Impact and Geology: spherules rule

Metageologist

One of the most striking changes in Earth Science in the last 20 years has been the way meteorite and associated impacts (or bolides and astroblemes, if you prefer) are viewed by Geologists. In the dark days of the 1990s they tended to be viewed as an annoying thing that people from NASA kept going on about. Now they are a standard part of mainstream Geology, not least because of the large number of geological features now linked with extra-terrestrial impacts.

The most dramatic form of evidence is a crater, but large or small, these only affect a small area of the earth. Excitingly, there are more subtle forms of evidence that are much more common and might be stumbled across by any geologist in the field.

You may have read about suevite, a deposit containing glass found close to impact craters. The glass was formed by shock melting at the impact site and is thrown out from the crater. Suevite is formed by material that falls close-by, but some of the glass is thrown high into the atmosphere and can land 100s of kilometres away. Big impacts can form a thin layer of bits of glass that cover most of the earth’s surface. The glass that goes a long way is most likely to have flown in the air as small droplets, known as spherules.

Black impact glass spherules, nothing to do with the Australian ones (see comments). Image from www.esrf.eu

These layers are thin and not particularly dramatic, but they are the evidence of an impact event most likely to be preserved in the geological record. The latest copy of Geology has a paper (doi: 10.1130/G31526.1) that nicely illustrates this. It describes a spherule layer, 2.57 Ga in age from Western Australia. The description of the layer itself is nice enough; the glass is long since devitrified, plus there is evidence that crystals grew in flight (before they were quenched by landing in the ocean). The glass is of mafic composition, suggesting the impact hit oceanic crust.

From Australia to Outer Space

The main interest of the paper is that this is by no means the first layer to be found. Three other layers are known, both in this Western Australian basin and in one of similar age in South Africa.  The age interval of the sediments containing these four spherule layers is 140 m.y., which suggests major impacts were quite often at this time. It is dangerous to extrapolate from so few data points, but it suggests an unusually high level of impacts at this time (Archean-Proterozoic boundary). Rates of impact frequency are generally thought of as a solar-system wide phenomena with the intensity reducing over time. This can be thought of as the solar system gradually getting tidier as old junk left over from the formation of the solar system is swept up by planets. Evidence that rates briefly increased suggests this picture is over-simplified.

Dating of features on other planets is generally done with reference to the number of craters cross-cutting them: the Earth is by far the easiest place to get absolute dates. The Geology paper’s suggestion that spherule layers on Earth can be used to infer variations in the rate of impacts is intriguing. If correct, it would mean that we can put constraints on solar-system wide processes by the detailed study of old sediments. Looking at dirty-looking outcrops in Australia can maybe help us understand other planets more.

Not just Australia

The paper shows a picture of an outcrop of the spherule layer. It is narrow and hardly jumps out at you. Yet it has been identified in three different places, which is impressive. The paper thanks a couple of mining companies for access to sites, and the area is the site of major iron ore mining, so I suspect there has been some very detailed logging of the sediments done.  Mining in the area boomed due to Chinese demand. Look around you, you probably see something made in China and if it has any iron in it, then Western Australia seems nearer than it did before.

There are bound to be many more spherule layers around that have yet to be identified. Phanerozoic ones may be harder to spot due to bioturbation, but there is an example from the Triassic of England (nice description here). So, the next time you are looking at sediments, keep your eyes peeled for spherules; you are the first generation of geologists to understand how significant they really are.

Categories: planets, Rocks & minerals

Impacts and Geology: deep peace?

Metageologist

Metamorphic rocks typically come from deep in the earth and form slowly. Simple physics shows that transferring heat into large volumes of rock (a key driver of many types of metamorphism) takes millions of years. Rocks that form the deep crust of stable cratonic areas lead the most placid of lives. They are heated for so long that they become annealed; they have achieved complete chemical, textural and thermodynamic equilibrium, like some sort of silicate-based Buddhist monk.

Some deep crustal rocks in South Africa were once in granulite nirvana and might still be there, if only they hadn’t been hit by the biggest impact known on earth. The slow and calm world of the deep crust was violently attacked from Outer Space and the shocking results are visible in the a thin-section.

The Biggest Impact Structure in the world

Vredefort from Space

Vredefort Dome in Google Earth

The Vredefort structure is a very old (2Ga), very big (>250km diameter) impact crater. It has the usual range of evidence (shatter cones, shocked minerals, coesite/stishovite) and unusually for Earth is multi-ringed. As well as being very wide, it also formed a very deep hole in the ground, which leads to a number of interesting things.

Geologically, the Vredefort structure is a dome, surrounded by an outer synclinorium. The dome itself shows an increase in metamorphic grade towards the centre, from amphibolite through to granulite facies metamorphism. This roughly corresponds to hotter rocks in the middle, so why should this be? Is this because deeper rocks are found in the middle, so we have a view of a cross-section through the pre-impact crust with pre-impact temperature differences preserved? Or, is the metamorphism related to the impact itself, with the intensity of the heating highest in the middle? To answer the question, we need to look into the metamorphic rocks in more detail

Metamorphic Petrologists find an amazing thing

I first heard about the Vredefort structure from a paper on metamorphic petrology by Stevens, Gibson and Droop from 1997 (see further reading below) . It is quite old now, but I still like it as it is such a Geological paper, and has photomicrographs of lovely rocks. It also does what Science should, gather together hard-won facts and rigorous argument to tell an exciting picture of the world. In short, it uses dry phrases like “petrogenetic modelling in the simplified KFMASH system” to show that at least 9 km thickness of rock was ‘instantaneously’ removed from the crater.

When it was published, there was still a debate about whether the structure was caused by an impact or by some other cause. It is significant as it helped to settle the argument in favour of an impact, by providing independent evidence from a detailed study of the rocks themselves.

Stevens et. al use the usual techniques of metamorphic petrology to describe the granulitic rocks towards the centre of the dome. They describe mineral assemblages and textures, and mineral chemistry. Often rocks contain multiple sets of mineral assemblages and these rocks are no exception. Using cross-cutting relationships, they constructed the following time-sequence of mineral assemblages (which I have greatly simplified):

  1. Peak metamorphism: rocks showing evidence of melting, containg garnet-orthopyroxene-cordierite
  2. Pseudotachylite veins: veins of glass, cross-cutting the peak metamorphic fabrics
  3. Post-shock overprint: recrystallisation of the pseudotachylite, coronas around minerals

Detailed analysis of these metamorphic assemblages allow them to reconstruct the history of conditions these rocks experience (a Pressure-Temperature-time  or P-T-t path). The peak metamorphism shows the kind of things that typically happen to rocks in the crust: an increase in pressure and temperature until the rocks melt, reaching 900°C and 0.6GPa pressure (about 20km depth). There is evidence of cooling, but this is not quantified. Later papers suggest the peak happened at around 3Ga, giving the rock about a billion years of peaceful cooling.

The pseudotachylite veins are linked to the impact event. They represent a fracturing event in the deep crust, where the energy released by the fracturing melts the surrounding rock.

The post-shock overprint is associated with recrystallisation of the glass at high temperatures, but also with coronas, new minerals forming around older grains. This texture is often associated with decompression, and thermobarometry on these mineral assemblages gives an estimate of pressure far lower than for the peak. Why has the pressure decreased? Stevens et. al. say because about 9km of rock has been nearly instantaneously removed from above the rocks being studied.

So to answer our question earlier, why are the rocks in the middle of the Vredefort Dome formed under hotter conditions? Most of the minerals in these rocks represent the peak metamorphism, the record of the later metamorphism is only really visible under the microscope.  The hotter rocks are in the middle, because that is where the crater was of the greatest depth.

Metamorphic petrologists continue to find amazing things

The Stephens et al. paper is quite old now and such interesting geology has continued to attract attention. This rather good online thesis gives a useful overview of current thinking. I’ll quickly draw out some interesting themes.

Post-impact rebound and shape of the crater

Theophilus Crater on the Moon. An analogy for the Vredefort structure, note central uplift and surrounding terrace. Image courtesy of Nasa via Wikimedia

Isostatic rebound is the phenomena where the crust responds to load placed upon it. The best example is in areas that were covered by major ice-caps during the recent Ice Age. Places such as Scotland and Scandinavia were covered by kilometres of ice, which pressed the crust down. Now the ice has gone, the crust is rising up again (on a scale of 1000s of years). This creates raised beaches as the crust rises and the sea doesn’t.

In a similar way, suddenly removing kilometres of rock from the crust will cause the remaining underlying rocks to rebound up over a period of time. In order to found out exactly how, you can either study the Vredefort structure, or go to the Moon or Mars.

As well as removing material, the impact puts a lot of heat into the remaining crust. For complex craters they start to behave as Bingham Fluids (like toothpaste). The centre of the dome rises up and the outer areas slip down towards the centre, creating a terraced rim. The large volumes of pseudotachylite in the Vredefort area are thought to be as much related to this post-impact movement as to the actual impact itself.

Shock metamorphism and recovery heating

One thing that can certainly be attributed directly to the impact, is shock metamorphism. This has been mapped across the area, with pressures of at least 30GPa (50 times greater than the ‘peak metamorphism’ above) found in the centre of the dome.

Shock metamorphism puts energy into the rocks, and as the pressure decays, this is released as heat. The post-shock overprint is prominent, both because the rocks were still hot, but also because they were further heated by the thermal overprint of the shock metamorphism. Temperatures reached up to 1000°C, with shock heating adding 500°C to already hot rocks. So, both the pre- and post-impact metamorphic events are at higher temperatures in the middle.

A natural laboratory

In order to estimate the conditions under which rocks are metamorphosed, petrologists generally have to assume they achieved chemical equilibrium. This is arguably true for slowly heated rocks, but clearly not true for shock metamorphism, or indeed for the post-shock overprint, where the rocks would have quickly cooled (since they are now close to the surface). The main thrust of this recent thesis is how getting a handle on metamorphic rocks that are not in full equilibrium can help us better understand all types of metamorphism. As with people, metamorphic rocks are always encountering changing conditions and rarely achieve total inner peace. Temporary partial equilibrium is the best most of us can manage and techniques that don’t assume metamorphic rocks achieved nirvana are to be welcomed.

References

Mid-crustal granulite facies metamorphism in the Central Kaapvaal craton: the Bushveld Complex connection: doi:10.1016/S0301-9268(96)00043-5. This is the 1997 paper I mention. The reference to the Bushveld complex is related to the idea that the local temperatures were elevated due to its intrusion.

Metamorphic studies in the Vredefort Dome, South Africa. This is a 2010 PhD thesis by Paula Ogilvie. I’ve mined it for its clear updates of current thinking but I’ve only started to begin to get my head around its main arguments. I can’t find this work published yet, but it will be soon, I suspect. If you’ve found my referencing a bit sparse, this is the place to come.

Categories: planets, Rocks & minerals

Cu – The Finale

Nina Fitzgerald

(NOTE:  I would like to thank Chris @ Highly Allochthonous for giving me this opportunity to cross-post a favorite sedimentary-stratigraphy class project (from 2007) on Earth Science Erratics.  All mistakes are mine and I welcome comments.  ESE is a great venue for up-and-coming geo-bloggers and I look forward to new and exciting posts from varied contributors as the site grows).

For this third and final post in my WATCH FOR ROCKS mini-series on copper mineralization and skarn, I pose a thought-provoking fifth question:

Is there anyone left in the building?

No, wait.  That was the fourth-and-a-half question.  Here is the final fifth:

What are the relationships between copper and other minerals such as chalcopyrite, cuprite, chrysocolla, bornite, malachite, and azurite, magnetite, and hematite, among others?

First, a word or two (I am not making these up!) about hypogene and supergene processes…

Primary minerals are those that form by the combination of elements rather than by alteration of a mineral.  Primary or hypogene minerals in a hydrothermal system can be extensively altered when exposed to oxygen in the near-surface environment.

Pyrite crystals

— Pyrite (FeS2), reacting with oxygenated groundwater, will form iron hydroxides and release the sulfur as sulfuric acid.

— Chalcopyrite (CuFeS2),  reacting with dissolved atmospheric oxygen and carbon dioxide, can produce cuprite (Cu2O) and siderite (FeCO3).

— Chalcocite (Cu2S) commonly forms from the alteration of primary copper minerals that are attacked above the water table by oxygen.

The net result of these and related reactions is to oxidize the metals found in the sulfides to form oxides, hydroxides, carbonates, and sulfates.  Because these reactions produce oxygen-bearing minerals at the expense of sulfides, the near surface zone is often referred to as the oxidized zone and the minerals produced are called supergene or secondary minerals.

Next is a simple listing of mineral names with each appropriate chemical formula.   These formulas tie a nice neat bow on the whole copper mineralization concept.  I hope you agree!

Common sulfide ores for copper include:

— Bornite – Cu5FeS4, Copper Iron Sulfide

— Chalcocite – Cu2S, Copper Sulfide

— Chalcopyrite – CuFeS2, Copper Iron Sulfide – the most common ore.

— Covellite – CuS, Copper Sulfide – an excellent ore of copper when available in economic amounts.

Aurichalcite

Copper carbonates include:

— Aurichalcite – (Zn, Cu)5(CO3)2(OH)6, Zinc Copper Carbonate Hydroxide – forms in the oxidation zones of zinc-copper deposits where it is deposited by circulating fluids of carbonate-rich solutions.

Azurite

— Azurite – Cu3(CO3)2(OH)2, Copper Carbonate Hydroxide  – considered a minor ore of copper, mostly because it is found associated with other more valuable copper ores.

Outer Malachite w/ inner Chrysacolla

— Malachite – Cu2(CO3)(OH)2, Copper Carbonate Hydroxide

Copper oxides include:

— Cuprite – Cu2O, Copper Oxide - Of all the copper ores except for native copper, cuprite gives the greatest yield of copper per molecule since there is only one oxygen atom to every two copper atoms.

Cuprite is found in the near-surface oxidized portion of copper-bearing hydrothermal sulfide deposits.  It generally is produced by the alteration and oxidation of primary sulfide minerals such as chalcopyrite.

Copper silicates include:

— Chrysocolla – (Cu,Al)2H2Si2O5(OH)4· nH2O, Hydrous Copper Silicate – found in the oxidation zone of copper deposits; associated with azurite, malachite, and cuprite.  It is an important surface indicator pinpointing the presence of disseminated copper deposits (porphyry copper ore).  However, it is not an important copper ore.

Iron oxides include:

— Magnetite – Fe+2Fe+32O4 – plentiful in skarns – in a metamorphic environment it is formed by reduction of hematite derived from the dissociation of sulfides and iron silicates.

— Hematite – Fe2O3forms under oxidizing conditions – remains stable in a low-temperature metamorphic environment where it often replaces magnetite.

To Sum Up ~~~

There are several possible sources of copper mineralization in skarns:

— It is uncertain whether the copper mineralization in the skarn came from the quartz monzonite itself or whether in the act of the skarn formation the limestones were altered in such a way as to produce the mineralization.

— Fracturing of the crust enabled hypogene or metasomatic mineralizing fluids to bring in copper and other metals from sources deeper in the earth.  This could be an undiscovered porphyry deposit or some other unknown source.

— Additionally, the copper atoms may have been transported (from somewhere) by ionic diffusion to ultimately bind with the sulfur atoms.

Thinking of prospecting for Cu?

— Ore bodies will have a layer of chalcocite which corresponds to the present or a past water table level and this layer is called a “chalcocite blanket”. The chalcocite blanket is richer in copper than the upper oxidized portion of the ore body and usually richer than the primary unaltered ores below. The chalcocite blanket represents a real “gold mine” to the copper prospectors.

— Look for the chrysocolla.

Cocktail party trivia:

— The Copper Age of Europe and the Middle East began around 4000 BC., a transition period as Neolithic stone tools gave way to the Bronze Age.

— Artifacts recovered from The Old Copper Complex of the western Great Lakes region have been carbon-dated at 1000-4000 BC.  Ninety-nine percent pure copper was discovered in the Lake Superior basin in vein form and in the form of nuggets in glacial outwash gravel beds.

— Native copper (copper found in a chemically uncombined state) was mined for centuries and now is all but depleted as an economically viable ore. Other copper minerals are far more economical to mine and purify into metallic copper that is used for wiring, electrical components, pennies and other coins, tubing and many other applications.

Cu

Selected References:

Mottano, A., Crespi, R., Liborio, G., (1978), Simon & Schuster’s Guide to Rocks & Minerals

Wray, W.B., 2006, Mines and Geology of the Rocky and Beaver Lake Districts, Beaver County, Utah in Bon, R.L., Gloyn, R.W., and Park, G.M., editors, Mining Districts of Utah:  Utah  Geological Association Publication 32, p. 183-285.

Categories: Ore geology, Rocks & minerals

Still Wondering About Cu?

Nina Fitzgerald

(NOTE:  This is the second in a series of three posts on copper mineralization in skarn.)

As a geology student several years ago, I had wondered why copper (chemical symbol Cu) shows up where it does.  I had wondered how copper gets to where it gets. I had found that copper could mineralize in rocks in certain areas of what is called skarn.

Native Copper (Cu) - scale in centimeters

These queries are turning out to be yet another chapter in my endlessly provocative geological quest:  How did that get there?  With this WATCH FOR ROCKS copper mini-series, I attempt to answer these myriad questions in ways that won’t compel anyone to leave the building.

My previous post addressed two burning questions:

First: What the heck is a skarn?

Second: What does quartz monzonite (a type of igneous intrusion present in my southern Utah study area) have to do with the skarn?

So now we arrive at the Third question — How are skarns related to copper mineralization, anyway?

Again, skarn is a type of rock formed by contact metamorphism and metasomatism of carbonate rocks (generally limestones) where hot, acidic, silica-rich fluids are driven from an igneous intrusion to react with the carbonates of the surrounding rock.  This is where hydrothermal veins enter the picture.  Skarn is considered to have formed along fractures in the “country or host rock” or bedrock.

Most major ores of important metals such as copper, lead and silver are sulfides. Sulfides are most commonly found in hydrothermal sulfide deposits, either in veins or disseminated throughout the deposit. While native copper was a source for the metal early in the development of human civilization, now most copper is extracted from sulfide minerals.

What is an ore?

“It is the naturally occurring material from which a mineral or minerals of economic value can be extracted at a reasonable profit.”

Fourth questions (a two-fer!) — How do sulfides form? How do we get copper from them?

MM900285332In a nutshell: Transition metals (such as Fe (Iron), Zn(Zinc), Cu (Copper), Pb (Lead), Co (Cobalt), and others) bond with Sulfur (S) by forming molecular orbitals that share electrons to satisfy the valence requirements of the transition metal atoms.

Yikes! Chemistry rears its ugly head!

Alas, there’s more…

Sulfides and related minerals are characteristic of hydrothermal vein and replacement deposits.

Essential features of hydrothermal systems include:

Water

Heat

A source for the metals and other elements precipitated by the water

Migration pathways

Precipitation

As you might suspect, these features (of hydrothermal systems) have features:

1— Water can be evolved from magma, be released during metamorphism (I could really go on about this one…), originate as rain or snow (meteoric), and that all-time favorite, be trapped in sediment pores (connate).

2— Heat is often provided by the igneous intrusion; it also can come from depth.

3—There are several ideas for a source of metals and other minerals precipitated by the water. Metals, sulfur, and other elements in the water may be derived from the crystallizing magma or they may be leached out of a large volume of country rock by the water.

The presence of the metals may also come about through ionic diffusion (ack!) from an unknown source (porphyry deposit?) or by gaseous emanation (but of what, from where?).

4— Fluids commonly flow along migration pathways through rocks along fractures, faults, and normal pore spaces.

5— Minerals may precipitate either by a) filling void spaces or by b) replacing other minerals in the rock through which the fluids flow. As the fluids migrate away from the crystallizing magma, they can encounter chemically reactive rock such as limestone, which may trigger precipitation of metal ions from solution.

Circulating fluids can scavenge elements from the rocks like a liquid Pac-man swimming through a stream. They can dissolve previously produced mineralization and re-precipitate those minerals.

The ultimate result here may be a progressively concentrated volume of valuable minerals.

Next post – the final question along with pictures of magnificent minerals:

What are the relationships between copper and many other magnificent minerals such as magnetite, chalcopyrite, cuprite, and chrysocolla?

C-u then!

Metallic Pyrite with Quartz (can you spot the darker Galena within the Quartz crystals?)

Categories: Ore geology, Rocks & minerals

What’s Up With Cu?

Nina Fitzgerald

Since returning two weeks ago from my four-day hiking extravaganza in southern Nevada and northwest Arizona, I have been doing a lot of thinking about mining and its long history in that part of the country.  In Arizona, in particular, there have been a lot of copper minerals mined over at least the past century or more.  Copper (chemical symbol Cu) is also mined in Utah in such places as the Bingham Mine up near Salt Lake City and smaller operations in the southern part of the state.  Copper is actually mined in many places all over the country and the world, Chile in particular.

Native Copper - scale in cm

With that in mind, I began leafing through my digital folders in search of a presentation I gave several years ago in one of my geology classes.  I had wondered why copper shows up where it does.  I had wondered how copper gets to where it gets. I had found that copper could mineralize in rocks in certain areas of what is called skarn.

I pondered the following:

What the heck is skarn?

What does quartz monzonite (a type of igneous intrusion related to granite that is present in my southern Utah study area) have to do with the skarn?

How are skarns related to copper mineralization anyway?

How do sulfides form? How do we get copper from them?

What are the relationships between copper and many other magnificent minerals such as magnetite, chalcopyrite, cuprite, and chrysocolla?

This pondering ultimately turned out to be yet another chapter in my endlessly provocative geological question:  How did that get there?  Naturally, it will be morphing into a series of blog posts. One or two questions will be answered per post over the next few days until I’ve run out of questions (for the time being).

As you read each post, please feel free to take a break.  Make yourself a sandwich.  Take the dog for a walk.  However, do come back!   I know this is a lot of “stuff” to absorb in one sitting.  Even I have to read it several dozen times to understand it, and I wrote it!

First question – What is a skarn?

Skarn is a type of rock formed by contact metamorphism and metasomatism of carbonate rocks (generally limestones) where hot, acidic, silica-rich fluids are driven from an igneous intrusion (such as a quartz monzonite, the above-mentioned relative of granite) to react with the carbonates of the surrounding rock.

If the carbonate component (CO3) of the country rock is dominant, marble forms as a result of the reaction.

If the carbonate component of the country rock is subordinate (impure limestones), the skarn may be composed of additional minerals such as Ca (calcium) – Mg (magnesium) – Fe (iron) – Al (aluminum) – Na (sodium) – and/or K (potassium), in which case these other rocks would form as a result of the reaction:

Diopside – CaMgSi2O6

Grossular – Ca3Al2(SiO4)3

Ca-amphiboles – Ca2(Mg,Fe)5(Si8O22)(OH)2

Vesuvianite – Ca10Mg2Al4(SiO4)5(Si2O7)2(OH)4

Epidote – Ca2(Al,Fe)3Si3O12(OH)

Wollastonite – CaSiO3

Second question — What does the quartz monzonite have to do with the skarn?

Image courtesy of Geokansas (website: kgs.ku.edu)

Skarn deposits almost always adjoin unaltered quartz monzonite or granodiorite igneous intrusions (“lamproite” in image to right is actually a volcanic rock but the diagram example gives a good visual). They are considered to have formed along fractures in the “country or host rock” or bedrock — this is where hydrothermal veins enter the picture.

What happens is this: the host rocks surrounding the intrusion are converted by heat and substantial metasomatic activity into wide calc-silicate skarn zones, locally dominated by iron and copper mineralization (along with other minerals).

Coming soon – Third Question:  How are skarns related to copper mineralization, anyway?

C-u then!

Selected Reference:

Wray, W.B., 2006, Mines and Geology of the Rocky and Beaver Lake Districts, Beaver County, Utah in Bon, R.L., Gloyn, R.W., and Park, G.M., editors, Mining Districts of Utah: Utah Geological Association Publication 32, p. 183-285.

Categories: Ore geology, Rocks & minerals

Bubbling Up… by Kathy Cashman and Alison Rust

Kathy Cashman

Gas bubbles (or pore spaces) are a fundamental component of many earth materials, yet processes that control bubble formation and migration are rarely addressed in basic earth science texts. Understanding bubble formation and migration is particularly critical for understanding volcano behavior, where gas expansion provides the primary driving force for volcanic eruptions. However, bubble behavior also affects magma chamber processes and ore deposit formation. The physical properties of bubbles that make them such effective drivers of magma motion are their buoyancy, their volume sensitivity to pressure and temperature, and their deformability, properties that are easily explored in the kitchen. Here we start our exploration of cooking analogies with bread, where the revival of artisan breads provides a wide array of bread textures and techniques.

Bubbles and baking – a tutorial[1]

Making and growing bubbles is a common baking technique; in fact, controlling the timing, rates and extent of bubble formation lies at the heart of most cake, cookie and bread recipes. In baking, bubbles are forced into the batter by beating or kneading; during baking, gas is added to those bubbles by CO2-producing reactions that involve either slow-acting yeast (for stiff bread doughs) or rapid chemical reactions (acid + base; used in quick breads and runny cake batters).  Times prescribed for bread to rise, and times and oven temperatures used for baking, are designed to balance bubble growth with changes in the property of the surrounding dough to produce the desired texture.

Baking causes both chemical and physical changes to the dough.  Heating initially causes the dough to become more fluid and expand by a combination of steam formation from water in the dough and CO2 migration from chemical reactions into existing bubbles. After the initial expansion, bubbles are trapped in place by stiffening of the dough and the formation of an outer rigid crust. Also important at this stage is rupture of bubble walls to form an interconnected (permeable) gas network that allows the gases to escape.  If the gas bubbles remained isolated, steam condensation on cooling would cause the structure to collapse.  The rate of heating is critical because it controls the relative rates of bubble expansion and crust formation – if heating is too slow, bubbles will expand, coarsen, interconnect and even collapse; if heating is too rapid, the crust will stiffen more rapidly than the interior expands, causing the crust to rupture.

Ciabatta bread

An illustration of the role of controlled vesiculation in bread baking is provided by the contrasting surfaces and interior textures of two artisan bread types: ciabatta and a multigrain bread[2]. Ciabatta has a smooth crunchy crust and large vesicles inside (see above). This combination of smooth outside and coarse bubbly inside texture reflects rapid bubble expansion in a sticky dough-  this is best done in a wood-fired oven, which reaches higher temperatures than a conventional oven, and the early introduction of steam to the oven, which increases the rate of heat transfer to the dough while at the same time maintaining a flexible crust that can accommodate the rapid volume increase of the expanding dough. Note that the bubble size is much smaller in the outer crust than in the interior, because it was “quenched in as the crust formed”, and that the largest bubble is in the upper half of the bread as a consequence of the rise and accumulation of escaping gases.

Crusty sourdough bread

In crusty sourdough bread, the original crust has been broken by expansion of the bubbly interior (see above). One could estimate the extent of volumetric expansion after crust formation by determining the surface area (and contained volume) of the unbroken crust relative to that of the finished (fully expanded) bread.  Examination of the internal structure of the bread shows that the bubbles are quite a bit smaller than in the ciabatta (a reflection of stiffer dough). Additionally, there is clear evidence of bubble deformation (representing flow accompanying expansion) oriented upward toward the opening crack.

Bubbles and basalt

Analogies to the bread forming textures can be found pyroclasts (literally ‘broken fire’ –), the products of explosive volcanic eruptions. Volcanic eruptions obtain their energy from expanding bubbles. As in bread, the bubbles are formed primarily by water and carbon dioxide, although in the case of magma, these gas phases were dissolved within the magma at depth, and come out of solution as the magma approaches the Earth’s surface. The rate of magma ascent, the amount of gas dissolved in the magma, and the magma composition and temperature (which control the fluidity) all determine when and where bubble expansion begins and ends. Pyroclasts formed by many basaltic eruptions continue to expand freely, with little restraint by the thin and flexible crust.

Stromboli potato bomb

These pyroclasts are analogous to ciabatta, in that they typically have a smooth crust and may have large interior holes. At Stromboli volcano (Italy), very frothy but smooth-surfaced bombs are informally known as potatoes, for their rounded shapes and smooth light brown surface.

In Hawaii, bubbly lava flows are transported through complex lava tube systems; when these flows travel slowly across the surface, they also form bubble-rich lavas with characteristically smooth surface crusts.

Bubbles and breadcrust bombs

It is no accident that an entire class of pyroclasts has the name of breadcrust bombs. Breadcrust bombs are characteristic of a specific type of volcanic eruption that is called Vulcanian after the volcano Vulcano in Italy (the original volcano!). These bombs form degassed magma that forms a plug on the conduit. When bubbles collect below this cap they exert pressure until the plug fails and an eruption happens.

Magma that formed the plug has enough residual gas dissolved in the magma to form bubbles, but only after the outside has cooled and solidified. Therefore, bubble formation and expansion causes the outer crust to crack.  Although there are many different types of fracture patterns, some bombs look very similar to the crusty bread shown above. If we break open these bombs, we see larger bubbles in the middle, just as we saw in the bread.

Breadcrust bombs from Guagua Pichincha volcano, Ecuador

Kathy Cashman and Alison Rust


[1] For more on cooking science, see On Food and Cooking: The Science and Lore of the Kitchen by Harold McGee; for bread specifics we suggest The Bread Baker’s Apprentice: Mastering the Art of Extraordinary Bread by Peter Reinhart

[2] Both breads featured here were made by Hideaway Bakery in Eugene OR, who employ much better bakers than we are!

Categories: volcanoes

Impacts and Geology: from lahar to suevite

Metageologist

As I’ve written before, the last 30 years has seen a big change in the way Geologists think about the Earth; this planet of ours does not sit in isolation in the Universe but is frequently hit and changed by meteorites (and other bits and pieces).  When I received by Geological education in the 90’s there was little mention of such things. Even more recently, after the evidence had piled up, it all felt a bit remote, nothing to do with the rocks I personally knew. Then in 2008 a paper came out that really brought the influence of extra-terrestrial events home to me.

In my parent’s garage is a pile of rocks collected when I was a lad. One sample was from the Proterozoic Torridonian sedimentary rocks of Assynt, Scotland, from a unit called the ‘Stac Fada member’. The guide-book described this as a lahar, a volcanic mud-flow deposit, but there was an air of doubt in the description, (there is no other sign of vulcanism in the entire sequence).  Let have a look at the sample:

Sample of Stac Fada Member

The green blobs are of devitrified glass (now chlorite), the pink a feldspar vein. The matrix is very poorly sorted and contains chunks of more normal rock-types. Calling this a lahar is not an unreasonable interpretation: rapidly cooled lava provides the glass and subsequent rain mixes it up together with whatever else is lying around and deposits it all in an untidy heap downstream.

However a paper in Geology presents a much more interesting interpretation: the Stac Fada member is a layer of debris formed close to a major meteorite impact. So, the glass was produced by the heat of the impact and the force of the impact threw debris over a wide distance, leaving the unsorted bits lying in an untidy heap on the ground.

The evidence is pretty compelling: the deposit contains shocked quartz and enriched levels of Iridium. Underlying layers are disrupted and locally ripped up into the impact deposit and it can be traced as a single deposit across the entire basin. The feldspar veins, and other evidence, shows that the deposit was hot when it was emplaced.

Suddenly, my little sample changes from run-of-the-mill lahar into glamorous suevite. Even better, the authors are able to infer some things about the impact. The geochemistry suggests that the impactor was a stony meteorite, which after being vapourised on impact is now distributed through the deposit. The patterns of overturning of layers underneath the deposit give evidence of the direction of impact and suggest the impact site is from an area now just offshore. They estimate the crater would be 6-8 km across.

Another buried crater?

Offshore from these rocks is a sedimentary basin, the Minch, so maybe a trace of the crater remains, buried underneath Mesozoic sediments? Well, maybe, but the Torridonian sediments that contain the impact layer show evidence of being deposited in a narrow fault-bounded basin, so the impact site might have been on the now-eroded flanks. Also between 1.2Ga and the start of the formation of the Minch basin Scotland enjoyed (from memory) the Grenvillian, Grampian, Scandian and Acadian orogenies. The area in question is in the ‘foreland’ of these events, but we are rather far from a stable cratonic situation here. So, the crater may be visible, still preserved deep under the Minch, but I wouldn’t bet on it.

The identification of the Stac Fada member as a trace of an impact is down to detailed field Geology. There is nothing startling about the deposit, it can’t be sensed remotely and the clinching evidence is based on microscopic and geochemical analysis. There must be a lot more of these subtle traces of impacts yet to be found in the Geological record.

Following its ‘transformation’ from lahar, my sample of suevite is no longer in my parent’s garage, but instead has been promoted to my office desk, next to some ichthyosaur bones, waiting to catch the attention and perhaps the imagination of passing colleagues.

Further reading:

A field guide to the deposit from Oxford Uni. Lots of nice pictures: http://www.earth.ox.ac.uk/~oesis/nws/nws-a98-st1.html

A write-up of the research from the Geological Society: http://www.geolsoc.org.uk/gsl/geoscientist/geonews/page6069.html

Categories: planets, Rocks & minerals

Meteorites and Geology: big holes in the ground

Metageologist

Many thanks to Chris and Anne for such a great opportunity to step into the geoblogosphere. Here goes…

One of the few advantages of having being on the Earth for a while (0.00004 Ma in my case) is that I’ve had time to see new things come along in Science. One of the big themes in Geology in the last 20-30 years has been the realisation of the importance of asteroid impacts in Earth History. A lot of attention has rightly been focused on the role of massive impacts in causing mass extinctions (K-T boundary, Chicxulub)  but since meteorite impacts follow a power-law relationship of size to frequency alongside a few ‘earth-shattering’ impacts there should be lots of *smaller* ones. Even a ‘small’ impact will make a trace in the Geological record. So where are they?

How many craters?

The classic trace of an impact is a crater, a circular hole in the ground with a distinctive rim and maybe a central uplift.

Other things make circular structures of course, such as volcanic activity or salt diapirs, so to unequivocally attribute a structure to an impact, other evidence such as shocked quartz or shatter cones is required.

Many craters are on the Earth’s surface, which either means they are recent or have managed to survive erosion, perhaps by being very large. Sedimentary basins are made up of a whole series of fossil surfaces, which have been preserved by a covering of later sediment as the basin fills and/or subsides. So if a basin was hit by a significant impact during its history, we can expect to see it preserved in the pattern of its layers of sediment. Oil companies spend a lot of money collecting seismic reflection data precisely to see patterns in layers of sediment.

A fascinating recent paper “Estimates of yet-to-find impact crater population on Earth” by Professor Stewart of Heriot-Watt University in Scotland discusses this matter in detail. Firstly, he discusses the number of craters currently recognised (178) and the rate at which they are being found (2 a year, which is why the paper says 176 and the database today has 178). Next, he heroically attempts to quantify how many there should be yet to find. Numbers are necessarily imprecise, but he produces three sets of estimates.

  1. The first is based on the number of objects we can see in space, the number of craters we can see on Mars or the Moon and the number of small meteorites we can hear or see burning up in the Earth’s atmosphere. This evidence is used to estimate the rate of impacts on Earth per size of meteorite (that power-law again).
  2. The second number is the ‘area-timespan’ preserved  in Phanerozoic sedimentary basins (e.g. those of interest to oil companies where seismic data is collected; he is “Professor of Petroleum Geoscience”, after all). As a simplistic example (the paper is *not* simplistic) a basin of 10km2 that was accumulating sediment for 10Ma has an ‘area-timespan’ of 100 million km2 a. His estimate for the whole world comes to pleasingly large 9 quadrillion km2 a (9×1015).
  3. Combining those two numbers gives the third: there are over 700 craters larger than 1km diameter yet to be found.

Putting it another way, on average over the last 542 million years, over 700 big asteroids would have hit a part of the earth with a subsiding basin covered by a shallow sea. This is rather cool, I suggest, particularly the idea that many of these craters may already have been surveyed, but the seismic data has yet to be appropriately analysed. It’s a nice thought that most examples of what happens when something very big falls out of the sky are to be found deep underground.

Professor Stewart mentions a number of examples of possible impact craters known from seismic data which are not part of the 178 ‘known’ total. This is because evidence from the shape alone is not sufficient to make the ‘official’ list. The craters are buried under kilometres of rock so it would cost millions to gather rock samples containing the other types of evidence.

Silverpit crater

To speculate about the human side of this research (without knowing any of the
people involved), I think the writing of this paper must have been motivated in part by Prof. Stewart’s experience with the Silverpit crater. This is a beautiful circular structure, visible only in seismic data, deep under the North Sea. In a 2002 paper Prof. Stewart and a co-author interpreted it as a meteorite impact. Over the next few years a number of papers appeared suggesting alternative interpretations, all of terrestrial origin. The tone of the discussion became rather bad-tempered, by the standards of academic debate. Prof. Stewart would not be human if the experience of being accused (in the journal of world’s oldest Geological Society) of suggesting an impact origin “without a shred of scientific evidence” (in doi:10.1144/0016-764904-070) didn’t motivate him to find yet more scientific evidence for it.

This paper does just that: of the craters estimated yet to be discovered, based on its area-timespan, three would be in the North Sea. This further strengthens the argument that the Silverpit structure (the only candidate yet identified) is a buried impact crater. Given the difficulty of getting more direct evidence, this may be the best that is possible.

If the debate over Silverpit got emotional, then that makes sense to me. My experience of academia is that nothing is disliked more than people from one discipline ‘moving into’ another area. My emotional response in the 1990s to astrophysicists arguing for the importance of impacts in Geological history was of scepticism and dislike. A training in Geology is all about the Earth: we spend our time looking at the ground. Things coming in from Space just didn’t seem relevant or necessary.

Of course, science at its best works across artificial human distinctions and makes connections between seemingly separate things. It turns out there is plenty of Geological evidence for ancient meteorite impacts and that is what I hope to talk about in future posts.

Categories: planets

Welcome to Earth Science Erratics

Admin

Like a rock plucked from its mountain and carried by a glacier into unfamiliar terranes, do you find yourself gravitationally pulled by the exciting force of the geoblogosphere, but unsure as to what happens next? Do you have something that you’d think you’d like to write about, but aren’t sure you have the time, energy, or material to build a moraine commit to a blog? Then come be an erratic with us!

Earth Science Erratics is conceived as a place for geoscientists or geosciences enthusiasts to be able to write one or a few blog posts, on any earth science topic of their choice, without the necessity of establishing their own blog. Think of this space as a field of erratics – a place where stones end up after the ice melts, but not in a thick enough deposit to constitute a mappable till unit.

We’d like to host an assortment of posts here – from the consulting geologist sharing tales of a field project in an exotic locale, to the grad student anxious to practice his science writing skills explaining a part of their discipline…from the amateur earth scientist who wants to write about the geology in her neighborhood to the researcher who wants to share her newly published results with an audience broader than journal readers.

We’ll work to seek out voices for this space, but we’re even happier to hear from volunteers. We’ll make it possible for you get the experience writing and publishing a blog post, by giving you the keys to the blog, but we’ll also hold your hand as needed and keep the technical demons under control. We won’t limit you to one post, and if blogging here whets your appetite for your own blog, we’ll cheer you on and throw you a launch party. But if all you want to do is write one or two or five posts, we’ll use the Highly Allochthonous blog, Twitter, syndication feeds, and the rest of our connections with the science blogosphere to make sure that those posts get the attention they deserve. We like hearing from new geosciences voices, and this is our way of trying to encourage those voices and make sure they are heard.

Plus, we think fields full of erratics are wonderful places to go exploring.

Field of erratics, northern Yellowstone

Field of erratics, northern Yellowstone (Photo by A. Jefferson)

Categories: Announcements