history of science - Metageologist

How to tell if the loaf of bread in your oven is cooked? You can see the outside is nicely browned, but you can’t see the middle – is it doughy still? Give it a tap and listen. If it sounds hollow then it’s ready. The sound of the tap passes through cooked bread differently than through dough.

As with loaves of bread, so with the earth beneath us. Earthquakes give the earth a tap and we listen to the earth with seismometers, but the principles are the same. The sound and shaking caused by the tap passes through places we can’t ourselves get to and the way they are changed tells us about the material they’ve passed through.

Simple seismology

Every time an earthquake ‘taps’ the earth, it sends seismic waves whizzing off in all directions. For those of us who live on the outside, the most dangerous types are those that travel along the surface, but we are interested here in body waves, the ones that travel through the earth’s interior. Imagine yourself watching a seismograph when an earthquake’s effects arrive1. The first set of twitches2 are caused by the arrival of the primary or P-waves. These travel fastest and work like sound waves – by compressing material in the direction of travel. Next arrive the secondary or S-waves which move forward while shearing material side to side (think of a slithering ssssnake).

What earthquakes can reveal

The first person to clearly identify P, S and surface waves on a seismogram3 was British scientist Richard Dixon Oldham. Born in 1858, he was child of the Empire. His father was Professor of Geology at Trinity College, Dublin. Following his father’s footsteps led him to the Geological Survey of India. Studying a 1897 magnitude 8.1 earthquake in Assam he spotted the distinct arrival of P, S and surface waves, due to their travelling at different speeds. What’s more, he linked these observations both to existing theory of how waves propagate in materials and to fault ruptures on the earth’s surface.

Ill-health took him back to Britain in 1903, where his research continued. In a lovely 1906 paper called “The Constitution of the Interior of Earth, as Revealed by Earthquakes” he showed what seismograms could tell you about the very centre of the earth.

In the introduction, he paints a picture of woe:

“Many theories of the earth have been propounded at different times: the central substance of the earth has been supposed to be fiery, fluid, solid, and gaseous in turn, till geologists have turned in despair from the subject, and become inclined to confine their attention to the outermost crust of the earth, leaving its centre as a playground for mathematicians.”

Taking data from a mere 14 earthquakes and similar number of stations, he plotted the interval between earthquake and wave arrival4 against distance in ‘degrees of arc’ (180 degrees of arc would be a wave that had passed directly through the earth to be measured at the point opposite the earthquake).

Figure 1 from Oldham (1906). The lower line is P-waves, the upper S-waves

Oldham’s insight, rescuing the centre of the earth from the mathematicians, was to use the obvious change in the pattern at about 150 degrees to infer a core of very different composition from the surrounding material. A seismometer at greater than 150 degrees from the earthquake must trace rays that have passed through the deep earth – so the times it records tell us about the very core of the earth.

Figure 2 from the same paper. Note how the paths taken by seismic waves are refracted by the changes in velocity at the core-mantle boundary

Oldham was confident that his data showed that “the central four-tenths of the [earth’s] radius are occupied by matter possessing radically different physical properties” and time has proved him right.

Seismology across the globe

Reading this paper is reminder that globalisation is not a modern phenomena. Oldham was drawing on recording stations from many place – not just from British Empire (New Zealand, Australia and South Africa) but also the Americas and the Russian Empire (Irkutsk, Tashkent and Tiflis). He refers to literature from Japan and papers written in both German and Dutch. The latter involved field work from the Dutch East Indies (now Indonesia).

While scientists across the world were communicating with each other, this was not necessarily done quickly. All the earthquakes he studied were between 12 and 4 years old.

A shadowy view of the inner core

As speculated upon by Oldham, but proved by others in the next few decades, S-waves don’t travel through the core, because it is liquid – seismometers on the opposite side of the earth don’t see an earthquakes S-waves as they are in the ‘shadow’ of the earth’s core. The patterns of refraction you see in Oldham’s diagram above mean that there is a shadow-zone for P-waves also.

In 1936, taking data from within the shadow zone of an earthquake in New Zealand, Inge Lehmann found waves that shouldn’t be there. Tracing their paths, she explained them by inferring a distinctive solid inner core that created more ways for rays to reflect and refract their way through (for more detail see this great write-up).

Seismic tomography

Seismic data can used for many things. Locating earthquake foci helps us trace patterns of faults across the globe and so better predict which areas of the surface are at risk. Making bangs at the surface and measuring the reflections from shallow sub-surface layers is a great way to find oil and gas. Using them to better understand the structure of the deep earth is known as global seismology, or seismic tomography.

The key principles of using seismology to study the earth’s interior were set in the early Twentieth Century. The time taken for waves to pass through the earth gives the speed they travelled, which tells you something about the properties of the material they passed through. Building up models of the interior of the earth allows you to trace the paths they took. Studying sets of paths that go through the same portion of the earth tells you something about the properties of that part of the earth.

Image of the deepest mantle from seismic tomography. Part of figure 7 from Steinberger et. al 2012

Armed with millions of data points and highly sophisticated mathematical models modern seismologists are able to image the earth’s depths in detail, down to scales of a hundred kilometres. As well as understanding how things change with depth, they are also able to spot differences between different places at the same depth. The very base of the mantle in particular contains a lot of distinct areas with unusually low velocity.

Variations in seismic velocity at the same depth can be explained by variations in temperature or in composition. Either way, to explain the features seismology shows us, we need to bring some other sciences to bear. Let’s move to chemistry next.

References

Oldham R.D. (1906). The Constitution of the Interior of the Earth, as Revealed by Earthquakes, Quarterly Journal of the Geological Society, 62 (1-4) 456-475. DOI: http://dx.doi.org/10.1144/gsl.jgs.1906.062.01-04.21

Steinberger B. & T. W. Becker (2012). Subduction to the lower mantle , Solid Earth, 3 (2) 415-432. DOI: http://dx.doi.org/10.5194/se-3-415-2012

I’ve been reading up on the story about cometary material found in Libya (odd pebble is cometary material fallen to earth). My plan to write about it myself has run into the desert sands – I have nothing new to add. However reading the source paper has taken me off on a tangent, inspired by the word quintessence. It is a word that rolls off tongue rather nicely. It also has so many interesting meanings that I want to talk about them a little.

Non-scientific

Quintessence has been defined1 as:

A thing that is the most perfect example of its type; the most perfect embodiment of something.
A pure substance.

For these everyday meanings you might use it like you do the more common word quintessential, for example to make a science fiction film seem profound: “Hope, it is the quintessential human delusion, simultaneously the source of your greatest strength, and your greatest weakness”

Proto-scientific

The word comes from Medieval french, meaning ‘fifth essence’. At this time, attempts to understand the natural world were often framed in terms of Ancient Greek concepts, based on the works of authors such as Aristotle. The world was seen as being made up of 5 elements, or essences. Fire, earth, air and water covered the everyday. The fifth, ‘highest’, essence, also known as aether was contained within everything and was also the material that made the heavenly bodies.

Hindsight is a wonderful thing. Having the benefit of hundreds of years of scientific progress, grounded in experimental observation, it is tempting for us to be dismissive of our ancestors ‘mistaken’ opinions. Take alchemy for example. Isaac Newton, has (largely since his death) been seen as the quintessence of a rational scientific genius. However in reality he was a keen student of alchemy (and of other occult studies).

Alchemy (like astrology) came from genuine efforts by intelligent people seeking to understand the world. There is an interesting thread in contemporary studies of the history of science tracing the connections between the concepts of modern science and these older beliefs.

The aether

One example of this is the scientific concept of aether. To the Ancients, it was linked to the heavens and the quintessence underlying everything. In alchemy the name became mixed up with the idea of elixirs or medicines. In 1730 German scientist W.G. Frobenius changed the name of chemical compound ‘sweet vitriol’ to ether, a name now attached to a whole related group of organic compounds. Having found a good way to manufacture ‘sweet vitriol’ he was keen to sell it – perhaps naming it ether (thereby invoking a heavenly universal medicine) was simply marketing. Ether has since been uses as a solvent, an anaesthetic and a narcotic to use while driving from LA to Las Vegas to find the American Dream.

The name aether also resonates strongly in the world of physics. Seventeenth Century studies of optics, notably by Christiaan Huygens led to theories explaining light in terms of waves. If light involved waves, then some substance must be present for the waves to act on. This was known as the aether. Only with the Twentieth Century understanding of the weirdness of wave-particle duality (light is caused by little particles (photons) that behave in wave-like ways) was the concept of aether no longer required.

Modern physicists attempt to reconcile observations of the incredibly large (cosmology) and unimaginably small (particle physics). Gaps between theory and observation lead them to infer the existence of dark energy, one proposed form of which is given the name of quintessence. Whether this a purely hypothetical concept that will go the way of aether is not something a mere ex-geologist can speculate on.

And finally

What about the Libyan cometary material, this interesting pebble found in the Sahara?

The scientists involved were only given a gram of material which they then subjected to a whole battery of analytical procedures2. One of these involved heating it and analysing the gases that come off. Typically chondritic meteorites – chunks of the primordial solar system – contain a component known as Q, short for quintessence. This is thought to represent the ‘ambient gas of the early solar nebular’. The solar nebular is the cloud of dust, gas and ice from which the everything in the solar system (including you and everything you’ve ever seen) was born. There is no quintessence in the Libyan fragment, suggesting it did not form in the inner Solar System, but can only have formed far out where there is little gas at all. Out there, between 30 and 50 times further from the sun than we are, is comet country.

Another name for the gas know as quintessence is ‘planetary 1’, which is as dull a name as you could imagine. I salute the person who named it quintessence. Science rests on the precise use of technical terms, all of which have names. Giving them beautiful names with resonance and historic significance keeps us in touch with the poetic. The Ancient Greeks yearned to understand the mysteries of the world around us. Using words like quintessence reminds us that we are driven by exactly the same quintessentially human urge.