Why do stratovolcanoes form

Their lower slopes are gentle, but they rise steeply near the summit to produce an overall morphology that is concave in an upward direction. The summit area typically contains a surprisingly small summit crater. This classic stratovolcano shape is exemplified by many well-known stratovolcanoes, such as Mt. Fuji in Japan, Mt. Mayon in the Philippines, and Mt. Agua in Guatemala. Mayon Mt. In detail, however, stratovolcano shapes are more variable than these classic examples, primarily because of wide variations in eruptive style and composition.

Helens, Mt. Rainier, Pinatubo, Mt. Fuji, Merapi, Galeras, Cotopaxi, and super plenty others. Although they are not as explosive as large silicic caldera complexes, strato volcanoes have caused by far the most casualties of any type of volcano.

This is for many reasons. First is that there are so many more strato volcanoes than any of the other types. This means that there will also be lots of people who end up living on the flanks of these volcanoes. Additionally, strato volcanoes are steep piles of ash, lava, and domes that are often rained heavily on, shaken by earthquakes, or oversteepened by intruding blobs of magma or all of these. This makes the likelihood of landslides, avalanches, and mudflows all very high.

Occasionally as well, entire flanks of strato volcanoes collapse, in a process that has been termed "sector collapse". Of course the most famous example of this is Mt. Helens , the north flank of which failed during the first stages of the big eruption. Helens was certainly not the only volcano to have suffered an eruption such as this, however.

Two other recent examples are Bezymianny Kamchatka in , and Unzen Japan in The Unzen sector collapse dumped a flank of the volcano into a shallow inland sea, generating devastating tsunami that killed almost 15, people along the nearby coastlines.

This is a photo of lahar deposits near Santa Maria volcano Guatemala. This used to be a wide, deep river valley, and you can see the far wall of the valley where the trees are growing. The lahar deposits extend from that far wall to behind where this photo was taken. You can see that between lahar events the river cuts into the lahar deposits, but every time there is another event, it fills up again. The people give an idea of the size of stones that can be carried by a lahar. Another very common and deadly hazard at most strato volcanoes is called a Lahar.

Lahar is an Indonesian word for a mudflow, and most geologists use the term to mean a mudflow on an active volcano. Volcanoes take their name from the island of Vulcano in the Mediterranean Sea. Long ago people thought this island mountain was the chimney of the blacksmith forge of the Roman God Vulcan. The steam and ash that came out of the vent was a sign that Vulcan was working at his forge making weapons for Jupiter and Mars.

In Hawaii and other Polynesian islands, local people once attributed volcanic eruptions to the Goddess Pele. They believed Pele was moving from island to island as she sought to escape her evil sister, Na Maka O Kaha'i, the goddess of the sea.

Today scientists understand that volcanic eruptions are surface reminders of Earth's still hot interior. Return to Top. Where do volcanoes occur? Volcanic eruptions do not occur just anywhere. Sixty percent of all active volcanoes are found at crustal plate boundaries such as the Pacific Plate, which has become known as the Ring of Fire because of the active volcanoes on its perimeter.

Earth's crust, like the cracked shell of a hard-boiled egg, is broken into a number of " plates. Where the plates are moving apart or colliding with one another, volcanoes may form. Many volcanoes form oceanic islands in the Pacific Ocean or Mediterranean Sea. These volcanoes formed over " hot spots " in the crust and mantle.

The Northern Hemisphere has approximately two thirds of the land-based volcanoes. Are all volcanoes dangerous? Not all volcanoes erupt and not all eruptions are explosive. Volcanologists label volcanoes active , dormant , and extinct , depending on the likelihood of an eruption occurring.