Landscape Change - How Fast?

A major question from the beginning of geology as a science has been how fast does change take place? From the Literalist read of the Bible, it would seem 6,000 years is far too short a time to permit the development of tens of thousands of meters of sediment, with remains of primitive life-forms at the bottom and advanced life-forms preserved at the top. The first rough estimates of the rate of sedimentation were made in England, by thoughtful natural scientists measuring how fast mud accumulated in a pond. These early geologists had already mapped thick stacks – thousands of meters of distinctive layers - of sediment in cliffs, road-cuts, and quarries. They had seen the same sequences long distances away, implying the same sedimentary process was happening over a very wide area. Finally, they had realized that for mud and sand to accumulate to thousands of meters of thickness, would take at minimum many millions of years. This was really the first baby step of geoscience.

Q: Hello, my name is Jurgen and I am currently enrolled in an AP Environmental Science class and have a question about river formation. I hope you can answer my question.

How long does it take for a gully or rill to be formed into a river if there is a constant stream or supply of water running through the land?  Thank you.

--Jurgen P

A: Time for a gully to become a river can vary wildly from less than a hundred to many millions of years. Generally, most terrains are in some sort of equilibrium and don't change much over time – unless disturbed by something, like a tectonic event. This is sometimes called "punctuated equilibrium." The change of a feature from one form to another (like a gully to a river) implies a permanent shift in the rainfall regime - some form of climate change – or tectonic uplift.

Change from a gully to a river could also have a lot to do with human intervention. I've walked down 10-meter-deep, steep-walled gullies that were really mini-canyons (Arroyos) in SE Arizona. These apparently didn't begin to form until man introduced cattle in the late 19th Century. Early journals from some of the first visitors describe “grass that was belly-high to a horse.” These cattle quickly wiped out the native prairie grasses by over-grazing the landscape. When Arizona earned its statehood in 1912, it had a human population of about 12,000 people, but an estimated cattle population of perhaps 10,000,000. Soils started disappearing rapidly with no roots to hold them, and small rivulets began to rip through the landscape and form small canyons in less than a century. Events like this, and the 1930's Dust Bowl, lead to the formation of the US Bureau of Land Management and the US Soil Conservation Service during the 20th Century.

Tectonic uplift can also weigh in powerfully, but tectonic shifts are generally relatively slow - slow at least in typical human time-frames. The Grand Canyon only began to form (cut down through pre-existing Precambrian to Mesozoic rocks) about 70 million years ago. The actual timing of the initial incision and the final down-cutting is still being argued today by geologists as more evidence accumulates, but it appears to have been quite rapid at the beginning.

Unconformity? Disconformity?

Here's a purely geologic question by someone who has already taken at least one course in geology. The question opens up and highlignts the three-dimensional aspect of geology - and why mathematics (especially geometry) is such a fundamental prerequisite for studying geology. Some people persist in saying that a geologist is just someone who didn't do well in physics or math. The hard reality is that physics, math, chemistry, and English composition are the building blocks - the basic tools - of a modern geologist. Some of the most sophisticated geology being carried out these days is done with computers. Drill-hole information is fundamentally three-dimensional, and the ability to construct three dimensional landscapes from surface mapping and drill-hole intercepts is just so very cool. To rotate this 3D landscape on one or several computer screens, showing how individual components evolved in time in a single giant cubic space... is absolutely essential to numerically assessing any resources the land under the geologic map may host.

Q: I have a question regarding identifying unconformity on geological map. I have attached a map as an example. How do we identify unconformity on such 2D geological maps if each colour represents a different rock? Please advice.

Thank you and hope to hear from you soon. Regards

- Hazel A

A: I have not downloaded your map and looked at it in detail, but just looked at it via the attached thumbnail. We are discouraged pretty strongly from downloading and opening any files from unknown individuals that might potentially be vectors for malware. For the purposes of this Q/A, a map is not really necessary, however. 

I'd like, instead, to address your question on a somewhat broader level: The inherent problem with a geological map is that it represents the surface of the land. It's a view looking downwards from space, which is not always the same as looking downwards in time. Sometimes, with tectonic and erosional events, older in time doesn't necessarily mean deeper in the Earth.

An unconformity is a gap in sedimentary deposition for one of several fairly specific reasons: non-deposition, subsequent erosion, etc. It is not easily represented in a geologic map, which only shows just one sub-horizontal surface - the part exposed to the sky. An unconformity means that there has been a time break in the geologic record. This is quite different from a juxtaposition of different geologic units due to, say, a thrust fault (though they could both be involved at the same time). 

In practicality, this means that the geologist who produces the map must somehow indicate or convey any unconformity (or disconformity, or nonconformity, or paraconformity, etc.: see http://en.wikipedia.org/wiki/Unconformity ) in her/his *Correlation of Map Units* columns on the side of the geologic map.

For most people not intimately familiar with a particular local or regional geology, it would be very difficult if not impossible to determine if some break between units is an unconformity or a fault juxtaposition just from looking at a geologic map alone. A change in rock-type could mean any of several much more common things: a change in sedimentary regime (like an ocean transgression), an intrusive event (like a big granite body punching up from the Mantle), a volcanic eruption, any of several different kinds of fault, etc., exposed at the earth's surface. 

It comes down to the fundamental difference between a map view (looking down at the ground from space),  and a cross-section view (looking at the ground side-ways, as if a giant trench had been cut in the landscape). However, even in an exposed cross-section, considerable sleuthing is required to determine if a break is an unconformity or not.

Volcano Questions from the 5th Grade

Some questions are just fun to get. Perhaps it’s the teacher in me that likes to see young eyes light up with intellectual excitement. I infer from the following that volcanoes first get talked about seriously in the 5th Grade. It's beyond the soda, vinegar, and food coloring lesson.

Q: Question for my 5th grade class!

My students have some questions,

-           Megan A

1. Why do volcanoes erupt?

A: Pressure builds up from rising low-density magma below the earth. The low density is caused by the heat from the Mantle and Core of the Earth convecting upwards, sorta like a pot of Cream of Wheat cooking, or a lava lamp. The path of least resistance is to break out through the Earth's Crust at its weakest point. Where are those weakest points? Well, where you now see volcanoes is a pretty good hint. Some geologists have speculated that when tectonic events leave faults, and two faults happen to cross, that may make the intersection a “target of opportunity” for rising magma. However, there are a number of other factors involved, including where is the magma rising, and from what source, is there some under-plating of the crust happening, are there some gross compositional differences in the crust, etc. 

2. What are volcanoes like?

Some volcanoes look like cones (Mount Hood in Oregon, Mount Fuji in Japan). Some look like giant bulges (Mauna Loa in Hawai'i). Some volcanoes don't look like much of anything. You just see black-gray lava that has broken out of fissures, then poured out and run across the land in all directions – but generally the "pouring" goes downhill. There are vast, nearly impassable volcanic fields in western Saudi Arabia. There are huge obsidian flows (volcanic glass, caused by lava emerging in water and cooling too rapidly to form mineral grains) at Medicine Lake volcano in California. These look like a giant painted the ground with swirling green-black glass.

3. What is lava like?

Lava is very hot initially when it first reaches the air - it glows yellow-red from incandescence in cracks and at the flow-fronts. You can walk on it, because it is denser than a human body, but it is pretty rough on your boots. It melts boot-soles while hot, and cuts them up when cold because lava (e.g., in Hawai'i) is really just black glass. As lava cools, it sounds like a bowl of Rice Crispies crackling. As the flow-front reaches trees and houses, it engulfs them and the very high heat sets them on fire. This often forms tree molds - molds of where trees once were before being engulfed by the lava, for instance in HAwai'i and at Newberry Volcano in central Oregon. On Mauna Loa, a fast moving flow-front in the 1950's burst out of a fissure high on the volcano's west flank. I talked with a man who watched the flow run down the volcano's flank and onto a forest. It clipped off the trees at the base, then stack them vertically like bunched toothpicks at the front of the flow as the whole thing raced downhill at 60 kilometers per hour into the Pacific Ocean. 

4. Have you seen a volcano erupt?

I was inside Mutnovskiy volcano in Kamchatka when it started venting. I watched Mount St Helens erupt several times in 2004-2005. I've walked over active (evolving, moving) lava fields from Kilauea volcano, tracking the growing flow-front using a GPS device.

5. Is your job dangerous?

Not any more dangerous than driving a car on a Friday night when there are drunks on the road. Most volcanologists know someone, a friend or a colleague, who has died while working on a volcano, so yes, volcanoes ARE dangerous, and must be treated with respect. Because volcanoes are so dangerous, we take extra precautions when working on one that is restive, and generally stay well away of they are erupting. It's sort of like wearing seatbelts when you drive in a car. If you don't you are being deliberately careless - and statistically you have a much higher chance of dying. 

How Can You Have NEGATIVE Earthquake Magnitudes?

Some questions require an explanation of a different kind of number that some students haven't yet seen before. These different ways of expressing numbers were developed to help explain very large things, very many things, very small things, or very complex things, among others.

Q: Hi, My name is Anthony. I was wondering how negative magnitudes can be recorded for earthquakes, and what is the smallest earthquake ever measured? Thanks
- Anthony N

A: Earthquake magnitudes are actually exponentials, so a negative exponential doesn't mean a "negative" value in the usual sense of the word. I'm hoping you've already had exponentials in school - or at least you can go ask a teacher what they are.

For instance,
10(exp)+2 = 10^+2 = 100.0
   The exponent here is +2 and it means one hundred. This is 10 to the second power.
10(exp)+1 = 10^+1 =   10.0
   The exponent here is +1 and it gives ten - ten to the first power.
10(exp) 0  =  10^0   =     1.0
   The exponent here is 0 and it means one - ten to the zeroth power.
10(exp)-1  =  10^-1  =     0.1
   The exponent here is a negative number, but it refers just to a SMALLER value than a non-negative exponent would. Here ten to the minus first power means one tenth.

The smallest earthquake ever recorded is a bit more difficult to answer. There are three parts to the answer:

1. It depends on the sensitivity of the instrument, and how close the hypocenter of the earthquake (the actual rupture point) is to the instrument. There are a lot of sensitive seismometers set up around the world as part of the global seismic network - they are designed to look for earthquakes in the magnitude 2 range or higher. There are also some really, really sensitive seismometers positioned on and around volcanoes. These are set up to look for earthquakes so tiny that earthquake people wouldn't really be interested in them - events so small that only one or two of the nearby instruments may even detect them, and no human would likely feel them.

2. I believe that the smallest recorded events are probably in the M= -2 range (negative two magnitude) for a very clean, noise-free station. That's also what two seismologists in my office tell me (independently!).

3. When you are looking at magnitudes this small, you are also dealing with a lot of noise: cars driving by on a nearby highway, people or animals walking nearby, wind vibrating trees and buildings, etc. In a sense, the smallest earthquake ever recorded is sort of meaningless, because it becomes harder and harder to even know if it's real - or just noise. Also, the smaller the seismic events, the more common they are. As an example, the US Geological Survey estimates that there were about 1,300,000 earthquakes worldwide in the 2.0 - 2.9 magnitude range. There are MANY more as you get to ever smaller magnitudes. See an earlier chapter on how many earthquakes are detected each year in each magnitude range (http://askageologist.blogspot.com/2012/11/earthquakes-how-often.html).

No one is really interested in most of the wiggles you see in these two examples:

http://www.avo.alaska.edu/webicorders/Veniaminof/  
This is an instrument set up on Veniaminof volcano in the Aleutians. At 8:30am PDT on 22 October, I can see a few distant teleseismic events (distant earthquakes) and a lot of tiny events that may or may not be small volcanic earthquakes, or in some cases just small rock-falls from the crater walls. I can also see some large swings of the recorder that are instrument noise - probably electrical noise, either human-caused or natural, like distant lightning.

Whereas, if you look at Augustine volcano's webicorder for that same day, you see only a huge amount of wind noise:

http://www.avo.alaska.edu/webicorders/Augustine/
This is an instrument set up on Augustine volcano in Cook Inlet in Alaska. At 8:30am PDT on 22 October I could only see masses of blue "ink" on the plot that indicate a lot of wind noise on this station. There is so much noise on this seismometer record at this point in time that any "real" earthquake would be impossible to see.

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How Many Volcanoes Are There In The World?

How many of X are there in the world? This is a common question that often provides a surprising answer. You might be surprised at how many drill rigs exist or once existed in the Gulf of Mexico, for instance (over 4,200!).

Q: how many volcanoes are there in the world?

- Josh S

A: The Global Volcanism Program of the Smithsonian Institution lists 1559 volcanoes with eruptions happening during the Holocene period (the last 10,000 years). This means they are listing active or potentially active volcanoes. There are MANY more volcanoes in the world than that, of course. Some of them are just older and long inactive, like parts of Craters of the Moon national monument in Idaho (however some features there are as recent as 2,000 years ago), or volcanoes that erupted in Venezuela over a billion years ago. Some volcanic features that are not on the Smithsonian volcano database are just too small to easily list. While serving as the chief scientist for volcano hazards in the US Geological Survey, I assigned one of my senior scientists to do a full assessment and summary of the Cascades volcanoes of Washington, Oregon, and California. I thought there might be 15 volcanoes there. He ended up with a list of over 3,500 - because he counted every focal point of volcanic activity including small scoria cones and maars. 

Is the Big One Coming?

The following query arrived just a day before the M=6.0 Napa, California earthquake of 24 August 2014. Unfortunately, I could not respond to the obviously nervous individual until after that event took place. The bottom line is that non-human risks change little over time – they are just there, but they can be dealt with. However, people tend to obsess over what scientists call “high-impact-low-probability” events – like a shark attack or an earthquake along the so-called Pacific Ring of Fire.

Q: Hello I live in California and currently I'm getting very scared with all the current activity around the ring of fire. My question is whether this is normal activity or warning signs for bigger earthquakes to come, or even the "big one"?

-          Dolores L

A: The activity you are noticing is normal - it has been going on for millions of years. Sometimes it seems more exciting in some locations than normal, but this is still normal.

When you write of a "big one", this is also normal: we are expecting a subduction mega-quake in the Pacific Northwest anytime in the next one day to 300 years. Some older schools are being earthquake-retrofitted in Oregon as I write this. The Earth's crust is an active place, and the truth of the matter is that no matter where you live there is some degree of local (potential) hazard everywhere, whether hurricanes, volcanoes, domestic violence, earthquakes, drought, floods, car-accidents, or tornadoes.

The rational way to deal with these is to evaluate each hazard carefully – and there are public agencies like the USGS that do this all the time, very conscientiously. Once you know what the risks are – and most of the "big ones" are high-impact-low-probability – then you can plan accordingly. I bought my home on a slope for the view, but I checked the foundations carefully before I paid for it. I also paid an extra 15% earthquake premium on my homeowners insurance.

To put things in perspective: people react strongly to learning that a swimmer was killed by a shark, but 10's of thousands of sharks die brutally at the hands of humans each year rather than the other way around. Compare the 1 - 3 human shark-bite deaths to over 35,000 highway deaths in the United States last year. While seat belts, speed limits, and no-texting are theoretically enforced, no one seems to get particularly excited about this huge killer.

Another perspective: My father, before he died of lung-cancer, lived in a high-rise apartment in San Francisco less than 10 miles from the San Andreas Fault. I once asked him if he worried about it much? His response opened my eyes. “Listen,” he said, “I could enjoy the view of the Bay from here, or I could hunker down in a basement somewhere and worry constantly. Long ago I chose the former.”

Bottom line: the world is NOT about to end. Study your own personal risks, and then take rational precautions to mitigate them as much as possible without going overboard. Just taking any steps will lessen your worries, because you will be actively doing something about them. This could include putting up a supply of drinking water and food to last you during a local or regional disaster. Or better yet, a years supply so you can also help your neighbors. 

RUBIES!

The US Geological Survey does not employ gemologists – while there have been several within our ranks historically, they have been amateur gemologists who have pursued their interest on their own time. Nevertheless, gems DO come from the ground, and could reasonably be construed to be an ultimate product of geology. The following question is typical of the kind we receive about gems.

Q:           Is there somewhere in California near Modesto that I can have a rock collection looked at? We are almost positive that we may have found some raw rubies! They have passed the scratch test and are very heavy and hexagonal shaped. 209-xxx-xxxx

- James

A:            We can't do gemology for you - the US Geological Survey is tightly constrained to work on only particular national objectives that Congress sets, including mineral resource assessments, volcano hazards, etc. 

My recommendation to you is that you contact a local gemological society and ask for guidance. I would NOT recommend going to any jewelry store, as they only focus and specialize on the end products. 

You might try:

http://www.americangemsociety.org/ ... but keep in mind that this is a trade association of retail jewelers, independent appraisers, suppliers, and selective industry members, and only incidentally will they have any component that might be of help to you. 

           You would probably do better with:

http://www.gemsociety.org/ 

...or even better with a local society of educated amateurs, like the San Diego Mineral and Gem Society:

http://www.gemsociety.org/

Finally, please keep in mind that there are beryls and other igneous minerals like garnet and eudialyte that can easily be mistaken for rubies by inexperienced people. A true ruby is a pink to blood-red (so-called “pigeon blood”) colored gemstone, a form of the mineral corundum (aluminum oxide). Ruby has a hardness of 9 on the Mohs scale, and is considered one of the four precious stones, along with sapphire, emerald, and diamonds. The red color in a ruby is caused mainly by the presence of the element chromium in the crystal lattice.

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