CSET Requirement 2.1c: Compare major types of Volcanoes in terms of shape, chemical, rock composition.
2.1d: Describe the location and characteristics of volcanoes that are due to hot spots and those
due to subduction.
CINDER CONES
Let's begin with the smallest form of volcanoes to describe composition and use a California site to help us with California geology as we go along. Cinder Cone (not much creativity in naming it) is a cinder cone volcano located in Northern California. It is a 750 foot high mound that spread ash over a 30 square mile range. The volcano was plugged by lava flows that ceased its activity and spread around it, this lava flow is known as the Fantastic Lava Beds. I think about another cinder cone in Central America: Cerro Negro (Black Hill) in Nicaragua.
As the name suggest, cinder cones are built from ejected lava fragments. These loose fragments have a high angle of repose (30-40 degrees) and as such volcanoes of this type have very steep slopes and small in comparison to other types. In addition, they frequently occur in groups, next to other volcanoes or inside calderas (Wizard Island in Crater Lake Oregon). Cinder Cone finds itself in a volcanic national park which includes four shield volcanoes.
Rock fragments that make up cinder cones, often called cinders or scoria, are glassy and contain bubbles as ejected magma exploded into the air and quickly cooled. These gas rich magma ejections are dark in color and contain cavities. Extrusive igneous rocks make up cinder cones which include basalt, andesite or a combination of both: basaltic andesite. Chemically, basaltic andesite rocks contain 53 to 57% silica (SiO2). Andesites contain between 57 to 63% silica. These two are chemically different but both are found at Cinder Cone and are common to cinder cones. Finally it is worth mentioning that due to the loss of gases when magma flows out from cinder cones, it does not do so from above the crater but oozes through the base or wall of cones and spreads out. This gave Cinder Cone its Fantastic Lava Beds which seeped through its base and snuffed the volcano.
As the name suggest, cinder cones are built from ejected lava fragments. These loose fragments have a high angle of repose (30-40 degrees) and as such volcanoes of this type have very steep slopes and small in comparison to other types. In addition, they frequently occur in groups, next to other volcanoes or inside calderas (Wizard Island in Crater Lake Oregon). Cinder Cone finds itself in a volcanic national park which includes four shield volcanoes.
Rock fragments that make up cinder cones, often called cinders or scoria, are glassy and contain bubbles as ejected magma exploded into the air and quickly cooled. These gas rich magma ejections are dark in color and contain cavities. Extrusive igneous rocks make up cinder cones which include basalt, andesite or a combination of both: basaltic andesite. Chemically, basaltic andesite rocks contain 53 to 57% silica (SiO2). Andesites contain between 57 to 63% silica. These two are chemically different but both are found at Cinder Cone and are common to cinder cones. Finally it is worth mentioning that due to the loss of gases when magma flows out from cinder cones, it does not do so from above the crater but oozes through the base or wall of cones and spreads out. This gave Cinder Cone its Fantastic Lava Beds which seeped through its base and snuffed the volcano.
COMPOSITE/STRATO VOLCANOES
Mount Shasta in Northern California is a stratovolcano. It is 14,179 feet high making it the fifth highest peak in California 329 feet smaller than Mount Whitney which is the highest peak at 14,505. Stratovolcanoes/Composite volcanoes are formed by alternating layers of lava and rock fragments. They are tall and conical and steep with periodic explosive violent eruptions. The lava that flowed or flows from these volcanoes typically cool and harden before spreading too far due to high thick viscosity. The magma that forms this lava is felsic, having high to intermediate levels of silica (rhyolite, andesite, and dacite). Rhyolitic magmas contain at least 70% silica (SiO2), a compound that causes magma to be significantly more viscous than ones composed mostly of basalt. Consequently, rhyolitic magmas can be difficult to extrude through volcanic vents and can sometimes plug the vent entirely. This characteristic increases the likelihood of a buildup of pressurized gases, resulting in explosive eruptions of ash. These volcanoes tend to occur along subduction plate boundaries and Mount Shasta is no exception as it happens to be in the Cascade Range formed by the subduction of the Juan de Fuca Plate under the North American Plate. As the Juan De Fuca Plate descends it begins to melt due to heat and pressure produced by friction at the zone. This melting produces magma which is less dense than surrounding material and rises to the top where it gradually forms volcanoes such as composite or stratovolcanoes. The most volcanically active zone in the world is known as the "Ring of Fire," which is an area of subduction zone sorrounding the pacific ocean.
SHIELD VOLCANOES
Shield volcanoes, the third type of volcano, are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile much like that of a warrior's shield. 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. In comparison, shield volcanoes are built from relatively weak viscous basaltic lava low in silica content (less than 52%) with a very small percentage of pyroclastic material, that erupt in longer cycles than that of composite volcanoes. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Shield volcanoes are distinctive products of hotspot volcanism (Hawaii), but can form at rift and subduction zones. Hots spots are volcanic regions fed by hot underlying mantle in comparison to mantle elsewhere. These spots may be on, near or far away from tectonic plate boundaries.
Some of the largest volcanoes in the world are shield volcanoes. In northern California, Medicine Lake Volcano has a diameter of 22 miles (East to West) and covers more than 770 square miles. 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.
Some of the largest volcanoes in the world are shield volcanoes. In northern California, Medicine Lake Volcano has a diameter of 22 miles (East to West) and covers more than 770 square miles. 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.