Great Geology in Google Maps: dunes

Google Maps is a great resource, particularly in satellite view. My favourite way to enjoy it is via the Chrome extension “Earth View from Google Maps“. This pops up a gorgeous image in every new tab. Many show human landscapes, but every now and then one appears that catches this geologist’s eye. This post is the first in a series exploring and celebrating these images.

This view is of the Rub’ al Khali or ‘Empty Quarter’ of the Arabian Peninsula – the largest sand desert in the world.

In dry environments sand is moved not by water but by wind. The characteristic landform is the sand dune. Common in deserts on earth, they are also found on Mars and even comets.
The shapes of dunes depends on the supply of sand, but above all the wind. Wind strength and direction, averaged over the year, will determine the shape of a dune. Various types of dune are recognised. The ones in this image are complex. Further east of here the dunes are clearly linear features, but here the lines are broken up into loops reminiscent of arabic script.

Look carefully and you’ll see that there are dunes upon dunes. The surface of the large sand bodies are covered in ridges and patterns with a wide variety of shapes. These themselves may have small ripples on, only visible if you visit in person.
 

This image is from the ‘Grand Erg Oriental’ in the Sahara desert in Algeria. It shows star dunes, which form when the wind is variable and simply piles the sand up into mounds 100s metres tall.

Both these areas of sand (by chance) sit above oil-fields, the oil-bearing rocks sitting deep under the ground. Ancient desert sands are of interest to oil geologists as the grains are very round and form sandstones that can contain a lot of liquid.
Ancient ‘aeolian sandstones’ are basically fossilised sand dunes and are often red. The ‘red sandstones’ of the UK, much Triassic sandstone in Europe and the classic Navajo Sandstone in the US are of this type.

Next time you see a red sandstone with big swooping cross bedding, think of these pictures.

Categories: Great Geology in Google earth, sediments

Tasting the earth: mantle geochemistry

If seismologists listen to the earth then geochemists taste it.

Like experts blind-tasting a glass of wine and recognising where it came from, geochemists studying the deep earth aim to find out where a particular liquid came from. Their liquid – basaltic magma formed from melting of the mantle rocks – is now solid, so ‘tasting it’ involves dissolving it in Hydrofluoric acid or vapourising it in the bowels of a machine with an unlovely name.

A wine buff can sniff out where a wine came from because they’ve already sampled lots of known vintages. Geochemists have a much harder job – basalt samples don’t have labels. They are formed from melting of the rocks below, but was the material that melted from the deep earth or shallow? Is it from oceanic crust that’s been subducted and remelted or material that’s sat around since the earth was formed?

Mantle geochemists still have more questions than answers, but that’s because what they do is really hard. They are like first-time wine-tasters who’ve been given anonymous bottles and only a fuzzy satellite image of France to work with.

Image stolen from the Basalt winery. I'm sure they won't mind.

Image stolen from the Basalt winery. I’m sure they won’t mind.

What to sample?

Most basalt is produced at mid-ocean ridges, where oceanic plates move apart and the underlying shallow mantle rises up, decompresses and melts. Known as MORB1 this is plonk. Widely produced, homogenised and of little interest to the true connoisseur.

Basalt from oceanic islands (OIB) is mantle geochemists’ favourite tipple. Found only in select areas far from plate boundaries it has many flavours but can be distinguished from MORB by a trained nose. Thought to be formed by material rising up in hot plumes from the deep mantle it carries whiffs of what is lurking down there.

Of particular interest at the moment are dark intense picritic lavas. Formed under higher temperatures in smaller batches they tell us more about what happens when a mantle plume first nears the surface.

sniffing wine

Tasting

The process of producing basalt from the mantle is complex, depending on the composition and mineralogy of the melting material plus the depth and pressure. Also a lot may happen to the magma before it cools as the surface as lava. Iceland has rhyolite lava flows – very different in composition to basalt, but ultimately formed from mantle melt2.

So to study the mantle that was melted tasting the basic chemistry of the lava is not enough as changes due to later processing can obscure the smell of the source material. More sensitive mechanical noses are required, that can sniff trace elements or isotopes that may be unchanged by later processing and hold the tang of the source mantle.

Terroir

Mantle composition, as inferred from basaltic melt, is very variable, leading to the identification of a ‘zoo’ of acronyms, from DMM and HIMU (sources for MORB) to EM and FOZO (for OIB).

A key concept is ‘enrichment’. Particular elements are ‘incompatible’ which means that if they are in a rock that melts, they are strongly partitioned into the melt. As the ‘enriched’ melt moves away you are left with a ‘depleted’ residue. Continental crust is extremely enriched, oceanic crust less so.

For this reason the churned up mantle contains portions which are depleted by having had oceanic crust melted from it (DMM) and other enriched portions which contain recycled oceanic crust (HIMU). Small amounts of continental crust may enter the mantle – perhaps the mantle frozen to the base of continents may fall off. Also continental material (sediment, stones frozen into icebergs, the Titanic) may end up on ocean floor destined to be subducted. EM and FOZO are sources that may have been enriched in this way.

Primitivo

Geochemists don’t just worry about the mantle, but the whole earth. Chondritic meteorites have long been thought to be a model of the bulk chemistry of the earth. Strip out iron and other elements into the core, account for the enriched crust and you can calculate the bulk composition of the mantle.3.

Compare known mantle compositions with the theoretical bulk composition and you get a gap, leading to the idea of a hidden reservoir of ‘primitive’ composition (e.g. closer to chondritic). Conceptually this is similar to the idea of ‘dark matter’ in physics – a thing invented to explain inconsistent pieces of evidence, but for which there is no direct evidence. Only time will tell if hidden reservoirs in the mantle will be found or go the way of the luminiferous aether.

Basalt in a vineyard

Basalt in a vineyard

Paradoxes and problems

The idea of hidden reservoirs was extremely popular over 20 years ago, when it seemed that subducting plates stopped at 660km depth, where a ‘phase change’ in minerals alters the stiffness of the flowing mantle. This suggested that the lower mantle could be of very different composition. But modern seismic imaging suggests whole-mantle convection is possible, suggesting that over billions of years the mantle will have been thoroughly stirred – with the exception of a mysterious layer at the base of the mantle.

Mantle geochemists often talk of ‘paradoxes’ – patterns of ratios between elements and isotopes that aren’t consistent. There is a lead paradox, and an Argon one, plus a ‘heat-Helium imbalance’. Explaining these in terms of a primitive reservoir is one way, but others are possible. Let’s look at Helium.

Helium comes in two flavours. The first 3He is just two protons and a neutron and from the earth’s point of view it’s primoridal, it’s always been there and never changes. In contrast, when the great hulking nuclei of Thorium and Uranium fall apart they leave small fragments – making 4He in the alchemical process of radioactive decay.

The ratio of the two Helium isotopes is fairly consistent for MORB sources, but wildly variable for OIB. Material with a high ratio has been interpreted in terms of a primitive reservoir, rich in primoridal 3He. An alternative explanation is that the source is extremely low in 4He due to it being depleted in Uranium/Thorium. Or maybe the 3He bubbled up from the core.

Tasting the earth does not give you all the answers, but it is vital part of the picture. As I continue my tour of the deep earth, geochemistry will often have an important role to play. The difference between OIB and MORB is a powerful argument in the armoury of those who favour mantle plumes and as seismologists start to see odd things at the base of the mantle, getting a whiff of the chemistry here becomes very important.

Tasting ‘black cherries’, ‘tar’ or ‘cat-pee’ in wine is a clever trick. Tasting blobs of 4.5 billion year-old rock or recycled oceanic crust in basalt is even cleverer. Cheers!

Further reading

This is a good overview, if a little old.

 

Categories: Deep earth, geochemistry

Metamorphic petrology under stress: round 2

Back in August I wrote about an extremely important paper by John Wheeler of Liverpool University called “Dramatic effects of stress on metamorphic reactions”. This uses a theoretical approach to show that differential stress (squashing rocks) is a very important control on metamorphic reactions. If true, this would imply that many estimates of depth of metamorphic conditions (that ignore the squashing) are wrong. Maybe eclogites don’t form at great depth after all.

Counterblast

Conceived as a provocative paper, it’s no surprise to find a “Comment” on it in the latest edition of Geology. Written by Raymond Fletcher of Penn State, it aims to “show
that Wheeler’s claims do not have a sound basis” by constructing a more complete mathematical model “for metamorphic reaction and pressure solution” (the two processes that Wheeler’s original paper wound together).

For both our sakes, I’m not going to get into the detail of the mathematics (all papers are open-source, so you can read it yourselves). It isn’t massively complicated maths – single lines of algebra only –  but what matters here is the assumptions and simplifications made and whether they are valid.

Fletcher’s comment picks on one aspect of the original paper – that in the section quantified the effects of differential stress, it focussed on a single way in which atoms can rearrange themselves called incongruent pressure solution. Fletcher’s set of equations are a more complete model that shows that Wheeler’s results are merely a ‘special case’ leading to ‘contrived outcomes’.

 A spirited defence

‘Comments’ on papers are often followed by a ‘reply’ from the original author. As here where the ‘comment’ is negative, they are effectively a form of public combat.

Wheeler is uniformly polite and positive. He starts by thanking Fletcher for his stimulating Comment and listing the ways in which they agree. Then this:

“But it is inappropriate to say that I am wrong, first because his model is not of the incongruent pressure solution (IPS) pathway, second because it actually contains
confirmation of some of my claims, and thirdly because it is extremely
restricted in scope.”

He then proceeds to show that Fletcher’s model doesn’t just model a single pathway and the he lists the assumptions made by Fletcher and demolishes each one. For this audience member, Wheeler starts to win the battle by the depth of context he brings to the discussion.

For each assumption he refers to existing research into real world complications. These include: defining 3-D stress as a single term in an equation is complicated – taking the simple average of the 3 dimensions is not correct; fluid pressure may control reactions, not stress; the topology of grains is important and the one chosen by Fletcher extremely unrealistic; diffusion of atoms is often a limiting factor in metamorphism; porphyroblasts often grown in specific shapes – ‘interfacial’ kinetics may also be important.

For this (slightly biased) reader the knock-out blow was the fact that 3 times the research into these complications is his own allowing Wheeler to write that Fletcher “may well have rediscovered the sorts of problems described above (Ford and Wheeler, 2004) but by ignoring these he reduces the value of his assertion that…” .

All good replies to comments look to the future:

In summary, Fletcher’s model is too restricted in scope to undermine my conclusions: we agree that a more general model is required. I challenge him and other interested readers (including myself) to construct such a model, which would be of great benefit to understanding how metamorphism and deformation interact.”

Science in action and in the open

I remind you again  of the important implications of Wheeler’s paper – existing estimates of metamorphic conditions – used to build tectonic models – are suspect. To quote another article in Geology discussing it “the potential inaccuracy of depth estimates based on minerals would question current paradigms in geology“.

Wheeler’s original paper was a huge challenge to metamorphic petrology. It has withstood the first attempt to refute it. This is science in action, in open source papers for all to view. I hope you’ll read the papers yourself and we can follow the unfolding story together.

Categories: metamorphism

Seismology in space

Seismology – using the propagation of waves through bodies to work out their internal structure – is extremely useful. You can use it to find oil, track active faults or understand what is at the centre of the earth. The principles and mathematics developed by studying the earth apply to other bodies too. The Moon, Mars, even distant stars: seismology can help us understand these bodies also.

Moonquakes

Let’s start close to home. As part of the Apollo moon-landings a series of seismometers were installed and collected seismic data for nearly 8 years. The vibrations were caused by small moon-quakes and meteorite impacts. To help things along (and to assist with calibration) a few pieces of rocket and the ascent stages of several lunar modules were deliberately crashed into the moon (once they were no longer needed, as the NASA page helpfully points out).

The moon is not tectonically active in the way the earth is – most moonquakes had Richter scale magnitudes of less than 2. Events not caused by collisions were clustered on a monthly cycle, suggesting they were caused by changes in tidal forces as the moon orbits the earth. The discovery of some recent tectonic features (found in imaging from the Lunar Reconnaissance Orbiter) suggests something else is going on. Perhaps cooling and contraction of underground melt is causing these surface features to form.

Diagram of the moon's interior. From Wikipedia.

Diagram of the moon’s interior. From Wikipedia.

This precious data was recently re-processed using the latest seismic techniques to tease out new details of the lunar interior. The seismic data was ‘messy’, due to smearing of signal in the upper 20 km of the crust, which is heavily fractured due to meteorite impacts. Modern processing allowed a clearer picture of the moons interior to be taken. It contains a metallic core, partly molten, but also a layer of molten rock at the base of the mantle.

Knowing the interior of the Moon is important for understanding the earth too. The most popular model for the Moon’s origin involves a massive impact between the earth and another body. We need to know what ended up in both the Moon and the earth to understand this process. It’s also interesting to reflect on the fact that the Moon is smaller than – and so would cool faster than – the earth. So why is it molten at the base of the mantle and the earth is not? A recent paper suggests the molten layer persists due to frictional heating of the moon from tidal forces. The same process (but stronger) heats up the moons of Jupiter, for example creating volcanoes on Io1.

Active vulcanism on Io, caused by tidal heating. No seismometers here (yet).

Active vulcanism on Io, caused by tidal heating. No seismometers here (yet).

Marsquakes

Mars is the only planet inhabited solely by robots2. The first Martian robot3, the Viking lander, had a seismometer stuck on the leg. Sadly it wasn’t very sensitive and didn’t detect any marsquakes at all (just a lot of wind). The NASA InSight mission aims to put this right, landing sensitive instruments (including a seismometer) in 2016.

Starquakes

Seismology is a useful tool for studying stars.

Our sun looking turbulent. Image from NASA.

Our sun looking turbulent. Image from NASA.

Yes, it really is.

There are no seismometers on the sun – they wouldn’t survive long4 – but it turns out that the same principles and mathematics we use to probe the earth also work on stars.

Asteroseismology is a form of seismology that uses pulsations in the light from stars to infer their internal structure. The outermost portion of a star is extremely turbulent and causes the entire star to vibrate like a dog waiting for a stick to be thrown. Instead of measuring the vibrations directly, we infer them from tiny variations in the intensity of the light, which we can measure from the delicious cool of our planet.

Sound waves (P-waves to seismologists) can be measured in both planets and stars. The sun has more exotic types of waves too and using these together gives us an invaluable view of the star’s internal structure. This in turn can be used to infer its age.

Similar techniques have been applied to Jupiter, which is also a ball of gas with something mysterious inside. Here the waves are detected by direct viewing of the surface5, as illuminated by the sun. The picture we’ve got so far is still rather fuzzy, but consistent with what we’d already guessed was there (a rocky core surrounded by metallic Hydrogen and then a Hydrogen and Helium atmosphere).

The Kepler missions search for planets orbiting other stars is linked to asteroseismology. The planets are discovered by the faint dimming effect as they pass in front of stars (the transit method). To do this we need to accurately know the size of the star and asteroseismology is the best way to do this. The ability to know the age of the star is useful too. Only a planet circling a relatively old star will have had enough time for life to evolve. The dream is of course to find a planet with intelligent life. If there is one, they are surely doing seismology. It’s such a useful technique to understand what lies beneath the surface of things.

Categories: space