly observe. It is only a few tens of miles thick. The center of the earth contains the core. This is mostly Iron. The in between part is the mantle. Fifty years ago little was known beyond the sizes of the surface, mantle, and core. Today thousands of seismometers are deployed. They are more accurate and we now have access to unbelievable amounts of computing power. So we know a lot more detail.
The inner part of the core is solid. Iron can hold a magnetic field and that's where the magnetic field of the earth comes from. The solid inner core is surrounded by a relatively thin liquid outer core. This allows a certain amount of independence between the rotation of the inner core and that of the rest of the earth. The mantle has two layers, the inner mantle and the outer mantle. Within each component (the mantle is liquid but just barely) are cells of rotating material. This allows warmer and cooler material to move around. There is a certain amount of radioactivity throughout all of the mantle and the core. This radioactivity produces heat and this heat has to go somewhere. That's what drives the movement of material. And this movement of material and need to get rid of the excess heat is what drives volcanism and ocean floor spreading. Note: Ocean floor spreading and Plate Tectonics were unknown at the time of the book.
At the time the book was written the major earthquake zones had been mapped out (Asimov supplies a nice map) and earthquakes were associated by proximity with mountain building. But the connection was a mystery. Plate Tectonics, which was developed in the decade after the book was published, solved the problem. The continents were composed of relatively light materials that floated over the mantle material. Ocean floor material was substantially denser (English translation: heavier). And the cell structure created upwellings where fresh material is brought to the top and downwellings where the opposite happened. On top of upwellings were spreading centers, areas where new crust was created. This new crust pushed the older crust toward trenches, which sat on top of downwelling zones.
The upwelling brings up hot mantle material which cools off as it radiates heat through the crust and from there through the atmosphere and into space. The now cool material is eventually returned to depth when it gets to a downwelling area. So that completes the process of getting rid of the heat generated by radioactive decay. Meanwhile continents float on top of the mantle material and are pushed around. This results in collisions. Denser ocean floor material dives below the continental material at a "subduction zone". The process isn't perfect. In particular water and other chemicals are squeezed out of the rock as it dives down under the edge of the continent. This material rises through cracks and channels in the crust and ends up emerging from volcanos like Mt. St. Helens.
And not all rock is the same. In other words, the chemical composition of the material that was squeezed out differs from place to place. So lava spewed by different volcanos behaves differently. The kind of lava that comes up at Mt. St. Helens tends to make volcanoes explode. The kind of lava that comes up under Mt. Etna in Italy or Mt. Kilauea in Hawaii tends to put on a spectacular display but not blow up. The study of the chemistry of lava was just beginning at the time the book was written.
At the time of the book there was a lot of talk about the "Moho". This was a seismic feature that looked interesting and appeared to be shallow enough that it could possibly be reached by drilling a very deep well. After Asimov's book was published this was attempted but the attempt was unsuccessful. Now scientists find the Moho less interesting and not much attention is paid to it these days.
To his credit Asimov mentions Wegner. He was a German geologist who pioneered the idea of "continental drift" which eventually morphed into Plate Tectonics. Asimov mentions that his ideas had been discredited. They were later revived because more information became available. The ocean floors were mapped using SONAR. This led to the discovery of the midatlantic ridge, a line of underwater volcanoes running roughly down the center of the Atlantic ocean. And a series of magnetic bands were discovered that indicated that new ocean floor material was spreading out from each side of the ridge. This seafloor spreading provided the mechanism that drove continental drift.
Asimov also mentions a theory of Darwin (astronomer son of the more famous Charles Darwin). He posited that the moon was somehow carved out of the earth. At the time of writing this idea too was out of favor. But it does contain a grain of truth, at least according to the current thinking on lunar formation. Current thinking is that a mars sized body made a glancing collision with earth. This threw a lot of material into orbit around the earth. This material, consisting in large part of earth crustal material, eventually coalesced into the moon. All the heavy minerals like Iron ended up with the earth. Only light materials ended up with the moon. This solves a puzzle that Asimov makes note of. The puzzle was why the composition of the moon looked very similar to the composition of the earth's crust.
Asimov makes note of a then controversy over whether the earth was ever completely molten. Modern thinking is that it was completely molten at some point in its early life. This controversy has been replaced by a contemporary debate about the origin of the water that makes up our oceans. I am not going to go into it but, trust me, the debate between the supporters of various theories is quite lively.
An early theory for why we have mountains is that the earth was shrinking because it was cooling. This resulted in a raisin effect. This theory is now discredited. Plate Tectonics works better and we now know the earth is not cooling. But as part of the discussion of this subject Asimov does go into radioactivity as a source of heat. At the time there was not enough known about this sort of thing to go from speculation to solid theory. At the same time the beginnings of the mantle circulation idea I discussed above were just receiving serious consideration. And there wasn't enough known about this sort of thing at the time to sort out the good ideas from the bad.
On to "The Ocean". At the time of writing earth's oceans (or, as Asimov correctly notes, ocean - they are all interconnected) were the only known ones in the solar system. This is still technically true. But it is strongly suspected that one or more of the moons that orbit the outer planets has a liquid ocean underneath its icy surface. No ocean has been definitively observed but a lot of very solid evidence points toward their existence. There is so much evidence that the discussion has moved on to the possibility that life might be possible there.
Asimov reels off some interesting statistics then notes "[a]lmost certainly the first forms of life originated . . ." in the oceans. This statement is still true. He then goes on to say "[e]ven today astronomers know more about the surface of the moon than geologists know about the surface of the earth under the oceans". This statement is now debatable but it's a close call. We now know a lot more about the ocean floor than we did then. But we also know a lot more about the surface of the moon. Consider this.
A jumbo jet crashed into the ocean somewhere off the coast of Australia a couple of years ago. If it had crashed on the surface of the moon we would know exactly where it crashed by now. But we don't know where on the ocean floor it is after spending more than a year looking very hard for it. All we have is a small amount of debris that has washed ashore thousands of miles from where it no doubt went down to show that it has not just disappeared into thin air.
Asimov credits the founding of modern oceanography to Maury. He then goes on to say that "the ocean currents have been thoroughly mapped." That was an exaggeration. The general outline of the major surface currents was known. But we now know that there are currents at every level of the ocean and almost nothing was known about these subsurface currents at that time. We also know that ocean currents are quite dynamic. They can speed up, slow down, change direction, perhaps stop altogether for a time. None of that was really understood back then. Scientists consider the modern study of ocean currents "early times". They think they still have way more to learn than what they know now. Scientists back then saw some hints of what was to come but only hints.
Asimov singles out temperature differences as the driver of ocean currents. Certainly temperature differences are a major factor. But wind patterns, tidal effects, Coriolis effects, and several other effects, also play a role. At that time the surface of what there was to learn in this area had not even been scratched. It has now been scratched but that's about it. The tremendous difficulty involved in working under the pressure and visibility conditions present in the oceans mean studying anything about them is a very slow and very expensive process.
Asimov associates the start of serious study of the deep ocean floor with an 1850 effort by Maury to develop a chart for use in laying the first telegraph cable to cross the Atlantic. The project took 15 years and the suffered many delays and setbacks along the way. In the 1870's the ship Challenger set out to do a more complete survey of all the oceans. But the only tool they had for measuring depth was a long cable with a big heavy weight on its end. The ocean is several miles deep in many places. It was a slow and difficult process to pay out and reel back in that much cable.
SONAR and its predecessors were introduced in the early twentieth century. By mid-century rough SONAR based maps of the entire ocean floor were available. But they were very rough and little was known other than depth information. Asimov notes that if you start counting from the foot of the mountain deep in the ocean then the highest mountains on earth are in Hawaii. (The solar system record is currently held by a mountain on Mars.) SONAR mapping of ocean floor has also indicated that some land features extend far into the ocean. He cites the Hudson river as an example of this. We now know that the theory that some ocean bottom features are "gouged out by turbulent flows of soil-laden water" is the correct one.
Very little was then known about the bottom of the ocean. Drilling devices had been lowered to the ocean floor and used to pull up "cores" of earth that could be studied. But this had only been done in a few places. More cores have since been pulled up but coverage is still extremely sparse.
Other investigations have turned up "great smokers" on the bottom of the ocean. These are places where hot spots (think volcanic processes) suck cold water into porous rock. The water flushes through and picks up all kinds of chemicals. This water, often discolored to the point where it looks like smoke, is then flushed out through chimneys. This process can take place in water that is quite deep. So it was shocking to discover "tube worms", crabs, and other creatures living so far away from sunshine. Some people now think life on earth may have originated in these extreme conditions.
Asimov observes that by 1872 scientists had determined that life permeated the depths of the oceans. (The earlier idea was that it was confined to within a few hundred feet of the surface.) We have still not filled the details of this picture out. We also know more about the strange (to us) metabolism of these creatures. But they are hard to capture and hard to study. As Asimov observes, these creatures "are so adapted . . . that they are unable to rise out of their trench".
He then mentions the giant squid. Scientists have since found many larger than average squids but have yet to find a truly giant one. He then moves on to "living fossils". He was talking about the coelacanth. For some time it was thought to be extinct. Then a fisherman caught one in 1938. At the time of writing only a few other examples were known. Now we know that it is relatively common. It just lives in deep water where no one normally drops a hook.
Asimov wraps the chapter up with a section on deep diving. 300 feet was then thought to be the limit for a diver wearing some kind of soft suit. Modern equipment and procedures allow people go deeper but only a few hundred feet deeper. By the time the book was written various deep diving submarines had been developed. This effort culminated in the Trieste, which was capable of (and did) going to the bottom of the Challenger Deep, the deepest part of the ocean. At the time of the book only a few deep dives had been done.
Deep dives are now more common but still relatively rare. James Cameron, the director of the movie "Titanic", built a one man vessel that allowed him to dive to the bottom of the Challenger Deep. Someone has developed a hard shell diving suit that is capable of diving to great depths but not all the way to the bottom of the Challenger Deep. But any deep dive is still extremely expensive. The current state of the art is robot submarines that are capable of diving to the ocean floor in all but a few particularly deep places. Not having to carry all the equipment necessary to keep people alive cuts the cost somewhat but they are still very expensive to build and operate.
On to "The Ice Caps". Asimov starts the chapter with a review of efforts to reach the north pole. It was finally reached by Perry in 1909. Now a trip back is much less of a big deal. The nuclear submarine Nautilus reached the pole in the '60s by going under the polar ice pack. The whole area under the ice has now been charted by the US, the Russians, and probably others. One reason behind this activity is that there may be oil there. The Prudhoe Bay field that feeds that Alaska pipeline in on the north coast of Alaska. Thinking there might also be oil a little farther north is not an unreasonable thought.
As Asimov notes, the original impetus for polar expeditions was to search for a northwest passage. After a lot of failure such a passage was presumed to be a myth. Its former mythical state is now, as they say, "greatly exaggerated". Global warming has caused the polar ice pack to shrink so much in the late summer that in most years pretty much any ocean going ship can transit from the Atlantic to the Pacific or, if they prefer, the Pacific to the Atlantic, with little difficulty.
Asimov's starts his discussion of the Antarctic with another list of explorers. The task of getting to the south pole was tougher because Antarctica is much larger. And there is a continent under the ice so you can't just submarine your way to it. The south pole was finally reached in 1911 by Amundson. The '20s saw the creation of the first Antarctic research stations. The amount of scientific research conducted on the continent jumped considerably during the International Geophysical Year (actually an 18 month period) that ended in December of 1958. Many countries made a big push to mount scientific expeditions to Antarctica during the IGY.
This burst of activity was quickly followed up by treaties to de-militarize (no standing armies allowed) and de-politicize (no country could make territorial claims) Antarctica permanently. Since then a number of countries including the US operate year round scientific facilities on the continent. They mostly beetle away doing science. But every once in a while people start paying attention when someone gets seriously sick during the Antarctic winter and a tricky evacuation must be performed.
But serious science is done there. A giant particle detector has been created by taking clever advantage of the fact that the Antarctic ice is more than a mile thick and very clear in a number of places. The Russians recently drilled a hole through miles of ice down to a lake that is still liquid to see what a body of water that was so cold for so long and so isolated for so long contained in terms of life forms. Those are just two scientific endeavors that come easily to mind. There are many more.
Asimov notes that 86% of all ice in the world is in Antarctica and another 10% is in Greenland. All the glaciers that are more accessible combine to total only 4%. But those more accessible 4% were the fodder for the science of glaciology. It was kicked off in Switzerland in the 1820s. Glaciers are like rivers. They can move rocks. They just do it slowly, at a glacial pace, one might say. Debris left by a melting glacier is distinctive. So once geologists started looking around they found it in many unexpected places. By the 1850's the study of this glacial debris led to the discovery of the ice ages. At various times in roughly the last hundred thousand years large parts of the earth have been covered by glaciers.
The repeated advance and retreat of the various ice ages has drastically changed the topology of the land in many places. In my area there are many valley features that run north to south. This is the result of glaciers scouring out deep trenches as they initially grew south then eventually retreated north. Asimov pegged the last glacial retreat as having happened between 8,000 and 12,000 years ago. You can select among a variety of dates for the end of the last ice age. It just depends on what is important to you. The maximum extent of glaciation was 22,000 years ago. By 13,000 years ago the glaciers were definitely in retreat. But there was still a lot of ice around 7,000 years ago. So pick whatever date you like best.
When the last ice age ended is important for many reasons. But one of them has to do with figuring out when humans got to the Americas. There is still serious disagreement as to when this happened. There is general agreement that they came from Asia and the presumption is that they crossed from eastern Siberia into Alaska and then moved south. But there is a big argument as to whether they took an inland route or a coastal route. Neither was an option 22,000 years ago when the ice age was at its maximum. But possible routes depend on details about when ice retreated from certain specific places. And that's tricky to determine. And there's another problem.
As Asimov points out glaciers took a lot of water out of circulation. That means that oceans were a lot lower than they are now. At one time sea level was 440 feet lower than it is now. So what level was it at when humans were crossing to America? It depends on when they crossed but it was definitely lower then. Why is this important? If they went along the coast and if the water level was say 50 feet lower then most of the traces of this migration are now under water. Scientists have gone looking for these traces. But as I noted above it's hard to search under the sea. They haven't found much of anything so far.
Back to Asimov. He notes that coal was found in Norway and signs of coal have been found in Antarctica. What's going on? He opines that there have been times when the weather was so warm that there was no ice anywhere on earth so maybe that's what was going on. Plate Tectonics lets us figure out what was going on with far more certainty than Asimov could muster. Continents move around. This means that what might now be at the pole could have been at the equator at some time in the past. And that's part of what's going on. Continents have also been broken up and jammed together at various times. At one time all the current continents were part of a single super-continent and it was oriented so that there were good growing conditions everywhere. Lots of plants plus a lot of geology gives you coal.
The "forcings" that began and ended ice ages were not really understood at the time the book was written. One component of this is called Milankovich cycles and Asimov discusses them. The angle between the earth's orbit and the axis of rotation is currently 22 degrees. That tilt results in our seasons. In the Summer the tilt causes the northern hemisphere to get more sun and the southern to get less. In the winter the situation is reversed. Various astronomical processes change this angle. The evolution of this angle is determined by the Milankovitch cycle. If the cycle forces the rotation axis to be straight up and down things work differently and the weather works differently. At the time of the book Milankovich cycles were known about but they didn't seem up to be strong enough to explain long term patterns by themselves. How the effects they do cause can be multiplied is better understood now. And we know about the movement of continents and that helps too. But there is still some "forcings" mystery left.
Asimov then explains a trick still in common use. You study the ratio of Oxygen-16 to Oxygen-18. Urey figured out how to translate this information into ocean temperature in 1950. Asimov published a graph of average ocean temperature for the last hundred million years based on this technique. The ocean used to be a lot warmer, the graph indicates. Asimov credits the cooling of the oceans over time that the graph displays as the reason the dinosaurs went extinct. We now know that they actually went extinct pretty much all at once when a large meteorite hit the Yucatán area of Mexico 65 million years ago.
Asimov also describe the greenhouse effect. Again it was well known in the '50s that increases in atmospheric carbon dioxide would result in increases in air temperature. He even calculates that a doubling of the amount of carbon dioxide in the atmosphere would raise temperatures by three degrees. Asimov uses Fahrenheit for our convenience. The Celsius equivalent that scientists use is 1.65 degrees. Conversely, the 2 degree Celsius change scientists talk about with respect to global warming is 3.6 Fahrenheit degrees.
And for all those "global warming was invented in the '80s" types, Asimov says that a drop of 3 1/2 degrees would bring on an ice age and an increase of 3 1/2 degrees would melt all the ice in Greenland and Antarctica. The 2 degrees Celsius that scientists now talk about is almost exactly the same as the 3 1/2 degrees Fahrenheit Asimov talked about in 1960. In actual fact the basic science behind global warming goes back to the '50s and has changed little since. What has changed is the political climate. Then no one cared. Now powerful forces want us to believe that global warming is some kind of hoax cooked up in the '80s for obscure nefarious reasons. Want more evidence that the science behind global warming dates back to the '50s?
If you melted all the water in Antarctica and Greenland, Asimov tells us, the oceans would raise by 200 feet. We are now arguing about a sea level rise of a few feet in the next 50-100 years if global warming goes the way scientists think it will. And in spite of the fact that this sounds like no big deal it would actually be devastating for reasons too complicated to go into. Almost all people live on or near coasts. And if a sea level rise of a few feet does not sound scary enough just think about the 200 foot sea level rise (not my number, Asimov's) that would be caused if we melted all the ice in Greenland and Antarctica. Scientists don't think it would all melt. Well, not in the next hundred years. But what if they are wrong?
He also gets into what is called the carbon cycle. Over geologic time periods there are processes that pull carbon dioxide out of the air and turn it into rock. There are also ways to turn the rock back into carbon dioxide gas in the air. So they can save us by getting carbon dioxide levels back to where they need to be, right? The problem is that these processes take tens of thousands of years. We don't have that long to wait.
Pretty much everything a scientist would need in order to put together a presentation on global warming is found in this chapter of Asimov's book. And that presentation based solely on data from Asimov would differ from one based on the latest data in only minor ways. The main thrust and general conclusions would be identical. And Asimov's book was written more than fifty years ago.
The next post in the series will be based on material from his "The Atmosphere" section.
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