VOLCANISM
Early Theories - The first geologists believed that volcanoes were formed by subsurface burning coal deposits which melted the surrounding sedimentary rock. Later geologists realized that volcanic rocks were primary products from the earth's interior. This led to the idea that the solid crust overlies a molten mantle, which could break through the crust resulting in active volcanism. It has recently been found that the mantle is almost totally solid, making volcanoes more difficult to explain. The development of the theory of Plate Tectonics has helped explaining why volcanoes form and why they are concentrated in certain areas.
I. Composition and Eruptive Style
A. Chemical Composition - magma s range in composition from basalt to rhyolite. There are important differences in the viscosity of the various magma compositions that have a strong influence on how the magma is extruded.
1. Mafic (basaltic) - apparently form by partial melting of the mantle (peridotite). Pyroxenes melt leaving an olivine-rich residue (dunite). These magmas have a very low viscosity and are hotter (1000-1200
2. Intermediate (andesitic) - intermediate viscosity and magma temperatures result in the common occurrence of both passively erupted lava and violently erupted pyroclastic rocks.
3. Silicic (rhyolitic) - the high viscosity and lower temperatures (800-1000
° C.) of silicic magma results in an abundance of pyroclastic eruptions and only rare lava flows.B. Gases - make up 1-5% of the total wt. of the magma. Several thousand tons of gases are released each day by active volcanoes each day.
1. Composition - about 70% H2O, 15% CO2 and minor S and N. (Sulfur forms sulfuric acid resulting in a very corrosive and dangerous gas).
2. Expansion - at temperatures near 1000
II. Products of Volcanism
A. Lava Flows - magma which flows out onto the earth's surface. Lava is normally a mixture of liquid and crystals. These flows generally move at a low speed (a few MPH); however, some flow velocities have been recorded at over 60 MPH. Most flows are intermediate to mafic in composition.
1. Pahoehoe - highly fluid flow which spreads out in thin sheets. These usually develop a thin, glassy elastic skin that is dragged up into ropy contorted folds as the flow continues beneath the surface. These flows are mostly basalt in composition.
2. Aa Lava - a more viscous, slow-moving flow whose outer layer is broken into an angular, jagged, blocky surface. Commonly basaltic, but usually more viscous due to lower temperature and/or loss of volatiles.
3. Pillow Lavas - ellipsoidal, globular masses of lava that are erupted underwater. A tough, pillow-shaped, plastic shell forms as the outer surface cools. These flows are also mostly basalt in composition.
4. Columnar-jointed Lavas - polygonal cracks form as thick lava flows or shallow extrusives cool rapidly and contract. The symmetry of the cracks are amazing. Ex. Devil's Post Pile - Devil's Tower.
5. Lava Tubes and Tunnels – cooler inner portion of flow continues to drain solidified outer covering. These were commonly used by the Indians to store food.
B. Pyroclastic Eruptive Products - silicic magmas are cooler and more viscous; therefore, they are often violently erupted as magmatic gases are released. Example - Mount St. Helen - 2.7 million cubic meters of pyroclastic material, Cerro Negro (Nicaragua), 1968, 13 million cubic meters.
1. Pyroclastics - particles ejected during an explosive volcanic eruption (includes volcanic glass, crystals and rock fragments). Ranges from dust to boulder in size.
a. Ash Falls - deposits of fine-grained ash. If this material is hot enough, it will become lithified and form a tuff. Tuffs are generally less dense than flow rocks and commonly contain pumice or lithic fragments.
b. Ash Flow - a flow of suspended tephra and gas that form welded tuff. These rocks display flow structures where pumice fragments have been collapsed and drawn-out in a subparallel arrangement.
c. Nuee Ardente - glowing avalanche of hot gas and tephra, which due to the increased density of this mixture, moves down the side of the volcano at high speeds (up to 100 km/hr). Example - Mt. St. Helem, Mt. Pelee (Martinique).
d. Volcanic Bomb - a streamlined pyroclastic fragment of lava which was ejected while partially molten.
2. Mud Flow - a mixture of ash and lithic fragments that becomes saturated with water so that it flows down the volcano as a low temperature mass (Lahar). Nevada del Ruiz - Columbia (November, 1985) - more than 23,000 people were killed by a relatively small volcanic eruption that set off a devastating mudslide. Renewed volcanic activity melted 10% of the surrounding ice cap producing a mud flow, 130 feet high which moved as fast as 40-50 km/hr.
III. Volcanic Landforms - makeup some of the most spectacular landforms in the world. Volcanic features are found in many of our western National Parks.
A. Shield Volcanoes - broad, gently sloping volcanoes which are built primarily of basaltic (low viscosity) lava flows. (Slope < 15
B. Cinder Cones - volcanic cones composed of pyroclastics. The steep slopes on these volcanic cones are controlled by gravity. Generally less common and smaller in size than the other types of volcanoes. Often found on the flanks of other volcanoes.
C. Stratovolcanoes (Composite volcanoes) - volcanoes composed of alternating layers of flows and pyroclastic deposits. *Most common volcanic form. Commonly associated with silicic and andesite volcanism (often along active plate boundaries). Characterized by violent eruptions. These are often dormant for long periods of time. Examples - Mt. St. Helen, El Chichon, Nevada del Ruiz.
D. Fissures - elongate cracks in the crust along which magma is extruded. Mostly erupt basaltic lava, often very excessive amounts. (Example -oceanic floor and Plateau basalts.)
E. Other Volcanic Features
1. Calderas - irregular-shaped craters at the volcano's summit. These are constructed by explosion and collapse. Composite volcanoes generally have several craters with some infilled with lakes.
2. Lava Domes - large masses of solidified lava within the crater which is usually made of very viscous lava (post eruption).
3. Lava Plateaus - large upland areas which are underlain by lava. These are usually more resistant, therefore they form flat plateaus. They are also commonly erupted from fissures. Example - Columbia River Basalts – these cover large portion of Washington, Oregon and Idaho (40,000 km2 and averages 30m. thick.)
4. Hydrothermal Deposits - water-rich metal-bearing magmatic fluids commonly deposit gold-silver-copper and other metals along veins and porous rock units that are adjacent to magmatic bodies. Explain the residual nature of these deposits.
IV. Benefits and Dangers of Volcanism
A. Dangers
1. Loss of Property and Life
2. Pollution of the Atmosphere and Climatic Changes - huge quantities of ash and volcanic gases are injected into the atmosphere. Due to the small size of the ash material it remains in the atmosphere for a long period and filters out solar radiation. Could cause a cooling trend.
B. Benefits
1. Created much of the ocean floor and continental crust.
2. Produces some of the most fertile soil. Example- Hawaii, South and Central America. (Ask Juan Valdez!)
3. Gasses emitted could have produced or changed the Earth's atmosphere and oceans.
4. Mineral Deposits
5. Geothermal Energy
6. Provide information about the earth's interior.