By Jonny Lupsha, Wondrium Staff Writer
Three well-known volcano types appear around the world. They are shield volcanoes, cinder cones, and composite volcanoes—which are also called stratovolcanoes. This week on Wondrium Shorts, learn to “lava” all three.
Volcanoes are just one of the many examples of geology we see all around us. Volcanic eruptions are beautiful but, quite obviously, dangerous. Lava flow can reach up to 2,200 degrees Fahrenheit, setting fire to much that crosses its path. The tragedy of Pompeii in 79 CE involved volcanic activity that happened so quickly that it killed people instantly and froze them in ash—where they remain to this day.
Most people know how volcanoes form and erupt, but the characteristics that separate each type of volcano aren’t as well-known. The three most common types of volcanoes are shield volcanoes, cinder cones, and composite volcanoes or stratovolcanoes. In his video series Practical Geology, Dr. James F. P. Cotter, Professor of Geology at the University of Minnesota, Morris, explains how they differ.
The three most well-known types of volcanoes form depending on the nature of their magma, the geologic setting, and the mechanism of volcano formation. The kind of lava that flows tells us which kind of volcano it is.
“Shield volcanoes are made up of an accumulation of lava flows [and] they have a broad, dome shape,” Dr. Cotter said. “The name shield volcano arose because people thought they look like a warrior’s shield. Most shield volcanoes develop on the ocean floor and are made up of basaltic lava flows—Basalt is the principal rock type of the ocean crust.”
According to Dr. Cotter, shield volcanoes are relatively safe because their eruptions can be predicted and their lava flows don’t build up and clog up the volcano, allowing their gases to escape easily. Iceland, the Galapagos Islands, and the big island of Hawaii are shield volcanoes.
The second common type of volcano is the cinder cone. Cinder cones are primarily of pyroclastic debris, which is a very dense mix of pumice, ash, hot lava blocks, and volcanic gas.
“The pyroclastic particles that make up cinder cones can be very small—smaller than a centimeter in diameter—or they can be much larger,” Dr. Cotter said. “The one distinguishing characteristic of cinder cone pyroclastic debris is the presence of vesicles.”
Vesicles are the result of gas bubbles getting trapped in lava as the lava crystallizes. Generally speaking, cinder cones are shorter but steeper than shield volcanoes.
Composite volcanoes, more often known as stratovolcanoes, are considered the most beautiful, but also the most dangerous, of the three common types of volcanoes. In regards to their appearances, they resemble large, symmetrical mountains often covered in snow and glaciers.
“Stratovolcanoes form from alternating lava flows and layers of pyroclastic debris,” Dr. Cotter said. “Stratovolcanoes are usually characterized by magmas with high silica content; although, this isn’t always the case. High silica content usually results in a very viscous magma. When the volcano starts generating layers of viscous lava and pyroclastic debris, the result can be like a concrete: It gets hard very quickly.”
However, additional magma is often forced up from below after it solidifies, leading to high pressures, which can result in dangerous and explosive eruptions.