Guest Post by Steven Goddard
The Guardian image below taken this week near Iceland has the caption “Smoke and ash billows from a volcano in Eyjafjallajokull, Iceland Photograph: Ingolfur Juliusson/Reuters”
The Guardian caption is for the most part incorrect. Note that the volcanic cloud is largely indistinguishable from the other clouds, except for it’s shape. The reason for the similarity is that the vast majority of the volcanic plume is water vapour, not ash and definitely not smoke. Where would smoke come from??? There aren’t any trees on Iceland to burn.
The abundance of gases varies considerably from volcano to volcano. However, water vapor is consistently the most common volcanic gas, normally comprising more than 60% of total emissions. Carbon dioxide typically accounts for 10 to 40% of emissions.
70% of the earth’s surface is covered with water. Where did that water come from? It is generally believed that most of it outgased from the interior of the earth during the first 700 million years of the earth’s existence.
Steam from the interiorToday most authors believe that early steam from the hot mantle but already cool atmosphere, caused the oceans in the very early stages of the planet. They reason from studies of chondrites (space rocks) in space that under compression, enough water could be released to form an ocean. Today one can observe the gases escaping from active volcanoes, and these too contain water. In this scenario, the oceans would still be increasing in size, a gradual process that would never really end.
The amount of water stored in rocks of the primary lithosphere is estimated at 25E21kg (Hutchinson G E, 1957), whereas the water in all oceans is 1.35E21kg, so it is quite possible that all this water emerged slowly after rocks were compressed and heated while the atmosphere had cooled already.
We know that the oceans could not have condensed out of the early atmosphere, because even a 100% water vapour atmosphere would only contain 10 metres of liquid water. People have hypothesized that the oceans came from comets, but the hydrogen isotope ratios in the oceans are different than that seen in comets Halley, Hyakutake and Hale-Bopp.
The only plausible origin of the oceans is from the interior of the earth. So why don’t we see oceans on other planets and the moon? Liquid water only exists in a narrow range of temperatures and pressures. Other planets are too hot, too cold or too small to hold liquid water, though some of the moons of the giant planets may have liquid water.
Why is the relationship between volcanoes and water important? Because steam pressure is the primary driver of explosive volcanic eruptions.
Below are some images of potentially explosive eruptions :
Mt. St. Helens 1980 : Mostly steam, some ash, almost no smoke.
The video above shows the moment of the big eruption May 18, 1980
Mayon 1984 USGS photo : Steam rising, ash cloud falling down the sides of the mountain.
Fourpeaked Volcano, Alaska 2006 USGS photo : 100% steam
Tungurahua 2006 NASA EO image : Steam, ash and lava
Eyjafjallajökull 2010 NASA EO image : Steam, lava, ice
Below are USGS images of non-explosive eruptions at Mauna Loa, Hawaii
Note in the image above that there is some smoke on the left side – from burning trees, and a little steam at the summit. So what is the difference between explosive and non-explosive eruptions? The difference is mainly due to the presence or absence of water. Water mainly enters volcanoes from two primary sources.
- Subduction on the sea floor, and transport upwards into a magma chamber. (Mt. St. Helens)
- Melt from snow and ice above. (Eyjafjallajökull and Mt. St. Helens)
Mauna Loa on the other hand has very little water mixed in with the magma, as it is neither near a subduction zone nor is it covered with snow most of the time. So eruptions from Mauna Loa tend to produce lava rather than steam and ash.
Looking at the mechanics, it becomes clear that explosive volcanic eruptions can not occur in the absence of large amounts of steam. Liquids (like magma) have very low compressibility and can not store enough mechanical energy to cause an explosion. Gases on the other hand are extremely compressible and can store vast amounts of energy. Steam has the unique property that it is liquid until it comes in contact with the magma (or the overburden pressure becomes low enough to allow it to switch to vapour phase) - then it converts thermal energy into mechanical energy very efficiently. The world used to run off steam engines based on this principle.
Most modern power plants still use steam to convert thermal energy into mechanical energy. Same principle that makes volcanoes explode.