What is a Volcano?




Volcanoes are named after Vulcan, the Roman god of fire. The Romans believed he lived under an island called Vulcano in the Mediterranean Sea. This island is a volcano. They thought he was a blacksmith and made weapons for the other gods. When Vulcan made these weapons, the earth would shake and the island would erupt.

The earth is made of four layers. We live on the earth's crust made of rocks broken into small pieces. The continents, ocean floors, mountains, and valleys of the earth's surface are in the earth's crust. Below the crust there are pockets of melted rock. Under the melted rock is the mantle. The mantle is made of solid hot rock with temperatures from 900 to 3000 degrees Celsius. The outer core is the third layer. It is liquid metal melted from temperatures between 3000 and 4000 degrees Celsius. The inner core at the center of the earth is solid metal and the temperature is between 3000 and 6500 degrees Celsius.

Many scientists believe that millions of years ago all the earth formed one big continent called Pangaea. They think the hot liquid mantle, called magma, under the earth's crust bubbles like water boiling on a stove. This process is called convection. The magma tosses and turns and breaks apart the earth's crust. Scientists have found 12 to 15 large pieces called plates in the earth's crust. The earth's crust looks like an eggshell that has been broken by a spoon.

The plates are like floating rafts, moving on layers of soft rocks under them. The huge plates of the earth's crust move all the time, but do not move more than an inch or so every year. The continents and the ocean floors are on these plates. The earth's crust is weaker at the edges where two plates meet and this is where most volcanoes are found.

Volcanoes are formed in different ways. The simplest to understand is when the hot magma breaks through a weak spot in the crust. As the magma shoots out of the crust, the cooling magma called lava becomes hard. After a while, this hard lava forms a volcanic mountain. The second way is more complicated. The convection process in the magma causes the earth's plates shift and move. The plates collide into each other. Sometimes one plate is pushed down into the mantle below the crust and melts. This melted material with the magma can then create a volcano.

There are also different kinds of volcanoes. The differences are the way they are made, the type of lava and the kind of volcanic material. The most common are the shield, the dome, the cinder cone and the stratovolcano.

Shield volcanoes are the type found in Hawaii. They are in the shape of wide, sloping mounds. The lava from shield volcanoes is liquid and flows from the crater and the sides of the volcano.

The most dangerous volcanoes are the dome volcanoes. The volcano in Lassen Peak, California, is a dome volcano that erupted in 1915. The lava from dome volcanoes is a thick lava that forms cones shaped like steep domes.

The cinder cone volcano can be explosive also. Their cones are formed from cinders, which are a small, jagged pieces of rock, and ash. When a cinder cone volcano erupts, these small cinders are scattered all over. The Paricutin volcano in Mexico is a cinder cone volcano.

Most volcanoes are stratovolcanoes. They are more active than shield volcanoes, but not as explosive as the cinder cone or the dome volcanoes. They are built in layers of lava, cinder and ashes. Mount Fuji in Japan is a stratovolcano.

Volcanoes are different sizes and shapes. Many volcanoes are shaped like cones. Others are long cracks in the earth's crust. Some are very small and very steep. Volcanoes that are very high mountains usually have been formed by earlier eruptions. When the lava cools, it makes the mountain bigger and higher.

Volcanoes are made of three parts. There is a reservoir inside the earth filled with melted rock called magma. At the top of the reservoir is a chimney that the magma climbs through to the top. The cone is a large hole on top of the volcano. The magma and other volcanic material come out of the cone when the volcano erupts.

A crater is formed from the lava and other volcanic material during an eruption. When lava erupts from a volcano, it falls in a circle around the chimney and makes a crater. If you throw sand in a circle around you, you will stand in the middle of a circle built up at the sides. Not all volcanoes have a crater. If the lava is thick, it will fall in strips down the mountainside and not form a crater. A lake will form sometimes from rain and snow in the crater. Crater Lake near Klamath Falls, Oregon, is a lake formed in the crater of a volcano. If the weather gets too cold, the water will freeze into ice. An eruption will melt most of the ice and can cause very serious mudslides.

Sometimes a volcano will die when the magma sinks back into the earth's mantle. The rock that supports the volcano becomes weak and falls into the volcano. If the center of the volcano sinks, a large crater forms. This is called a caldera. Caldera will fill with rain water and become a lake just like a crater.

Types of Volcanoes

There are pretty much 3 types of volcanoes, but as with all kinds of classifications you can find exceptions. The three types that I am thinking of are: 1) shield volcanoes (such as we have here in Hawai'i); 2) stratovolcanoes (such as Mt. St. Helens and Pinatubo); and 3) large rhyolite complexes (such as Yellowstone and Taupo). You might also want to add the mid-ocean ridges, flood basalts, and monogenetic fields to make that 6 types in all. "Volcanoes of the World" by Tom Simkin and Lee Seibert, lists 26 different types, but that's probably kind of extreme.

1.Shield volcanoes--the largest of all volcanoes on Earth (not counting flood basalt flows). The Hawaiian volcanoes are the most famous examples. These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. For this reason these volcanoes are not steep (you can't pile up a fluid that easily runs downhill). These volcanoes are only explosive if water somehow gets into the vent, otherwise they are characterized by low-explosivity fountaining that forms cinder cones and spatter cones at the vent, however, 95% of the volcano is lava rather than pyroclastic material. Shield volcanoes are the common product of hotspot volcanism but they can also be found along subduction-related volcanic arcs and out by themselves as well.

2.Stratovolcanoes--making up the largest percentage (~60%) of the Earth's volcanoes, these are characterized by eruptions of cooler and more viscous lavas than basalt. The usual lavas that erupt from stratovolcanoes are andesite, dacite, and occasionally rhyolite. These more viscous lavas allow gas pressures to build up to high levels (they are effective "plugs" in the plumbing), therefore these volcanoes often suffer explosive eruptions. They are usually about 50/50 lava and pyroclastic material, and the layering of these products gives them their other common name of composite volcanoes. Stratovolcanoes are commonly found along subduction-related volcanic arcs.

3.Large rhyolite caldera complexes--the most explosive of Earth's volcanoes. These are volcanoes that often don't even look like volcanoes. They are usually so explosive when they erupt that they end up collapsing in on themselves rather than building any tall structure. The collapsed depressions are called calderas, and they indicate that the magma chambers associated with the eruptions are huge. Fortunately we haven't had to live through one of these since 83 AD when Taupo erupted. Yellowstone is the most famous U.S. example of one of these. Their origin is still not well-understood. Many folks think that Yellowstone is associated with a hotspot, however, a hotspot association with most other rhyolite calderas doesn't work.

4.Monogenetic fields. These also don't look like a "volcano", rather they are a collection of sometimes hundreds to thousands of separate vents and flows. These are the product of very low supply rates of magma. The supply rate is so slow and spread out that between the times of eruptions the plumbing doesn't stay hot so the next batch of magma doesn't have any preferred pathway to the surface and it makes its own path. A monogenetic field is kind of like taking a single volcano and spreading all its separate eruptions over a large area. There are a number of monogenetic fields in the American southwest, and there is a famous one in Mexico called the Michoacan-Guanajuato field.

5.Flood basalt provinces--another strange type of "volcano". Some parts of the world are covered by thousands of square kilometers of thick basalt lava flows--some flows are more than 50 meters thick, and individual flows extend for hundreds of kilometers. The old idea was that these flows went whooshing over the countryside at incredible velocities. The new idea is that these flows are emplaced more like pahoehoe flows--slow moving, with most of the great thickness being accomplished by injecting lava into the interior of an initially thin flow. The most famous U.S. example of a flood basalt province is the Columbia River Basalts, covering most of SE Washington State, and extending all the way to the Pacific and into Oregon. The Deccan Traps of northwest India are a much larger flood basalt province.

6.Mid-ocean ridge volcanism occurs at plate margins where oceanic plates are created. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins--some folks consider this the largest volcano on Earth. Here, the plates are pulled apart by convection in the upper mantle, and basalt lava intrudes to the surface to fill in the space. Or, the basalt intrudes to the surface and pushes the plates apart. Or, better yet, it is a combination of these two processes. Either way, this is how the oceanic plates are created. A recent mid-ocean ridge eruption took place along the Gorda Rise--the mid-ocean ridge that separates the Juan de Fuca plate from the northern part of the Pacific plate.


Return to Table of Contents