Shield volcanoes are common at spreading centers or intraplate hot spots Figure below. Mauna Loa Volcano in Hawaii in the background is the largest shield volcano on Earth with a diameter of more than kilometers 70 miles. The volcano forms a significant part of the island of Hawaii. The lava that creates shield volcanoes is fluid and flows easily. The spreading lava creates the shield shape. Shield volcanoes are built by many layers over time and the layers are usually of very similar composition.
The low viscosity also means that shield eruptions are non-explosive. Cinder cones are the most common type of volcano. A cinder cone has a cone shape, but is much smaller than a composite volcano. Cinder cones rarely reach meters in height but they have steep sides. Cinder cones grow rapidly, usually from a single eruption cycle Figure below. Cinder cones are composed of small fragments of rock, such as pumice, piled on top of one another.
The exact composition of a cinder cone depends on the composition of the lava ejected from the volcano. Cinder cones usually have a crater at the summit. In , a Mexican farmer first witnessed a cinder cone erupting in his field.
Composite volcanoes are also called stratovolcanoes because of the alternating layers, or strata , of which they are made Figure 8. The magma that creates stratovolcanoes tends to be more viscous, or thick.
Viscous lava creates greater pressure which, in turn, tends to create explosive eruptions. In addition, the viscous lava cannot travel far down the sides of the volcano before it solidifies. This viscous lava thus creates steep sides on stratovolcanoes. When a stratovolcano erupts, it ejects a great deal of pyroclastic material into the air, which then settles back down on the Earth.
After an initial explosion, lava then flows from the volcano creating a second layer of material. As these layers solidify, they create alternating levels, or strata, of material. Ash from the volcanic eruption is also present between the lava layers along the edge of the volcano. Composite volcanoes are common along the Pacific Ring of Fire and other major tectonic plate boundaries where the presence of water in the magma chamber creates explosive eruptions. Shield volcanoes get their name from their shape—a huge shield laid on its side.
You can see that shield volcanoes do not have the steep mountainous sides of composite volcanoes. They have a very wide base and are much flatter on the top than composite volcano. Although they are not steep, they may be very large. The Mauna Loa Volcano has a diameter of over kilometers 70 miles and forms a significant part of the island of Hawaii.
The Mauna Kea Volcano, also in Hawaii, is another shield volcano that is over ten kilometers 6 miles high from its base below sea level to its peak. Shield volcanoes are more common at spreading centers or volcanic hot spots in the middle of tectonic plates Figure 8. The magma that creates shield volcanoes is less viscous, so it flows much more easily. For this reason, the eruptions of shield volcanoes are non-explosive.
They are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava that spread widely over great distances, and then cool as thin, gently dipping sheets.
Lavas also commonly erupt from vents along fractures rift zones that develop on the flanks of the cone. Some of the largest volcanoes in the world are shield volcanoes.
In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1, to 2, feet. The Hawaiian Islands are composed of linear chains of these volcanoes including Kilauea and Mauna Loa on the island of Hawaii-- two of the world's most active volcanoes.
The floor of the ocean is more than 15, feet deep at the bases of the islands. As Mauna Loa, the largest of the shield volcanoes and also the world's largest active volcano , projects 13, feet above sea level, its top is over 28, feet above the deep ocean floor. Mauna Loa Volcano, Hawaii, a giant among the active volcanoes of the world; snow-capped Mauna Kea Volcano in the distance. I n some eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaus.
Lava plateaus of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern Idaho. Along the Snake River in Idaho, and the Columbia River in Washington and Oregon, these lava flows are beautifully exposed and measure more than a mile in total thickness. Schematic representation of the internal structure of a typical volcanic dome.
V olcanic or lava domes are formed by relatively small, bulbous masses of lava too viscous to flow any great distance; consequently, on extrusion, the lava piles over and around its vent. A dome grows largely by expansion from within.
As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. Some domes form craggy knobs or spines over the volcanic vent, whereas others form short, steep-sided lava flows known as "coulees.
The nearly circular Novarupta Dome that formed during the eruption of Katmai Volcano, Alaska, measures feet across and feet high. The lava spreads in a thin layer before cooling. A third type of volcanic cone is a composite cone. Composite cone volcanoes are also called stratovolcanoes. They form when different types of eruptions deposit different materials around the sides of a volcano. Alternating eruptions of volcanic ash and lava cause layers to form.
Over time these layers build up. The result is a cone that has a gentler slope than a cinder cone but is steeper than a shield volcano. Helens is an example of a composite cone volcano. Also called a scoria cone. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.
Tyson Brown, National Geographic Society. National Geographic Society. For information on user permissions, please read our Terms of Service.
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