Seafloor Ooze, Subduction, and Oil

When I was a young man, I thought that having my PhD meant that I was now a scientist, that the advanced academic degree was somehow the dividing line between scientist and not-scientist. If I had been a little better at history, I would have realized that some of the greatest minds in science – people like Michael Faraday and James Clerk Maxwell – did not have PhDs. What they DID have was a tendency to think about things. The following two queries came from someone I call Patrick the Plumber Scientist.

Q: I've read the seafloor  "ooze" contains a fair amount of carbon based material. When this ooze is carried along with the seafloor downward in subduction zones wouldn't the combination of heat and pressure along with the presence of water form hydrocarbons aka oil?

- Patrick D

A: You are an unusually thoughtful person to arrive at that conclusion. Not all the ooze, as you call it, actually goes down with the subducting oceanic crustal slab – some of it gets scraped off and in some cases rafted onto a continental margin. You can find some of these strange remnants on the northern California and southwestern Oregon coastal area, among many other places in the rest of the world.

At some point the carbon from the seafloor muck that DOES go down with the oceanic crust probably passes through an oil/hydrocarbon maturation phase, but at depths and circumstances where it could not be economically extracted (even if it could be located). The muck continues down even deeper with the oceanic crustal slab to depths where even greater heat and pressure subsequently break it down to even more primitive constituents. With the water and sulfur also found in these seafloor sediments, this leads to partial melting – the lighter constituents rise through the crust (like a lava-lamp), somewhere in-board of the subduction zone to form volcanic chains like the Cascades, the Kamchatka Peninsula, the Andes, the Indonesian Archipelago, etc.

The magma that actually rises is driven at least partly by CO₂ and H₂S gases that derive from that original seafloor muck and seawater. These constituents, along with the iron, manganese, and silica of the Mantle, comprise the rising magma.  As it comes closer to the surface of the Earth, the pressure decreases and the gases come out of solution (like uncapping a bottle of soda) in that rising magma to form bubbles. This has been studied in one of our laboratories in a hot-high-pressure cell. The increasing nucleation of bubbles expands the magma volume and this causes the whole mix to accelerate upward faster and faster toward the surface. There it can often reach a runaway explosion that we call a Plinian eruption (named after Pliny the Elder, who died at Herculaneum trying to rescue friends during the eruption of Mt Vesuvius). This bubble-filled magma becomes a froth exploding violently upward into the atmosphere; it cools in the air to form the ash and tephra that (along with effusive lava) form the slopes of stratocone volcanoes like Mt Fuji, Mt Hood, and Mount St Helens.

Volcanologists work hard to measure and track volcanogenic H₂S (the burnt-match smell) and CO₂ gases to get a sense of where a restive volcano is in its possibly-pending, probably-not eruption. When Mount St Helens erupted in 2004-2006, it was relatively non-violent (though you would have died if you had been inside the crater at the time). An earlier almost-eruption in 1998 never quite reached the surface. Seismologists could see the volcanic conduit below MSH "light up" with the rock-breaking activity of a magma approaching the surface, but it never broke through. In the intervening 6 years, apparently these gases largely escaped, reducing the explosive danger from the volcano when it finally did erupt on October 1, 2004. One way to know if the CO₂ is volcanogenic, or from the modern atmosphere, is to measure its isotopic makeup. Atmospheric CO₂ has ¹⁴C ("Carbon-14"), ¹³C, and ¹²C isotopes. Volcanogenic CO₂ has only Carbon-12 (¹²C), the stable isotope in it. The other two radio-isotopes have long since decayed during the millions of years passed while the carbon was deep inside the Earth. 

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When Will the World End?

I have received episodic queries asking if the world is about to end? Sometimes these correlate with apocalyptic movies being released. Sometimes they are triggered by an uneducated conspiracy theorist (an oxymoron) somewhere with nothing better to do than to look at seismic data freely available on the web. For instance, does the latest seismic activity in Yellowstone portend the end of the world? That one turned out to be an instrumentation issue not understood by the conspiracy theorist. Do the huge earthquakes off the coast of Chile and Japan mean that the End Times are approaching? We’ve all seen trailers for movies like “Volcano” (“The Coast is Toast”), and “2012”, and I have little patience with these attempts to make money.

But when will the world really end?  Or at least become unrecognizable to us, or even uninhabitable?

Current understanding of the evolution of the Sun suggests that it is about 5 billion years old and will likely continue burning for another 5 billion years. It may start fusing helium to carbon and turn blood red before then, but the time is so distant as to be irrelevant to us.

What about things heading south on somewhat shorter time scales? An article by Wolf and Toon (http://onlinelibrary.wiley.com/doi/10.1002/2013GL058376/abstract) suggests that there will first be a “moist greenhouse runaway” event, followed by the loss of all water from the surface of the Earth, followed by a runaway thermal greenhouse situation – like Venus is currently experiencing. The Sun increases its energy output by roughly 1% every 100-110 million years. In other words, it will continue growing slowly hotter on the planet Earth (see an earlier chapter on the Faint Young Sun Paradox here: http://askageologist.blogspot.com/2012/06/snowball-earth-faint-young-sun-paradox.html).

As solar output grows, the Earth’s surface temperature should steadily rise. When it does, water vapor concentrations in the lower atmosphere will increase, and this will lead to an increase in water vapor in the Stratosphere. Solar radiation there will break down water molecules, and the Solar Wind will then blow them away into space, leading eventually to a waterless surface.  This may be what happened to Mars billions of years ago, made to happen faster and earlier due to its weaker gravity. 

Some earlier research had suggested, based on computer simulations, that a “moist greenhouse runaway” process would start about 170 million years from now, and that a full thermal runaway (the “Venus Effect”) would start around 650 million years from now. However, Wolf and Toon factor in ocean-atmosphere moderating effects from those same surface waters, and calculate something more like 1.5 billion years before the onset of the “moist greenhouse runaway” event. 

Somehow I find this difficult to worry about.

What about bad things happening on shorter time scales? For instance, what is climate change really leading to? There is no shortage of either Climate Doomsday or Climate Rubbish prophets. A recent article in EOS (Transactions, American Geophysical Union, Vol. 95, No. 18, 6 May 2014, Wuebbles et al, link here: http://onlinelibrary.wiley.com/doi/10.1002/2014EO180001/abstract) provides several illuminating graphs included here for interested readers. Figure 1 shows the severity of weather in the United States on a decade-by-decade basis starting in the 1950’s. It’s hard to argue with a graph like this: climate change is clearly well underway (see the earlier chapter on Climate Change – is it real? Here: http://askageologist.blogspot.com/2013/07/climate-change-is-it-real.html).

Figure 1. Extreme weather events in the United States by decade since the 1950's (Wuebbels, et al., 2014).

Figure 2 actually lays out the consequences for climate change: what things will look like for different parts of the country for the 2070-2099 timeframe. A short summary: it all gets hotter (no surprise), and the precipitation generally increases (surprise), except for the southwest, where precipitation will decrease (no surprise). More and greater hurricanes are projected (no surprise), but the numbers of severe tornadoes and severe East Coast winter storms have not increased in six decades and may not with the increasing CO2 and methane in our atmosphere (surprise). The minimum temperature in Alaska will be between 12 and 15 degrees (Celsius) warmer – not bad for people like me who don’t like white stuff on the ground. Perhaps more surprisingly, the northern tier of the Continental United States will get warmest – by about 6-11 degrees Celsius by the end of the century. Mean precipitation will stay pretty much the same in the Southwest – but it will be 6-8 degrees Celsius hotter, leading to drier conditions even with that precipitation. This will make those Phoenix afternoons somewhat less survivable as the century develops. 

Figure 2. What we can expect, region by region, from climate change if CO2 and methane continue to be produced by fossil fuel consumption at current rates (Wuebbels et al., 2014). 

What about economic impacts? The American Breadbasket of the central and northern plains will be seriously threatened by increasing drought conditions. Perhaps we should stop wasting 10% of our corn crop for ethanol. 

What can anyone do on their own? You should consider investing in land in the Canadian Prairie Provinces – but NOT anywhere near a modern coastline. Estimates of seawater rise vary – but they are all on the positive side, and low-lying areas like the Jersey coast, Florida, and New Orleans will be the Big Losers. An attempt to rationalize flood insurance following Hurricanes Katrina and Sandy lasted just two years – then appeals to congresspersons for relief from dramatically increased flood insurance rates “won” again. The end result is that people are rebuilding in low-lying areas, and the American taxpayer will be expected to bail them out at enormous expense yet again.  Hurricane by hurricane. Science deniers apparently don’t believe in gravity, either.

Ultimately, if the world was going to end in 1,000 years, how would that be different from 1,000,000 years or 1,000,000,000 years? How would you change your life?

If you’re rational, you would not worry about the End of the World too much - unless you live on the Jersey Shore, or Florida, or New Orleans. If you are both rational and responsible, you would consider replacing your gas-guzzling SUV for something that gets better mileage. If you are still bothered, go help at a Sharing House for people who cannot get enough to eat, and you’ll feel quite a bit better afterwards.  

You will have increasing opportunities for this with time.

Aquamarine

Some people may be sitting on a gold mine – literally. I’m acquainted with some once-hard-scrabble ranchers in Arizona whose lands sat atop what would eventually become a gold or copper mine. They live in large houses and drive late-model pickups now. Other people may have stumbled on a rare fossil (a woman in Montana accidentally stumbled onto what turned out to be the most complete T Rex fossil ever found), or a rock that turns out to be a gem in more ways than one. 

Q: I have an aqua marine stone, approx 15 pounds . I would like to know it's value.

- Terry M

A: If you mean "aquamarine", then there are several possibilities:

a. a pale blue or greenish gem variety of beryl,

b. an aquamarine sapphire,

c. an aquamarine topaz, or

d. an aquamarine tourmaline.

15 pounds of any of these would be worth quite a bit, depending on the grade and quality. However, in the US Geological Survey we do highly applied research in geology and geophysics (some field offices work on ecosystems and biology). We have very specific line-item assignments in this agency, assignments set by Congress, and they do not include dealing with gem stones. As a result, we have never hired a gemologist per se as far as I know. 

I wish I could provide more help, because this is fascinating to me. In Bangkok, Thailand, there is a Wat (temple) that houses something called the "Emerald Buddha" that is apparently a carved statue of rough-grade emerald. In several senses of the word, this is a priceless artifact. Your stone would not be on par with this (it's not carved or sculpted I assume), but it is still worth something - if only as a source of material that gemologists can cut/extract high-quality raw gems from. 

You drank WHAT?!??

Most people have no clue where their drinking water comes from. I once contracted Giardia from a drinking fountain in Ocean City, Maryland, and after a pretty terrible week of vomiting and diarrhea, have been much more sensitive to what I am drinking. I’m also much more aware of where my water comes from. 

Q: Why is it important to clean and recycle water & where does drinking water come from i finally can hope that these only two questions can hopefully been answered and be removed of my mind. Kind Regards.
- Natalin I

A: There are many reasons why we need to clean and recycle water. Fundamentally, they all come down to the fact that there is relatively little naturally pure water left in the world. About 3.4 million people die each year from water related diseases.

Most drinking water comes from springs, streams, and rivers (surface water) or from wells (groundwater). In some places (such as NE Thailand) it is trapped from rainwater. However, all of these have potential problems. For example, if you collect rainwater from your roof, how do you keep birds off that roof?

In Saudi Arabia and a number of other arid and/or coastal countries, most drinking water is provided by immense desalination plants. As you can imagine this makes that water rather expensive. A side effect with this kind of water is that it is usually disposed of as waste into septic tanks... waste which seeps quickly into the local shallow groundwater. The groundwater in and around Jeddah, Saudi Arabia, for instance, is highly polluted with industrial chemicals and biologic contamination, and the groundwater levels are rising because of the dramatically growing human population. This polluted groundwater is now sapping building walls, and at least one hospital must pump water 24/7 out of the surrounding ground – and then dispose of it elsewhere so the hospital walls do not collapse. 

Consider surface water: if someone pollutes a stream near its source (e.g., cattle or other animals defecating), then everything downstream is contaminated. Giardia (sometimes called "Beaver Fever") and Clostridium (which shut down the Minneapolis city water supply for several weeks) are particularly nasty examples, and both are resistant to chlorination. In the 19th and early 20th Centuries, it was common for campers and hikers to drink stream water in the Rocky Mountains and Cascades Range with impunity. Not anymore: when I camp or hike I bring my own (safe) water, or a powerful micropore filter. Industrial feed lots or pig-raising farms are particularly dangerous offenders - major threats to safe drinking water. 

Now consider groundwater. I live in the (very wet) Pacific Northwest of the United States, and my groundwater comes from a well field deep under a large, 12 million-year-old basalt flow north of my city. The water originates as rain, and has been subsequently filtered by soil and basalt rock before it reached the aquifer where it is now pumped from. However, there are places in the US and elsewhere in the world where hydrocarbons (both NAPL and DNAPL forms), dioxins, and other terrible chemicals have seeped into the groundwater due to human carelessness: an abandoned gas station with rusting tanks, or a military base dating from the past century when waste was not thoughtfully disposed of. 

Recently, large parts of West Virginia have not had safe drinking water for weeks due to an "accidental" dumping of chemicals by a coal mine service company into a reservoir. I put "accidental" in quotes here, because the offending company has a long history of deliberately violating the Safe Drinking Water Act, including recent helicopter photos taken by CNN of highly illegal pumping and disposal of toxic wastes into nearby streams. In several places in the US, hydrofracking ("fracking") wells were not cemented in properly, and residents can literally light with a match the methane that has seeped out of their kitchen faucets. There are Superfund sites where highly carcinogenic dioxins, acids, and other exotic industrial chemicals have been released into the earth. These chemicals tend to move as plumes through the aquifers towards any well that is pumping water out of the aquifer. While biological contaminants can often be filtered (or boiled) out of drinking water, chemical contaminants that are in solution usually cannot. 

The United States and the Developing World have some of the highest standards on water quality in the world - but the large majority of the human population does not have these protections. There is a cholera epidemic in Haiti that has been going on for years, caused by fecal pollution of drinking water sources following the 2010 earthquake there. Cholera is a major childhood killer.

Here are several helpful websites that will guide you in your study of drinking water and pollution:
http://water.epa.gov/lawsregs/guidance/sdwa/basicinformation.cfm
http://health.usgs.gov/dw_contaminants/
http://water.usgs.gov/edu/groundwater-contaminants.html
http://www.unwater.org/water-cooperation-2013/water-cooperation/facts-and-figures/en/
http://water.org/water-crisis/water-facts/water/

I hope this adequately answers your questions(s).

Landslides

Catastrophes have a way of catching our attention. A single nearby disaster can lead us to believe that this is the only important threat to us. A case in point: the 1980 Mount St Helens eruption in Washington State killed 57 people, and led to a dramatic increase in volcano research and infrastructure over the ensuing years. Wildfires and floods were on the back burner for awhile in the Pacific Northwest, and people bought a lot of masks that were never used. However, as the technology resulting from the research spreads worldwide, it is becoming increasingly unlikely that a volcanic crisis will ever again evolve into a volcanic disaster.

Sometimes we do not want to learn the lessons. Just two hurricanes in the United States (Katrina and Sandy) killed between them around 1,100 people in 2005 and 2012. Yet people are rebuilding homes on exposed New Jersey coasts and below sea level in New Orleans as if these events never occurred.

More recently, the OSO/SR 530 landslide killed at least 35 people, with 11 still unaccounted for. How are we as a society going to react to this? We are riveted when a woman is devoured by a shark off an Australian coast (New South Wales, March 2014). However, the United States in 2012 had 34,080 traffic fatalities. This contrasts with more than 51,000 deaths in 1980, so it’s clear that if society focuses on a threat long enough, many deaths can be prevented. But do we expend our resources in mandating seatbelts, airbags, and speeding-and-texting enforcement, or do we construct hundreds of kilometers of shark fence? What is a proportional response to a rare, unforeseen disaster?

Q: I live in Australia, but heard of the recent tragedy in Washington State where many people were killed in a landslide. I have some family who live on a steep hillside in the Pacific northwest and am wondering if they are in danger and if it is possible to predict when a landslide will occur. Thanks

- David I

A: No matter where one lives, there is always what I call “locality risk”. If you live in the woods, there are opportunistic and hungry bears and cougars – but far more commonly there are rocks to slip on. If you live in a city, there are people driving over-sized SUVs while texting. I had a very close call last year with a lady combing her hair with one hand while using the other to talk on a phone. On a curve. Locality risk is obvious to people who live in eastern Australia (truly apocalyptic firestorms), the southeastern US (continent-scale hurricanes), the central US (Force 5 tornadoes cutting swaths more than a kilometer wide across entire states), and California (earthquakes to magnitude 7.2 are not uncommon). Every once in a while our attention is caught by a “new” surprise, such as the 1980 eruption of Mount St Helens in Washington State. Volcanoes? We have volcanoes in this country? In December 2004 relatively few people on the planet had ever heard the word “tsunami” – until 250,000 people died around the margins of the Indian Ocean from a single event.

Bottom line: There. Is. No. Safe. Place. 
In flood-prone areas, or in hurricane-risk areas, in earthquake zones, etc., one can buy event-specific insurance to garner at least some protection. However, these policy riders are always expensive, and usually have large deductibles.

Your query probably has to do with the “Oso Landslide” (technically, the “SR 503 Debris Avalanche and Debris Flow”) in Washington State, on March 22, 2014. I listened to a senior scientist in our office who worked there describe what happened, and his speculation as to why, and learned a number of new things about landslides in general, and the Stillaguamish Valley in particular. I learned that typically the landslide height-to-runout ratio - the height where the cut in the hillside began vs the distance from that cut to the toe where the debris flow eventually stops  is commonly greater than 0.3. However, the Oso debris flow moved nearly three times as far as it should have, based on a database of previous landslide events worldwide. It may have reached speeds of 100 kilometers per hour. It removed and displaced a large section of the Stillaguamish River from its bed, creating a blockage that built a temporary lake. I learned that this particular area had experienced small to medium landslides in the past. I learned that the region had experienced unusually heavy rainfall for months preceding the event. Most importantly, I learned that the surrounding hills were not Cascades Range volcanic rocks like most everywhere else in the region, but were instead a large glacial outflow terrace. In other words, a big pile of (wet) dirt and rock.

What appears to be new in this case – and perhaps the reason for the unusually long and destructive runout – is that these glacial terrace sediments apparently were perched on a layer of clay-rich ancient lake bed material. Under the shock of the initial collapse, this may have (along with the overlying water-saturated glacial material) been liquefied by increasing the water pore-pressure in the sediments. Clearly everything was water-saturated, because even after the event, investigators began calling one scarp face “the weeping wall.” This scientist who led our discussion directs a research group that uses 4D mathematical modeling, laboratory-bench-scale physical modeling, and a 90-meter flume to experiment with debris flows. Their research concentrates on how debris flows behave differently with different composite materials and water saturations – and how they start. With all their years of experience, these scientists are only just starting to get a "feel" for when a debris flow in their flume will begin... but it's still impossible to predict. The leader of this group may be the most experienced landslide/debris-flow expert in the world, and he told us that he had never seen or heard of an event before like Oso. 

Are your friends and family at risk?

What can they look for? Is there a lot of open ground up-slope from their house that could be exposed to heavy rain? Do their foundations anchor in any sort of bedrock - or just thick soil? If there are old trees in the area, do they have bases that appear to bend into the hillside? This latter is a sure sign of ground creep. If the slope above their home is mostly other houses, paved streets and sidewalks, and the trees above and below them are straight, they probably have little to fear.  

As discussed in an earlier chapter, we cannot predict earthquakes. We can generally predict tornadoes by a few minutes to hours, and hurricanes with perhaps a few days warning. We can forecast these if we have enough data on previous events, especially in the case of large regions like the southeastern US, southern California, or the San Francisco Bay Area. By forecast, I mean to provide a percentage likelihood that an event of a certain magnitude will take place within a fixed span of time (usually 30 years). Forecasting is different from predicting, however. Predicting implies foreknowledge of the where and the when of an event. It implies that a warning can be given (like a siren for an impending flood) and people can be evacuated beforehand. Ideally, a disaster can thus be mitigated to be “only” an economic crisis. Forecasting, on the other hand, is largely suited to inform building codes, emergency preparedness, and to calculate actuarial data for insurance rate purposes. It may help you make a better-informed decision about accepting a job somewhere.

One of the few destructive events that scientists CAN consistently predict in the medium to long term are volcanic eruptions – if the volcano is adequately instrumented. However, even this is imperfect – we can often predict an approximate time of an eruption, especially as the magma approaches the surface, but we do rather poorly when it comes to predicting size and duration of a volcanic eruption.

Can we predict landslides? 

No – no more than we can predict earthquakes. Can we forecast landslides? Not really – they are localized events, and not regional events where we can gather meaningful statistics. Each landslide is like a human or a bear – it has its own unique characteristics, or “personality.”

If you are living in a flat area, however, it’s probably safe to say you need not fear a landslide. With sufficient geological mapping, we can get a sense of whether a landslide is possible in a given area: Are there steep slopes nearby? Are the steep slopes hard rock like granite, or are they hydrothermally altered or mixed rock types like we commonly find in volcanic terrains? A more dangerous end member is something like the unconsolidated glacial terrace deposits surrounding part of the Stillaguamish Valley. It is even more dangerous if there is geologic evidence of previous slides in the area. It gets more dangerous still if the area is prone to earthquakes or heavy rains, such as in Los Angeles. And it could get even worse: if there has been a huge fire or clear-cutting, followed the next year by heavy rains (such as Vernonia, Oregon, in 2007), then you lose even the limited protection of vegetation anchoring the soil of a slope.

In retrospect, the Oso area had several of these risk factors: heavy rains, unconsolidated sediments piled 180 meters high, evidence of previous landslides. However, there had not been any recent clear-cutting, nor had there been a fire in the area. There had not been any seismic activity, nor any human activity that could have triggered the mass movement. It just happened.

Perhaps we can say that landslides/debris flows are a risk one assumes when building in a place with a nice view. A son and a cousin who live near mountains in different parts of the Los Angeles area each separately experienced a large wildfire nearby, followed the next year by large mudslides. Neither regarded the mudslides as worth much thought – but they were not living in expensive hillside homes, either. It was the smoke and flames earlier that caught their attention and distressed them the most. For both, the fire was the more immediate and palpable threat, even though both fire and landslides were probably equally as dangerous to human life.

Oso is apparently just one of those rare, remarkable anomalies that could not have reasonably been predicted. It just happened - in one tiny fraction of the all the landslide-prone areas in Washington State. Initial mapping suggests that it’s an isolated situation - the glacial outwash terrace deposits to not extend very far up or downriver from Oso. The area is being monitored now with helicopter-dropped USGS “Spider” instrument packages and time-lapse cameras, but these don’t help the 46 dead or missing. It may help protect the survivors - however it’s hard to imagine people rebuilding in this area. 

Your friends and family are probably as safe as you are – or anyone else.  

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What the Earth Giveth, the Earth Taketh Away

Seafloor spreading centers and volcanoes create new land every day; seafloor subduction trenches gobble it back up. So who is winning – the land or the sea?

Q: Hi my question is: If you were to add up the length of all the convergent and divergent plate boundaries, would they approximately be equal?

- Julienne Y

A: The mid-oceanic ridge system - a divergent tectonic plate boundary - is the longest mountain chain in the world, extending through all global oceans (including the Sea of Cortez and the Red Sea, but not the Mediterranean Sea). All these divergent boundaries together are estimated to be about 80,000 kilometers in length.

There are estimated to be about 50,000 km of convergent plate margins, mostly around the Pacific Ocean (the so-called “Ring of Fire”). This total includes oceanic (subduction) trench systems, but also land features like the Himalayas and the Alps.

In principle, one would think the different boundaries would average out to be the same, but this doesn't incorporate either fractal behavior nor does it incorporate actual geography (and spherical geometry). From basic fractal theory we know that a 5 kilometer endpoint-to-endpoint segment of any boundary can be equal to or substantially longer than 5 kilometers depending on its rugosity (irregularity). Also, in a simplest topological model, you could have an outer rim of divergent seafloor spreading, and an inner rim of trenches and plate convergence. This may help explain why the latter (trenches) would necessarily be smaller than the former (seafloor spreading centers) in our modern Earth. By the way: this modern 50,000km/80,000 km ratio may have been very different - substantially reversed - when the Pangaea supercontinent was just starting to break up about 500 million years ago, because the divergent margins were inside the proto-continent, and most convergent boundaries would have had to be outside. 

Note that I’ve discussed only the lengths of convergent and divergent tectonic boundaries here. The calculation of volumes of materials “created” or “consumed” at these boundaries is far more difficult. This requires making a rather daunting number of assumptions, in lieu of actual data that are very hard to come by.

Crystals in Rocks

Q: My science teacher and I had a conversation about the crystal formation  on rocks and we got confused. Do crystals form on igneous rocks or  do they form on metamorphic rocks?  - Justin W

A: Crystals form in both kinds of rock. 

A way to think of crystal formation is to envision a crystal mush: As the intruded magma slowly cools (slowly because if it is underground it is well insulated), crystals will begin to form. The slower the cooling, the larger the crystals, in general. Some crystals will settle to the bottom of the crystal-magma mush if they are denser - and if there is no circulation happening in the crystal mush. These precipitated-out crystal accumulations can sometimes be seen in some ultramafic bodies exposed by later weathering - this bottom layer looks like a mat of "crystal toothpicks."  

More commonly, however, the crystal mush is  very active - convecting or circulating with repeated injections of magma from depth, and/or gas coming out of solution - until the percentage of crystals is too great for further circulation. As crystals continue to form, the percentage of the fluid decreases until the entire intrusive body is solidified. Often in late stages of this crystalization process, cracks will form in the intrusive body itself and in the surrounding host rock, and these cracks will fill with the last bits of fluid in the crystal mush, forming veins. Because there is a preferred order of crystal growth, the last-gasp fluid tends to be different from the average composition of the original magma body that entered the crust from the mantle in the first place.

With metamorphic rocks it's a bit different, because the material was solid to begin with, but under deep tectonic or sedimentary burial (or contact with a hot intruding body), the original material (which could be sediments, or could be older intrusive rocks) heats up and partially melts. Then something called recrystalization takes place. 

If you are ever in Tucson, Arizona, look north towards the Catalina mountains. From a distance you can clearly see the original sedimentary layering, but these rocks have been buried at least 15 kilometers deep and then uplifted by tectonic processes. When you get up close, you will see that coarse crystals have formed during this burial-heating process, so it looks more like a granite than a sedimentary rock. It's actually called an "augen gneiss", words derived from the German language where this sort of rock was first described. Even more fascinating is that as you walk farther north in the Catalina mountain complex, the augen gneiss gradually becomes a classic granite. This means that the more northern sedimentary rocks were buried even deeper. Old time miners would say that these rocks were "stewed and cooked."

It's a lot more complicated than this, of course, because there is heat and fluid released when crystals form. There is also contact metamorphism, where a hot intruding body will heat up the edges of the surrounding rocks and change them chemically both via heat and via fluid and chemical transfer across the boundary. 

I hope this answers your questions. 

==Jeff Wynn