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.

How close is too close?

More questions about working in and around volcanoes

Q: Hi I was wanting to know how close do y'all get up to a volcanic eruption.

Thank you for your time.
- Jade B

A: I have personally walked over an active moving flow southeast of Kilauea volcano in Hawai'i. However, this is an effusive flow, not an explosive one. It's pretty rough on your boots, but if you don't stay there long you will be OK. The air above the flow is very hot, however, and the discomfort usually limits our time measuring the edges (or sampling) an active flow. 

We take the dangers of volcanoes very seriously. There are people here in the Cascades Volcano Observatory who personally knew people who are now dead - killed by volcanic explosions or pyroclastic surges. We monitor the telemetered seismic, GPS, and gas data from a restive volcano closely, both to minimize risk to our scientists and to protect the public. For example, during the 2004-2006 eruption at Mount St Helens, both aircraft and hiking exclusion zones were established. The size of an exclusion zone depends on the previous eruptive history of a volcano, something not hard to get in the US, but not readily available on all world volcanoes. Most high-risk and very-high-risk volcanoes in the United States and its possessions have volcano hazard reports written for them. These include maps showing where the danger zones will likely be in case of an eruption. 

Hope this answers your question. 

Prediction vs. Forecasting

More detailed questions about earthquake PREDICTION and earthquake FORECASTING

Q: hello, i have looked at your website for information about predicting earthquakes and i learnt that scientists cant tell when an earthquake will exactly happen but they can assume were one will hit (the probability of a major earthquake occurring in the San Francisco Bay over the next 30 years is 67%).

i still had a question asking: what do you use (tool, machine etc) to predict earthquakes ?

i would also like to know who is answering this question.  thanks.

- Louis C

A: You are correct. After more than a century of full-time research by some of the finest minds on the planet, it is apparently not possible to PREDICT an earthquake. It IS possible, however to FORECAST an earthquake, and the example you gave is an excellent one. 

By predict, one means to know beforehand the time, location, and magnitude of an event. To forecast is to calculate the future likelihood of an event in a region that has a history of earthquake activity. In simplest terms, a geologic and instrumental record is assembled of activity in, for example the San Francisco Bay area or the Los Angeles Basin. With sufficient data, one can assemble a statistical distribution of magnitude and frequency of events. From this a forecast can be made.

The details of how this is done, that you may have already seen, can be found here:
http://earthquake.usgs.gov/regional/nca/ucerf/
http://earthquake.usgs.gov/regional/nca/wg02/index.php

Hope this helps answer your question.

More information on the person replying to you can be found here:
http://profile.usgs.gov/jwynn 
The short version can be found here:
http://en.wikipedia.org/wiki/Jeff_Wynn

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

Rocks drying?

Some questions to Ask-a-Geologist are so off-the-wall that they rock me back on my heels and make me really think. Here's an example:

Q: is there a type of rock that dries faster than another?

- Shea P.

A: That's an interesting - and delightfully atypical - question. There are at least two issues involved:

1. 1. The rugosity (or ruggedness) of the rock's surface. The smoother it is (like obsidian), the less surface is exposed to water and the less available in the way of nooks and crannies to trap and hold water. A sandstone would likely keep some moisture on it's surface longer than obsidian would.
2. The surface tension/hydration of the rock's minerals. Certain minerals like clays adsorb (some also absorb) water on their surfaces in a pretty strong manner. Bentonite, for instance, will expand on contact with water and it takes forever to dry it out. When I lived in Denver I noticed that the ground sloped up to foundations of our tiny new house. I was emphatically warned by the realtor to not disturbed that grading. If water got under the eaves of my house, he told me, it could be funneled up against the side, and I could get heaving and major cracks in the basement walls as the swelling clay crushed into the concrete.

There's more to the issue than this, of course - there are other variables that include:

• Is the entire rock in contact with air, or is some of it buried? 
• What is the humidity of the air? 
• Is the air circulating?

As an example of how important these are, I was once working in the Saudi Arabian desert. After a day in 40+ C temperatures, I felt sticky with dried sweat, and was determined to bathe. We carried plenty of water with us... but we were also being engulfed in a three-day sandstorm at the time. It was dark, so I took a 5-gallon Jerry can of water, a metal chair, and a towel about 100 meters out into the desert. I tied my clothes to the metal chair to keep them from blowing away, and used a heavy metal sauce-pan to load and pour water over myself. I lathered up and then poured more water over my hair and body to get the soap off. The wind was blowing so hard that I felt stinging sand up to my chest... and realized that I needed no towel after all. The air was so dessicated and moving so strongly that I was dry almost immediately.

~~~~~

Sinkholes and Plate Tectonics

Q:

Dear Geologist,

Our names are Liam and Allison and we are sixth grade students Preston Middle School in Fort Collins, Colorado. In Science class our current unit is Constructive and Destructive Forces. This has made us curious. We generated multiple questions that could only be answered by a professional geologist like you. For example, what forms sinkholes? Also, how were tectonic plates discovered? And finally, what do you do for your job? We ask this because we are interested in becoming geologists ourselves when we mature. Thank you for your time.

- Cordially, Liam and Allison



A:

1. Sinkholes usually occur because of dissolution of carbonate rocks. A variant on this is when the carbonate cement in a sandstone is dissolved away. As an experiment, drop a tiny bit of acid on a limestone (a carbonate rock) - or scratch the rock with a knife and pour Coke over it - and it will fizz. Florida and other states have a lot of limestone underlying their surface soils, and if there is even a slight acidity to the groundwater (for instance it filters through a swamp of rotting vegetation first), then it will slowly dissolve the limestone. As a practical matter, the sinkholes generally (not always) form when there is a dry spell. Then the water saturating the damaged rock under a house will drop lower, and without the water saturation, the roof over a solution cavern will more easily collapse.

2. The idea of Tectonic plates was first proposed by Alfred Wegener, a German geophysicist and meteorologist, in 1912, He noticed that the west coast of Africa would make a pretty good fit to the east coast of South America. In the 1960's, aeromagnetic data acquired by aircraft showed distinct, symmetric banding in the mid-Atlantic (paleomagnetism). Isaacs, Oliver, and Sykes in a paper published in 1969 showed that this could only be caused by the growth of the Atlantic floor as it spread apart. Iceland is just an above-water part of this mid-Atlantic ridge spreading center, which may extend over 25,000 kilometers around the Earth. In the 1990's people started using GPS to directly measure the actual motions of the tectonic plates. Where I am sitting right now (Vancouver, WA), the North American continental plate is riding up over the Juan de Fuca oceanic plate at about 2.5 centimeters a year. In Kamchatka, in the Russian Far East, the plate movement is greater than 8 cm/year. Because it's faster there, the Russians have many more large earthquakes and many more active volcanoes than we have here in the United States.

3. To answer what do I DO, you can check out the profile here: https://profile.usgs.gov/jwynn


I look forward to you joining the ranks of geoscientists - we need smart young people like you to move the field ahead. Who knows? Perhaps YOU will discover a way to predict earthquakes.

~~~~~

Earthquakes only during the day?

According to my calculations, the 6th grade means students are around 11-12 years old. If so, then the Rising Generation is full of people a lot smarter than I was at that age. The example below is just one of many like it:

Q: Dear Geologist,

Our name is Arianah and Cray and we are sixth grade students at Preston Middle School in fort Collins, Colorado. We are currently learning about how the Earth’s surface changes over time. We are curious about earthquakes. We have a couple questions for you. Is there a common time when earthquakes happen during the day? Also, why did you become a geologist?

Yours sincerely, Arianah and Cray :D

A:

1. Earthquakes are essentially random. We understand why they happen, we understand where they happen, but we do NOT understand WHEN they will happen. There are always aftershocks following a main event, of course, but the main event cannot be predicted. Extensive research has shown that there is no correlation between earthquakes and certain times of the day or external* events - for instance there is no correlation with either the location of the Sun, or of the Moon, or with tides (alignments of celestial bodies, which cause neap tides or spring tides, is called syzygy). Some of the brightest minds on this planet have been searching for more than a half century for some evidence that main event earthquakes can be predicted, but without success. They can be forecast#, but not predicted.

2. I was a solid-state physicist and realized that if I didn’t do something drastic, I would be stuck inside a laboratory all my life with radioactive sources and high-pressure cells. This was brought very much to my attention one day when I had a high-pressure cell blow out and spew Cobalt-60 all over the inside of our lab, and had to call in a special Spill Team. Also, by this time physics as a profession was drifting into a dead end with string theory, and I saw relatively little value to humanity to spending billions of dollars to see if another exotic particle existed. I checked out breakoffs of physics, including astrophysics, hydro-geophysics, weather physics, and geophysics, and found the last one to be very exciting. It also got me out into exotic places, like the Venezuelan jungle, the southeastern Alaska panhandle, the Empty Quarter of Saudi Arabia, etc. Geoscience gives me amazing opportunities to visit these places and many more. But even more interesting to me is to be a detective – to be the first to discover something beneath the ground or the seafloor. I was the first to say where the groundwater was beneath the San Pedro Basin in Arizona and Sonora, Mexico, and the first to map where titanium sands lay beneath the seafloor off the coast of South Africa. That’s ever so cool. 

* It has been shown that if you inject fluids into certain formations (e.g., deep sediments northeast of Denver, CO), you can trigger swarms of micro-earthquakes. Basically this is the ground shuddering to equilibrate and adjust itself to a slightly new stress regime. However these sorts of events are so small that they are almost never felt.They really are not earthquakes as the general public understands earthquakes.

# A forecast: in other words, there is an X% chance that there will be a magnitude Y event on the Z fault zone in northern California within the next 30 years. This is very, very different from saying that there will be a Magnitude Y event at Z location on X day - that would be a prediction. We can't do that.

~~~~~