Volcano Terminology   

What is a volcano anyway?

Volcanoes are fissures in the earth's crust through which the earth's magma reaches the surface. Volcanoes form a cone by way of  depositions of solid and molten rocks (lava) ejected or flowing from the earth's inside through a chimney and then exiting from its crater opening. You could also imagine the volcanoes being safety valves of the earth's hot interior permitting extra pressure of gases and materials to escape.

What makes a volcano work?

Plate Tectonics (or the movement of the earth's major plates) are the driving force behind volcanoes and earthquakes, which go hand in hand. Two major interrelated belts, which divide earth's surface into 12 major crustal plates are found and responsible for most quakes and volcanic eruptions. One, the Mid-Ocean Ridge, runs through the Atlantic and Indian oceans and is an area, where plates move away from each other. The other one, called Ring of Fire (see map) circles the Pacific Ocean and there the plates converge or subduct (see map). The Andes Mountains were folded upwards by this subducting of the plates and therefore is a major region of volcanic activities. Ecuador, being part of this ring, boasts therefore of many volcanoes in different stages of activities in the Andes (see map). Galapagos are recent volcanic islands, located above a so-called Hot Spot, which is an open area on the seafloor, from which huge lava blocks of several tons emerge from earth's interior and settle at the bottom. Over million of years those blocks build themselves up and sometimes rise above sea level as in the case of Galapagos. This action, which is also called mantle plume is associated in regions where tectonic plates spread away from each other.

What kind of volcanoes are there?

Geologists group volcanoes by their formation process and other criteria. Cinder or Scoria cones, the simplest form of volcanoes, are formed by accumulation of volcanic material ejected from a single crater. They are usually low but steep sided with a large crater opening and erupt usually only once in their lifetime. Composite or Stratovolcanoes, which you find in high mountain ranges like the Ecuadorian Andes are built alternately by volcanic lava flows, lahars, pyroclastic flows and so on and have often high, steep-sided and symmetrical cones with a relative small crater on top. They happen to erupt often violently and more than once in their life span but there are long periods of dormancy between the active phases. Shield volcanoes, which can be found on Galapagos, are built up entirely by fluid lava flows and therefore have broad bases and gentle slopes. They are the volcanoes which are erupting most frequently and for a long period but often non-violently (e.g.: Hawaii). Apart from these 3 main types of volcanoes, there exist Lava Domes, which are built up by masses of lava, which are too viscous to flow any large distance and therefore built themselves up into hills. You find those in existing larger craters and Calderas, which are huge craters, created by cataclysmic explosions or because they could not support anymore the weight above themselves as its chamber was emptied by repeated eruptions, and therefore collapse into themselves.

When is a volcano considered active?

Another classification of volcanoes is according to their activities. There are extinct volcanoes, which once were active but in the past ceased to be and are believed to stay that way. Dormant ones are presently inactive or asleep but have the potential to re-activate again any time in the future. Active volcanoes emit presently gases and show other signs of volcanic life, like earth tremors and uplift or had an eruption in the last 500 years. Eruptive ones have flow of lava and continuous emissions of volcanic materials. The borders between the various stages are often unclear or unknown as most of the events take place in the earth's interior. All major peaks in Ecuador are of volcanic origins and some are in the active stage (see map).

How does a volcano erupt?

The different types of the eruptions, from non-violent lava flows to violent explosions, depend on the composition of the magma, its silica content, viscosity and temperature and the presence of gases and water in the magma chamber. The magma of the stratovolcanoes of the Andes have a high silicate content and are acid and viscous and therefore lend themselves to explosive eruptions. The type of volcanic rocks found in the Andes is called Andesite, which is a dark and fine grained rock. Its composition is of feldspar and hornblende, which puts it between rhyolite and basalt. The basic magma on Galapagos on the contrast is low in SiO2 and therefore fluid and most often non-violent eruptions. Basalt which is  found on Galapagos, is the most common volcanic rock and consists of feldspar and magnetite and is dark gray to black. At an eruption you have pyroclastic material, like bombs, lapili and ash which are thrown into the air and lava, which flows down the flank of a volcano. Lahars is a term for an avalanche or slide of fire, mud, water and ice which occur in the event of big a eruption in the high stratovolcanoes and are often the most devastating. Fumaroles are emissions of gas and water vapor from steam vents in the crater.

Information was taken from the internet

Website by Dr. Vic Camp at the San Diego State University.


and the following book:

ACTIVIDAD VOLCANICA   Patricia Mothes & others