13 Questions

I suppose we’ve all heard the expression of saving the best for the last. Here is another example: a series of remarkably eclectic questions from a group of 12-yr-olds in Indiana. These kids are ‘way out ahead of where I was at that age.

Q: My 7th grade science students had several questions they would like answered. They have been working hard studying the forces of the earth and have generated these questions collectively.

- Brianne G (7th grade science, Otter Creek Middle School)

A: I'll try to respond to each question below them.

Q1. How do tectonic plates move if they are right next to each other? (Madison)

A: The plates move with difficulty, as you might expect. The forces driving them, however, are immensely powerful – the strongest forces in this part of the solar system. Take an Oreo cookie and try to slide the two dark sides apart - it's not easy. Once it "goes", it goes with a sharp jerk... just like the San Andreas fault. Now take one of the freed-up dark sides and break it in half. Try to force one half back against the other, perhaps with a little skewed sliding action, on a flat table. There is a lot of breakage at and near the contact between the two halves... just like the Himalayas or the Andes range in South America.

Yes, the plates are right next to each other, but they must move in response to the ginormous forces driving them. The result is faulting (and huge earthquakes) and mountain uplift. It's a rough life if you are the lead edge of a tectonic plate.

Q2. How do fossils stay around after all this time? (Marisa)

A: Fossils "stay around" only if they are silicified of form a mold with another material around them. That is, the original bone or scale or feather material is (a) imprinted on a long-lasting mold in the resistant material around it, or (b) slowly converted after death to a rock made up of mostly silica - the same stuff in your car window. This happens because fluids from rain move through the rocks all the time, and they carry small amounts of dissolved silica from the overlying rocks that these fluids percolate through. The original bone material is more easily dissolved, and thus slowly "replaced" by the silica. This is especially the case if the water is slightly acidic – for instance if it came from a swamp above the buried bones. The bony/silica fossil record "turns on" at the beginning of the Cambrian period, about 542 million years ago. Before that, all life forms were apparently soft tissue, but even some of *those* were imprinted onto silica-rich (or at least weathering-resistant) material that formed molds, for instance a carbonate mud that then solidified. These sort of imprints are why we now know that some dinosaurs had feathers. 

Q3. Is there any evidence that all the continents will form back together and make a super continent? (Kobe)

A: If you were able to stay around long enough, you would see the Pacific Ocean slowly squeezed out of existence by the continents over-riding it from nearly all sides. Tectonic plates do not always go in the same direction all the time (note the Hawaiian Island chain and the Emperor seamounts before them - they form volcanic chains that meet northwest of Oahu but are are different angles). If we assume that the plume swelling up beneath Kilauea, Mauna Loa, and Loihi volcanoes right now is "fixed" in position with respect to the Mantle, then it's clear that the Pacific plate must have changed movement directions sometime before Oahu Island appeared.

This is a long answer that I can put in shorter form: no geologist can say for sure. The surface of the earth is incredibly complex, and it has been evolving (changing) with time. Even if you made the assumption that all the plates will keep moving in their current directions (a bad assumption: it's clear that no one can say that if you just look at previous geologic history), it would still be messy to try to figure out where things will finally end up.

But it does look like a conglomeration of continental crustal plates is one future possibility... if you don't worry too much about the details of what a "super-continent" or its complicated margins actually are, and have 500 million years to wait around and see...

Q4. How far can a sinkhole go down? (Wyatt B.)

A: The sinkhole can go down as far as the bottom of the soluble carbonate rock that it forms in. That is to say, if the rock below your house is largely limestone, and the water table drops due to a drought and water is able to move more easily through it in a lateral direction and expose more voids, then theoretically all that limestone can be dissolved away. In fact all the limestone will not be dissolved, because not all the limestone will be exposed to water moving through it - that's why you have sinkholes and not sink-counties. There are clearly geometric considerations, in other words, plus the water dissolving the limestone has to be at least slightly acidic and stay acidic even as it mixes with the carbonates. However, there is always a bottom edge of any limestone or carbonate geologic unit. That bottom is as far as your house can possibly go down, at least vertically.

Note that I have not addressed here the issue of sinkholes caused by human mining of salt domes.

Q5. How do you define a resting volcano? (Evan N.)

A: Generally if a volcano has not erupted in 10,000 years (that is, within the Holocene period) then it is considered dormant. This isn't a perfect rule, as "Fourpeaked Volcano" in Alaska erupted in 2006, and apparently had been dormant for more than 10,000 years beforehand. To know if a volcano is dormant or restive, you must do careful geologic mapping to identify all previous flows – including buried flows – and age date them. The US Geological Survey has gotten very good at doing this, but its funding has been steadily declining, making it harder and harder to map and instrument all the potentially dangerous volcanoes in the United States and its possessions.

Q6. How do volcanoes create islands? (Chris M.)

A: Volcanic islands (and Guyots) are formed by magma welling up beneath the seafloor and breaking through. There are several reasons why this might happen, including seafloor spreading, like what we see in the middle of the (expanding) Atlantic Ocean (Iceland is an example), and mantle plumes, like we believe are building the Hawaiian Islands. Over time the lava and other volcanic materials will build up until it all breaks the sea surface, creating an island. There are many examples of this through geologic history, as well as modern, currently evolving features like the Hawaiian Islands. Another proof that this process is active are the huge pumice "rafts" seen floating in the Pacific Ocean by mariners periodically since the 17th century. Southeast Alaska, in fact, appears to be a series of volcanic island arcs that have been "accreted" (slammed together) since the Ordovician period as the North American continent encroached upon ocean seafloor where they originally formed. These island arcs were essentially "rafted" onto the approaching continental margin.

Q7. How hot are volcanoes on average? (Peyton L.)

A: It depends on what the material is and how close it is to the source. Rhyolite (silica-rich lava) can be solid at the same temperature where basalt (silica-poor lava) is flowing as a liquid. Some estimates of the bottom of the crust put temperatures at around 900 Celsius, but the bottom of the mantle is estimated to be at least 4,000 Celsius. Lava at Kilauea volcano in Hawaii is mantle-derived, and can be at least 1050 degrees Celsius – it glows bright yellow to your eyes even in broad daylight. With time at the surface, it cools to a dull red and then to black... but it is still hot enough to destroy boots for weeks afterwards. I lost a pair of boots walking out a lava flow lobe. An interesting anecdote: the cooling gray-black lava sounds like Rice Crispies after you pour milk into a bowl of it. This comes from thin flakes of volcanic glass popping loose as the cooling flow contracts.

Q8. How do we know when there's about to be an earthquake? (Devin A.)

A: We do not know. Moreover, some of the best minds on the planet working on this earthquake prediction problem say that we may never know. After over 150 years of intensive scientific study, no one has ever been able to figure out how to predict an earthquake. We can forecast an earthquake – that is, we can estimate a 63% probability that there will be a magnitude 6.7 earthquake in the San Francisco Bay Area within the next 30 years. However we cannot predict when it will happen, nor exactly where, nor can we say how big it will really be.

Q9. Where can we find faults, even when they aren't on a boundary of tectonic plates? (Nic)

A: There are several ways to find faults – and they are everywhere, so it should usually be easy. Geologists can map faults in the field (broken rock, or mismatched units adjacent to one another are evidence), and geophysicists can "see" them via small earthquakes if the faults are active (for instance, most faults in southern California are presumed to be active). If the faults are buried under dirt, swamp, water, or forest, it becomes very difficult to map them, and sometimes geologists must rely in digging deep trenches across ground segments where they think a fault may lie. This is expensive to do and dangerous to then crawl into and map, and thus is not commonly done unless fault timing is really critical (for instance, in southern California). The 1994 Northridge earthquake in Los Angeles was a big surprise to everyone. The seismic data indicates that it was caused by a break on a flat-lying fault lying many kilometers below the ground surface. This fault was not exposed anywhere, and was thus labeled a "blind" fault.

Q10. What do you have to do to become a geologist? (KC)

A: There are people who are just interested in rocks and fossils and land-forms all their lives, and they might rightly consider themselves to be geologists if they are studying these things as amateurs. However, to be a *professional* geologist – someone who can be hired by a company and paid to do geology work – you would need a college degree in geoscience or geologic engineering. A bachelor's degree might not get you more than an entry-level, low-paying job, so a Masters degree or PhD might make more sense. In either case, you cannot be a "real" geologist until you have studied physics and chemistry, learned math to at least the calculus level (you will need a *lot* of trigonometry for structural and economic geology), and you will need to get good grades in English. English!?! Yes – if you cannot write a clear and coherent report it makes no difference how much you have studied, because no one will know what you know, or what you have discovered. I personally know a brilliant PhD geophysicist who could not write his way out of a paper sack. He never got promoted, because he had to depend on others to write his reports and scientific research papers for him.

Q11. Why is your job as a geologist so important? (Hunter)

A: Are you warm and comfortable right now? Do you drive a car? Did you eat breakfast? Are you reading this with a hand-held device, or a computer, or with electric lights? Then thank a geologist who helped find the petroleum to power your car, and coal to power generators, and the copper and other critical elements for the wires and the computers and the harvesters. USGS geologists saved at least 800 lives in 1980 when they told Washington State that Mount St Helens was about to erupt catastrophically - and governor Dixie Lee Ray ordered that a "red zone" be set up around the volcano. USGS geophysicists saved the lives of hundreds of thousands of people in the Philippines in 1991 when they monitored and then recommended a massive evacuation around Mount Pinatubo. They also saved billions of dollars of aircraft that would have been destroyed by the ash and ejecta from the volcano at Clark Air Force base. We have a tsunami watch system all around the Pacific Rim now, saving untold lives. In 2004 we did not have a tsunami warning system in the Indian Ocean, and 250,000 people lost their lives after the Aceh, Indonesia, mega-quake.

Q12. How and why is the water in the ocean salty? (Andrew)

A: Try pouring a pitcher of water through fresh crushed rock. Then pour it through again and again. The mineral content in the water will continue to rise until pretty quickly you can taste it.

Another way of looking at this: put a tiny amount of salt in a pot of water and you may not even be able to taste it. Now boil the water down to just a few milliliters of water remaining. That remaining water will be very salty to your taste.

Over billions of years rain has poured down on the continents and leached out all the easily dissolved minerals, which then passed down through rivers to the sea. Salt is easily soluble, and even if found in just tiny amounts on the continents, it will just keep accumulating and accumulating in the seas over time. It’s more complicated than this, or course, because there are chemical interactions and changes involved, but you get the idea.

Q13. How does the crust of the Earth divide into plates? (Megan)

A: There are really two questions here, a how and a why. One way how you can divide up the crust into plates – to figure out where the plate boundaries are – is to use continuous GPS units to track which direction the ground beneath one station is moving with respect to the ground beneath another station. If the distance between two stations doesn't change, then to first order they are on the same plate. If they do change, you either have different tectonic plates (a boundary between the stations), or a volcano between the stations is about to erupt. In the case of two relatively close GPS stations on a volcano, if they are moving away from each other, then the volcano is inflating. How fast do plates move with respect to each other? The Kamchatka Peninsula of east Russia is moving about 8 cm/year eastward. The North American continent is moving roughly westward at about 2.5 cm/year. The Caribbean is moving about 2 cm/year with respect to South America. These are movements that are easily detectable with modern GPS technology.

As to why the crust is divided up, try closely watching cream of wheat cooking. Watch the surface of the goop in the sauce-pan... and notice that when there is heat applied below it, the surface moves in complex ways. The surface is responding to convection (one form of heat transfer), which moves heat (and with it material) from the bottom of the pan to the top surface, displacing material already at the surface in complex ways. This is continental drift writ small.

Q: We appreciate your time answering these questions, and we are looking forward to any replies!

A: It's my pleasure. Helping young minds grow may be the most important thing that you and I can accomplish in any given day. However, you get to do it all day, 5 days a week.

Reply (next day): Thank you! My students will be thrilled to hear these answers! :)

- Brianne

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