Friday, April 25, 2014

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

Saturday, April 19, 2014

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:

I hope this adequately answers your questions(s).

Wednesday, April 9, 2014


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.  

Saturday, April 5, 2014

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.