Scientists classify volcanoes into three basic categories on the basis of size and shape. Let’s take a closer look at these types of categories. Shield Volcanoes While many people think of volcanic eruptions as being explosive, many volcanic areas produce quiet, oozing lava. Fissures and hot spots are two examples. Fissures are long fractures in the earth’s crust. Instead of erupting from one central vent, lava erupts gently like a fountain from the fissure in a long line. Fissures normally form in areas where two plates separate, such as along a mid-ocean ridge. Like fissures, hot spots produce quiet eruptions. Most hot spots form under a plate instead of along its boundaries. Other hot spots coincide with mid-ocean ridges. Both fissures and hot spots produce a runny lava that spreads out to form a wide, broadly sloping volcano. These volcanoes are called shield volcanoes because they resemble a warrior’s shield. The slopes of a shield volcano are rarely steep; these volcanoes are wide and flat. Over thousands of years, shield volcanoes can reach massive size, for example, 9 kilometers high and 193 kilometers wide. The Hawaiian Islands and Iceland are examples of shield volcanoes.

Mauna Loa in Hawaii is an example of a shield volcano. Composite Volcanoes Composite volcanoes are tall and pointed. They are some of the most picturesque volcanoes in the world because of their height and snow-capped summits. They form from alternating eruptions of lava and ash. Steep near the summit, a composite volcano is flat toward the base. The lava is sticky and does not flow far before it solidifies.

Mt. Fuji, in Japan, is an example of a composite volcano. These tall volcanoes usually form where two plates collide

and one overrides the other. Thick magma and water from the sinking oceanic plate cause the volcano to be explosive. Water dissolves within the magma and travels upward as small bubbles, like the bubbles in a carbonated soft drink. When the magma explodes from the volcano, it breaks the lava and rocks along the vent into pieces. Alternating layers of lava and fragmented rock pile up. Mt. St. Helens in Washington state and Mt. Fuji in Japan are composite volcanoes.

Sunset Crater in Arizona is an example of a cinder cone volcano. Cinder Cone Volcanoes Cinder cone volcanoes are smaller than shield volcanoes and composite volcanoes. If the eruption contains thick magma, the gas pressure shatters the rock within the volcano into small pieces. In other cases, the lava in the air may harden and fall as fragments. These small pieces are called cinders. These cinders accumulate around the opening, or vent, of the volcano. These volcanoes tend to be explosive, which is why the rock breaks into fragments. Cinder cones can also ooze lava at the base of the cone. Eldfell in Iceland and Sunset Crater are cinder cone volcanoes.

Plate tectonics processes help produce the different types of volcanoes. Can you tell which volcano type forms in the different areas?