What are the Traits of a Shield Volcano

There are several different types of volcanoes but the one that forms some of the largest volcanoes in the world belongs to the group called “shield volcanoes.” A shield volcano can be as wide as three or four miles, and can stand as tall as 2,000 feet. They are truly the king when it comes to being the biggest. Surprisingly, there are still four active shield volcanoes in the United States.  Three of these active volcanoes reside in Hawaii: Mauna Loa, Kilauea, and Haleakala. The only other active shield volcano in the United States lies in Umnak Island, Alaska: Mount Okmok.

There are also six dormant shield volcanoes and one extinct; they are found in Hawaii, Oregon, Arizona, Washington and California. The honor of the largest active volcano in the world goes to Mauna Loa in Hawaii, with its last eruption in 1984; Mauna Loa has erupted 51 times since 1832. Mauna Loa is also considered the most massive mountain on the planet Earth; it rises 31,677 feet above the sea floor and its overall volume is seemingly immeasurable, as it would be at minimum 18,000 cubic miles. Mauna Loa’s neighbor Mauna Kea is considered the tallest mountain on Earth, as it tops out at 31, 796 feet above the sea floor.

The shield volcano gets its name from the shape it takes; it appears to take the shape of an ancient warrior’s shield, hence the name shield volcano. The reason for this type of formation is that shield volcanoes are relatively calm compared to a more explosive volcano. The lava from a shield volcano will in most cases go farther than that of other types of volcanoes as it is most often a fluid lava flow. There are three main types of lava flows that occur with a shield volcano.

The one least often seen is what is termed the “pillow” eruption; this eruption comes from under the sea or can be caused from lava flowing into the sea. As the sea cools the lava it forms a sort of shell that allows the lava to still flow but inside a tube of cooled lava. As it cools and forms, it makes what looks like pillows on the ocean floor. There are two types of eruptions that can happen above the ocean floor, the pahoehoe and the a’a.

Technically these two forms are made of the same thing, but one is much thicker than the other. A’a lava is thick and flows slowly. It is still basaltic lava, and the cooled lava is still a dark color, but a’a has a chunky appearance when cooled, almost like chunky peanut butter spread out. Pahoehoe, on the other hand, is much thinner, causing it to flow much faster than the a’a. It has the consistency of long ropes when cooled. It makes a great “sidewalk” for easy walking.