Lava Flows and Pyroclastic Flows

Explosive eruptions involved gas and high amounts of magma and tephra ejected into the sky. Effusive eruptions involve outpouring of basaltic lava without much of any explosive activity. The most extreme effusive volcanic eruption is the Hawaiian eruption, while the most explosive eruptions are the Plinian eruptions. Not all eruptions are emitted out of the top of the structure. Remember that Hawaiian eruptions happen along the rift zones and not necessarily from the top.

There is a scale called the VEI or volcanic explosivity index, which is a scale that runs from 0 to 8. It is used for both modern and prehistoric eruptions in ways like we now use the Richter scale. It is not linear but logarithmic. This wide variation means that most eruptions today are between 0 and 2 on the VEI scale.

LAVA FLOWS AND PYROCLASTIC FLOWS

Let's talk about the different type of lava flows and eruptions you'll see in more detail. You can see how they range from fairly mild to very dangerous eruptions.

Magmatic eruptions are those that have some intensity and gas behind them. Tiny ones are seen as lava fountains and big ones are seen as high as 30 kilometers in the sky. This is bigger than the eruption that destroyed Pompeii in 79 AD when Mount Vesuvius erupted.

Hawaiian eruptions are the effusive kind you see in Hawaii. These are relatively tame volcanoes that erupt with small volumes of basaltic lava. Most eruptions are not in the center of the volcano but erupt all over the volcano in rift zones. Because of the way these eruptions look, they are often called a curtain of fire when they erupt. Hawaiian eruptions last a long time. A volcanic area on Kilauea has been erupting for more than 35 years.

Strombolian eruptions are named after Stromboli, which has been erupting for centuries now. These are mostly driven by gas bubbles that burst through the magma. These bubbles will burst and pop loudly with magma ejected in the air. You will hear these occasional blasts when the bubbles pop. Despite the sound, they do not cause a great deal of damage. Expect a few volcanic bombs to be ejected in the air as well.

The term "vulcanian eruption" comes from the volcano called Vulcano, located in the Tyrrhenian Sea. There is magma of intermediate viscosity so that gases have a hard time escaping. Gas will build up and will eventually pop to the surface in a more violent eruption than you'd see in a strombolian eruption. These do not get ejected into the air as much, however, and will simply create eruption columns that are up to 6 miles in the air. These are andesite eruptions and not basaltic like Hawaiian eruptions. Expect shortlived events with plenty of blocks and volcanic bombs being ejected into the air.

Pelean eruptions are named after a volcano called Mount Pelee in Martinique that erupted in 1902. It was a major volcanic natural disaster with plenty of dust, gas, ash, and pieces of lava fragments that blow out of the volcano's middle, leaving behind a large central crater. These are rhyolite and andesite lava fragments driven out of the earth by expanding gas. Often, there is a great deal of pyroclastic activity. These are very dangerous and capable of killing thousands of people. More than 30,000 people died when Mount Pelee erupted at once.

Plinian eruptions are similar to the one that was ejected from Mount Vesuvius in 79 AD. There are dissolved gases that build up within the magma. These bubbles are smaller and in vesicles until they rise and come together in the upper chamber. Once they get big enough, they explode out of the narrow conduit. These reach very high levels – up to 45 kilometers in the air. The plume gets into the stratosphere and the ash can spread widely. Those with rhyolitic lavas are most explosive, with eruptions lasting up to several days. The difference between these and strombolian or vulcanian eruptions is the length of time of the eruption. These tend to be longer in duration than most gaseous type of eruption. Pumice will fall everywhere around this type of eruption, just as it happened in Pompeii. The pyroclastic flows and lahars are common and deadly as well.

Other Plinian eruptions have been the Toba eruption 70,000 year ago, the eruption of Pinatubo, and the eruption of Mount St. Helen's. These are very destructive in general and are the kind that can potentially kill millions of people.

Phreatomagmatic eruptions involve magma as it interacts with water. Magmatic eruptions are driven by thermal expansion, while Phreatomagmatic eruptions are driven

by thermal contraction because water will quickly contract any hot object that cools rapidly. Expect the magma pieces ejected from these latter eruptions to be fine-grained and more regular in their shape. Rapid cooling might widen nearby cracks made by stress waves as the initial magma cools.

The Surtseyan eruption comes from shallow water interacting with lava. These form large water-vapor clouds similar to strombolian eruptions. Water also gets superheated and forms large steam expansions lifting into the sky. They can happen on land if the magma comes in contact with underground aquifers. Most are basaltic and many are relatively continuous when they erupt.

Another feature of Surtseyan eruptions are radial clouds that hug the ground due to their density. You might see dunes of deposits in rings around these radial clouds. Accretionary lapilli are accumulations of spherical ash that build up around the volcano. You know you've seen one of these lapilli when you see tons of small spherical ash or lava balls near the volcano or imbedded in the rest of the lava. There are a few of these eruptions in the world, including one in Surtsey, Iceland, one in Mount Tarawera, New Zealand, and one along the Mediterranean Sea in a seamount called Ferdinandea.

Submarine eruptions occur under the water. About three-fourths of the total volume of magma ejected through these deep underwater eruptions come out from below the earth's surface. Most of them come out of mid-oceanic ridges. You can get island chains and seamounts from these over time, or if they are very large. Most of these are basaltic but if they are near subduction zones, you will get thicker, more viscous, felsic-like rock ejected and more violent eruptions. These are extremely common but were undetected until the 1990s when equipment was made that could listen for them underwater. Pillow lava is the kind of lava you get from these, but you might also get sheets of lava flowing out of them. There are at least 100,000 of these deep-water volcanoes on earth, but you will probably not see any of them and most are beyond being active at all.

Subglacial eruptions are interactions between ice and lava beneath a glacier. They usually occur at higher latitudes on earth and at higher altitudes than most other glaciers. Some make instantly melted water that can lead to flooding and lahars in lower ground areas. A main feature is a flat-topped volcano with steep sides, called a tuya.

You might see these in Iceland or in areas of Northern Canada. As glaciers become more unstable in a warmer earth, landslides are possible as part of volcanic activity beneath them. Figure 33 shows what a tuya looks like:

Figure 33.

Phreatic eruptions come from steam. You can also call them steam-blast eruptions. If the ground is cold or water is near hot magma, it become superheated, fracturing rock as it explodes. The stuff that erupts is not new magma but preexisting broken rock pieces. You might not see a real eruption but only a lot of rock cracks hidden within rock faces that somehow withstood the high pressure circumstances.

Phreatic eruptions can precede giant Plinian eruptions; they can also cause toxic gas leaks, mudslides, and avalanches that can be very dangerous by themselves. You would have been able to see these before Mount St. Helens erupted in 1980.