Wednesday, 27 April 2016

Death from Below: Supervolcanoes and What Makes Them Tick

A couple weeks ago we learned about how rocks from space can destroy cabins, cities, and even civilizations with little to no warning. Very few things in nature hold as much destructive potential as a wayward hunk of solar system leftovers on an unlucky path, but there is one other event that comes close and you don’t need to look far to find it. Approximately 30 km (18 miles) beneath you right now is a hot, churning mass of semi-liquid rock we call the Earth’s mantle and in a few select places around the planet, it has found a way to say hello in the most terrifying of ways.

Mantle plumes are columns of magma that rise up from deep within the Earth and form reservoirs of molten rock relatively close to the surface. The reservoirs contain the full range of materials that make up the inner-Earth, including solid rock and dissolved gases. The trouble with these reservoirs is that as more material flows into them, pressure builds. Sometimes, it builds to the point where the Earth’s crust cannot contain it and it explodes upward with startling force. This process is similar to what happens with the Earth’s many volcanoes, except it tends to be much, much bigger, and for that reason, we call these reservoirs supervolanoes.

The name is a little misleading because the processes behind (or more accurately, beneath) supervolcanoes occur on such a scale that they only vaguely resemble their smaller cousins. When these babies go off, there isn’t much you can do except head for your doomsday bunker. The generally accepted lower-bound size limit for a supervolcano is a reservoir with the potential to erupt 1000 km2 of material. By comparison, the 1991 eruption of the regular volcano Mount Pinatubo  released 5 km2 of material; just enough to circle the Earth a couple times and reduce average temperatures in the Northern Hemisphere by half a degree C for a year or two afterwards.

Supervolcanoes erupt fairly frequently in geologic time and when they do, the effect goes a little beyond needing a sweater for a few extra days a year. Supervolcanoes release enough ash to block out the sun and usher in the ice ages. The most recent eruption from one of these beasts was 26,000 years ago in New Zealand. Another event at Lake Toba in Sumatra occurred 74,000 years ago and nearly wiped out the human race – geneticists have pointed at the Toba eruption as an explanation for the lack of diversity in the human genome. Apparently, our species was reduced to a few thousand people in the wake of the blast and the subsequent volcanic winter. The biggest eruption we know of took place 28 million years ago in Colorado and left behind over 5,000 km2 of deposits, roughly the size of the island of Trinidad.

So where will the next world-shaking eruption happen? Basically, we have no idea. Despite being enormous and built into the planet we live on, supervolcanoes are hard to study. Actually, they are pretty hard to even find. The problem is that the destruction occurs on such an unimaginable scale that we tend to overlook it. The most telltale sign of a sleeping supervolcano is often a gigantic lake (flooded crater) or an absence of mountains where you would expect some to be. The latter is what allowed scientists to identify the caldera (aka magma reservoir) below Yellowstone National Park in the American west. Yellowstone’s last eruption blew up 50 km of mountains and left a caldera 50 by 70 km (30 by 50 miles) in size.

If you really want to figure out the odds of a supervolcano erupting, Yellowstone is the example to look at. On average, the hotspot beneath the park has produced an eruption once every 730,000 years. That puts the odds at around 0.00014% for any given year. The last eruption at Yellowstone was around 640,000 years ago, so you’ve probably got at least a few more years to go see Old Faithful and herds of bison. That could change though; scientists continually monitor Yellowstone for disturbances. The park experiences between 1,000 and 3,000 earthquakes per year as the caldera churns beneath it, so an increase in activity could mean an increased risk of eruption … or, it could mean pressure is being released and everything is safe.

Much like with death from the sky, supervolcanoes are unnerving in their ability to surprise.

Wednesday, 13 April 2016

Asteroid Rage: Big Impacts and Why They Are Scary

One of the more interesting things science can do for us is help us to imagine the end of the world. Whether it is born out of fear or just a sick fascination with our own demise, the number of disaster movies that hit theatres around the world every couple of years speaks to the fact that people like to imagine something big and bad going down. Among the most popular doomsday fantasies is the notion of a space rock smashing into the Earth.

This is only a fantasy however because of the brief span of human life and anecdotal memory. Stuff from space hits the Earth all the time. Over the 4.5 billion year history of our planet, we have probably been struck by tens of billions of meteors. That is to say, many find their way to Earth every year - even every day. Thankfully, most of these objects are small and don’t bare noticing beyond making a wish on a shooting star. Larger objects (the size of a house, for example) hit the Earth once every hundred or two-hundred years on average. Objects big enough to cause mass extinctions (like the dinosaur’s nightmare rock) show up every 50 to 100 million years or so.

But it isn’t just the size of a meteor that determines the damage it will cause. A number of factors go into calculating what happens when we welcome space rocks to the neighbourhood. Among the most important factors are the object’s speed, the angle at which it hits the atmosphere, what it is made of, and where it hits (water or land). Fortunately, there is an online tool called Impact Earth that is hosted by Purdue University and allows you to model any death-from-above scenario you want to dream up. With that in mind, let’s do some imagining:

Scenario 1: 30m object made of porous rock travelling at 30 km/s hits at 45 degrees over land 20 km from where you are standing.

In terms of large asteroid impact scenarios, this is one of the ones to hope for. According to Impact Earth, this object would begin to break up at an altitude of 81,600 meters. No crater is formed, although chunks do hit the Earth. Mostly what happens is the object explodes 21,700 meters above the ground with a force approximately equal the to bomb dropped on Hiroshima at the end of WWII. About a minute and a half after the explosion an air blast as loud as heavy traffic blows by, but you survive the event despite being relatively close by. The interesting thing about this scenario is that it played out in reality only a few years ago over Russia in February of 2013. A number of buildings close to the explosion had their windows broken and people were knocked off their feet, but no one was killed.

Scenario 2: 500m wide object made of dense rock travelling at 20 km/s hits at 70 degrees in the ocean 200 km from your beach house.

This is a bad day to be at the beach. This object hits the Earth with a force ten times for powerful than the largest atomic bomb ever exploded - the Zhar Bomb. The first effect you feel is a 7.1 magnitude earthquake that begins 40 seconds after impact – imagine a truck crashing into your house. The walls crack, dishes break, but that’s the least of your worries. Even before the shaking begins you would see a fireball that appears 4 times larger than the sun (in reality it is 3.6 km across). For a minute and a half after the blast the heat of the fireball is double the heat you feel from the sun. Three and a half minutes after touchdown you are hit by a dusting of super-heated particles that used to be the seafloor and ten minutes after impact a blast of air shatters any windows left standing. If you survive all that, you have about an hour to get as far away from the coast as you can before a wave between 9 and 17 meters (30 to 60 feet) high arrives to finish you off.

Scenario 3: 5 km wide object made of iron travelling at 35 km/s hits the Rocky Mountains at 90 degree angle to the ground while you watch from Vancouver.

You’ve pretty much had it with this one. You won’t have to worry about the air-blast that will knock down every building and tree  for hundreds of kilometers, 35 minutes after impact. You won’t have to worry about the fiery particles that used to be a mountain range reaching you 7 minutes after the blast. You don’t even have to worry about the magnitude 9.9 earthquake that begins 2.3 minutes after touchdown. What will finish you off in short order is the blast itself, which will go off with 76,600,000 MT of force (766,000 more powerful than that puny Zhar Bomb). The heat given off by the 100 km wide fireball will give you third degree burns over most of your body, ignite your clothing and even set any glass around you on fire. This impact would throw enough material into the atmosphere to block out the sun for about a year and leave a crater 136 km across and 1.3 km deep. Fortunately, even this is not a world-ender. The rock that marked the end for the dinosaurs was roughly twice this size.

We’ve only just scratched - okay, maybe severely dented - the surface of what meteor impacts can be like, but as you can see it is rarely a pretty picture. Worse still is that something Scenario 3 sized could surprise us, giving little to no warning before impact. Keep that in mind next time you’re trying to decide whether to splurge on your next vacation.