Saturday, December 25, 2010

Volcanoes!

I thought I’d start with something that I know.

Volcanoes are awesome. They look cool, are incredibly deadly, and Dr. Evil built a hideout in one. So, what are they? How do they work? Exactly how dangerous are they?

Ok, I bet you all think volcanoes are nice, big pointy mountains rising out of the scenery. For some, that is very true. But, there are many other kinds of volcanoes. These tall, often snow capped peaks are definitely the prettiest, but they are not necessarily the most interesting. But hey, here’s Mt. Fuji in Japan, probably the best example of a snow capped volcano.

Mt Fuji, Japan. Ooo, pretty!

So, firstly, what is a volcano? Well mostly, they are an area on the Earth which contain molten rock and have the ability to bring that rock up to the surface. This molten rock, when it’s underground, is called magma, and when it reaches the surface is called lava. So, all volcanoes have lava of some kind. They come in all shapes and sizes, and range from being rather friendly to being able to wipe out an entire continent.

There many different kinds of volcanoes out there, some more dangerous than others, so I thought I’d take a walk through the volcanoes from least deadly to most.

We begin our journey close to home (for me). Small volcanoes, such as Mt. Elephant in the picture below, tend to erupt only a small amount of lava in a single eruption. They look pretty, and can have some really cool things in them as well. I am very familiar with these beasties, as there is a large province in Western Victoria full of the things. In fact, there is technically still-active volcanoes in that province, but they haven’t erupted for about 5000 years. These volcanoes typically have steep sides, and cover an area around bout a square Kilometre. This is because they don’t have much to throw out, and there isn’t much doing the throwing, so it all lands close by.

Mt. Elephant, Victoria, Australia. I can't see the resemblance either.

For our next encounter, lets meet the biggest volcanoes around. Us geologists call them shield volcanoes, because they kind of look like a shield that’s been put on the ground. We are an inventive lot.
These volcanoes erupt a fair bit of lava over their lifetime, and it is normally pretty runny stuff. Because of this, the lava can flow away from the eruption point for a ways before it solidifies. This makes the slopes of these volcanoes very shallow. Probably the best known of these volcanoes is Hawaii. All the islands in that picturesque archipelago are old volcanoes, the older ones are so heavy that they have begun to sink.  But, we’ll get onto this amazing chain of volcanoes later.

Next, we come to the classic volcanoes. These volcanoes, known as strato-volcanoes, are normally arranged in big arcs near the ocean. This is basically because they need to be near a subduction zone on the Earth, but more on that later. These volcanoes are relatively active, and build up their steep sides over many different eruptions which deposits ash and rock on their sides. Here’s one, Mt. Ngauruhoe in New Zealand, during a small eruption in 1974.

Mt. Ngauruhoe, but you don't just walk there.

Interestingly, this volcano has the claim to fame of being Mt. Doom in the Lord of the Rings movies.
These volcanoes tend to be rather dangerous, because they tend to be near where people like to live, and can erupt quite dramatically. Mt. St. Helens erupted quite spectacularly in 1980, but luckily it was situated in a national park. That didn’t stop it killing 57 people, however.

Probably the most spectacular and deadly eruption to be captured on camera was the 1991 eruption of Mt Pinatubo in the Philippines. There were several villages on the slopes of the volcano, and major cities were relatively close as well. The major eruption took place on the 15th of June just after a typhoon struck the coast only 75km away. This mixed with the volcanic ash in the atmosphere and eventually the eruption claimed over 800 lives after throwing between 6 and 16 cubic kilometres of ash and rock into the air. Here’s a photo!

Boom!

Now, we’ve seen most of the volcano types. These strato-volcanoes sound pretty deadly, don’t they? But they are small players compared to our next contender. The extremely large explosive calderas don’t even look like volcanoes. They are so powerful they basically rip apart any hint of a mountain which has built up around them. They are also called supervolcanoes, and have the ability to erupt more than 1000 cubic kilometres of stuff into the air. Yellowstone, or instance, once erupted and covered the entirety of the continental USA in ash. You don’t want to be anywhere near these puppies when they go off. Everything in this next picture is part of one volcano.

Lake Taupo, New Zealand

Now we’ll now have a look at why these beasts like to blow their tops. Firstly, to get a volcano to erupt, you need to make a volcano. There are really two ways to make on, by subduction and by bringing really hot things up to the surface. Let’s look at these in turn.

One of the easiest ways to make a volcano is to merely bring a whole bunch of hot rocks up to the crust. Now, the hottest rocks in the Earth are right down next to the core, and periodically, they tend to rise up as a large mass through the mantle of the Earth. When it reaches the crust, it is generally hot enough to melt. This will then work its way through the crust and erupt through the surface. These volcanoes tend to be like the Hawaiian Islands, or like Iceland. They tend to make large, shield volcanoes, and will make lines of these over the Earth as the plates they erupt through move over the top of this “hot spot”. In fact, they are really useful for working out how these tectonic plates have moves in the past. These hotspots sit relatively stationary below the plate which moves over the top of it, leaving a trail of volcanoes behind it.

The other way to make a volcano is through a process called “subduction”. This is where old crust dives under new crust or continents and interacts with the mantle underneath. Now, old crust is usually covered in water and sediments when it isn’t part of a continent. This water is then introduced to the mantle, and makes the melting point of the rocks in the mantle lower. It’s kind of a bit like putting salt on roads to melt the ice. This molten rock is a fair bit lighter than the solid stuff, so it rises up through the mantle and crust. It eventually starts to interact with the crust, picking up all sorts of additives, and starts to flow more like honey than water. This lets gases in it build up and eventually cause it to erupt rather violently as the pressure become too much.

Well, that’s how to make a volcano, but how and why are they dangerous?

Lava, Hawaii

Firstly, let’s look at lava. This gloopy stuff is basically molten rock that has reached the Earth’s surface. Before it shorts out of a volcano, it’s called magma. This stuff is pretty dangerous up close. Some lava, like that in Hawaii, comes out at about 1350°C. Also, it eventually cools and become rock, so anything you get stuck in it is stuck for good. But, when it’s on the surface, lava is pretty easy to see and steer clear of. It only really gets dangerous when the top solidifies and makes a tube. The crust on these can be quite thin, and can fall through at any time, dropping whatever is on them into the lava. This guy really has some balls then.

Wow, that crust is thin.

What is really dangerous with all this stuff is something called a pyroclastic flow. These are basically a bunch of rocks and ash which can be quite hot rushing down a hill. They can reach speeds of over 400km/hr, and can contain house sized boulders. Really, there isn’t anything else which is more deadly. Pyroclastic flows are what killed all those people in Pompeii and Herculaneum, and covered them up for nearly 2000 years. Here’s what one looks like racing towards the camera.

Yes, they got away.

The ash from volcanoes can also be dangerous, but mainly from the weight of it, and breathing it in. This stuff is made from tiny shards of glass which has fractured in the eruption and can float away from the volcano essentially causing the same problems as asbestos. It can pile up quite drastically, and can collapse roofs, and when mixed with rain can cause quite drastic landslides. Here’s the result of the Pinatubo Ashfall in 1991.

Mt Pinatubo ashfall.

The last hazard I want to touch on is called a Lahar. These occur when volcanoes have either lakes or snow on their upper reaches. When the volcano starts to become warmer, this can cause the water or snow to start to flow down the sides of the volcano in a massive landslide. These can be quite hot in them, but mainly consist of water mixed with the loose rock and ash from the slopes of the volcano. There is usually no warning for these, simply because they can occur during an eruption, or they can occur when there is just too much water in a crater lake at the top. Lava, pyroclastic flows and ash need a volcano to erupt before they are a problem, but Lahars only really need some water to be near the top in some form.

So, that’s volcanoes. They are pretty safe when not erupting, but they are pretty interesting.

Comments? Corrections? Ideas? Email me! Go Science

3 comments:

  1. Great entry, mate!! Very informative... reminded me of my childhood when I read all those cool stories about how vulcanoes build and erupt, and all these cool pictures - man, you have a hell of a cool job .)

    MERRY CHRISTMAS!

    Take care - looking forward to your next article.
    Horst

    ReplyDelete