More About Volcanoes

by Deb on April 30, 2010

To round off the week I thought I’d better do something general on volcanoes, how they form and the different types.  I’ve already talked about tectonic plates and how they affect earthquakes, it shouldn’t be a surprise that moving giant blocks of the earth also affects volcanoes.

Volcanoes and plate boundaries

The little red dots are volcanoes.  There are a couple out in the middle we call ‘hot spots’, but most of them are right where plates are going under each other and lots of surface rock is melting, or where plates are being pushed apart by lots of new magma coming up from below.  Around the Pacific is called the ‘Ring of Fire’ because there are so many volcanoes.

Types of volcanoes

Most volcanoes have their roots in the crust and upper mantle.  For volcanoes at plates that are meeting, the crust going underneath melts when it reaches the hot mantle and provides a source of magma (melted rock).  For hot spots and at spreading plates, the heat coming up from the lower mantle and core melts the rock above them.

Inside a Volcano

The magma rises because it is less dense than the cooler rock around it and collects in a magma chamber.  There are many things that can set off an eruption – more magma, earthquakes, the gases dissolved in the magma build up pressure.  Eventually the pressure builds up enough and the magma comes up out of the chamber and out through the caldera, when it comes out it is called lava.  We think of eruptions as big and violent, but they aren’t always.  There are many volcanoes that erupt just by having the lava flowing out.  Eruptions are violent when there is a lot of gas or water dissolved in the magma, as it gets to the surface there is less pressure and it bubbles out, just like when you open a bottle of cool drink and the carbon dioxide bubbles out.

There are three main types of volcano, they depend on the type of lava and gases.

Shield volcano

A shield volcano is large and flat with a caldera in the middle, so it looks like an enormous shield.  They form from lava that goes a long way and spreads flat.  The type of lava depends on how how hot it is and the rocks that make it up.  Hotter lava is runnier, just like chocolate.  Olympus Mons on Mars is an enormous shield volcano, so are the Hawaiian islands.

A cinder cone is usually small and steep, it is made up of lots of rocks and ash.  There is a lot of gas in the lava, as it is blown into the air it cools and solidifies as small rocks or cinders.  Cinder cones are very common in volcanic areas.  Paracutin is a famous cinder cone volcano, it began growing in a farmer’s field in Mexico in 1943 and over 9 years covered around 100 square kilometres in rock and ash, scientists learnt a lot about volcanoes by studying it.

The classic volcano we are all used to seeing is called a composite volcano, it has some runny lava and some rocks and ashes, often in layers.  They are in between shield volcanos and cinder cones – larger and flatter than cinder cones, but smaller and steeper than shield volcanoes.  A very famous one is Mt Fuji, or Fuji-san in Japan.

Mt Fuji

There is a very rare type of volcano called a volcanic pipe.  I don’t think there are any active today, they come from much deeper in the mantle than other volcanoes where it is much hotter and pressure is higher.  This is where diamonds form, so ancient volcanic pipes are sought after because they are the only commercial sources of diamonds.

Volcanic pipe

They do not have large cones, only small rings of debris and a classic carrot shape of solidified magma.  Xenoliths are foreign rocks, in this case deep mantle rocks including diamonds brought up by the magma.  The names Orapa, Jagersfontein and Kimberly are diamond mines in South Africa that are at different levels of an ancient pipe that has been eroded.  As well as diamonds, they help us learn about the deep earth.

Effects of volcanoes

Volcanoes have obvious immediate and dangerous effects like lava and explosions!  But there are others as well.

The reason that the current eruption in Iceland has shut down air travel is because of the dust it has released.  It is not a visibility problem, volcanic dust has lots of silica, a form of glass.  Even at very low concentrations large amounts will be sucked into jet engines – that’s what they do, suck lots of air in.  The heat in the engine re-melts the dust, which can then solidify in the engines as glass.  This can have a terrible effect on a plane!

Dust and gases in the stratosphere is also a huge problem weather wise.  As it spreads around the world it reflects sunlight back into space.  Many of the gases that come from volcanoes are acidic and kill people directly, or some like Sulfur dioxide spread and react with other atmospheric chemicals to produce acid rain.  Many of them form aerosols, ultra fine suspensions of dust that reflect sunlight and cause measureable cooling.  This may seem good at the moment to counteract the greenhouse effect, but it is unpredictable and short term.  As the particles settle out of the atmosphere the effect is lost.

Volcanoes do have a good effect – old lava erodes to produce extremely rich soils.  This is why so many people have lived on or near volcanoes in spite of the danger.  They are one of the earth’s recyclers, all the minerals they bring up from the earth act as fertilisers in the soil and allow rich ecosystems to thrive around them.

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{ 4 comments… read them below or add one }

Brenda April 30, 2010 at 9:42 pm

Hey Deb, stopping by to say Happy FYBF. I know it’s 10 pm already but it’s still early morning in the US. Heheh.

I should totally ask my children especially my 8 year old boy to read your blog. It’s chock-full of info. Well done!
.-= Brenda ´s last blog ..FlogYoBlog Friday #9 =-.


Kristin May 1, 2010 at 1:45 pm

Hey Deb, I didn’t realize you lived in Tennant Creek (checked the AMB website just now). On statcounter your IP address comes in as Adelaide! Funny. Lucky you. I’m hoping to make it back out to Australia again next year (fingers crossed) if I can swing it financially. On my last trip I didn’t make it up north (just to Alice) but next time would like to get back up to Darwin. Actually, would love to drive across the country again. That’s the best way to see it, hands down.

Last weekend I did paper mache projects with my kids and my daughter chose to make a volcano! Had to get creative to make it, using a styrofoam dome and cone, hollowing out a hole in the former to stick the latter then filling in the gaps with crumpled newspaper. Anyway, it looks like a giant hershey’s kiss to me. We’re going to paint it this weekend.

.-= Kristin´s last blog ..Finding our way home =-.


Annette Piper May 2, 2010 at 7:31 am

I’m going to have to sit my son down in front of your blog and let him read it himself, rather than continually disseminating the information!
.-= Annette Piper´s last blog ..Blink … and another week is gone =-.


PlanningQueen May 2, 2010 at 8:11 pm

some great diagrams here Deb. I find they really help to explain stuff to kids. Will show this to my boys too.
.-= PlanningQueen´s last blog ..Easy Cookie Cutter Recipe =-.


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