It is the most picturesque and yet the deadliest of the volcano types. Also known as composite cone, stratovolcano is the most common type which account for 60% of all volcanoes (J. Seach). The lower slopes of a stratovolcano are gentle but get steeper to the summit containing loose rocks. Being tall and conical are the most common charateristics of a stratovolcano besides having periodic and explosive eruptions. Among the well-known volcanoes that are classified as stratovolcanoes are Mt. Mayon and Mt. Pinatubo in the Philippines, Mt. Adams, Mt. Rainier and Mt. Baker in Washington, Mt. Haruna and Mt. Fuji in Japan, Volcan de Agua and Volcan de Fuego in Guatemala.
This steep cone was built by both pyroclastic and lava-flow eruptions (Britannica.com). Stratovolcano has stratified appearance with alternate lava flows. These lava flows are called airfall tephra, pyroplastic flows, lahars or volcanic mudflows and may also consist of debris flows. Stratovolcanoes are commonly found along subduction zones. They usually form arcs along tectonic plate boundaries a “wherein oceanic crust is subsumed under continental crust (www.geology.sdsu.edu). Such feature is referred to as Continental Arc Volcanism of which common examples are the Cascade Range and that of the central Andes. There are also cases where a curved volcanic mountain chain forms along the subduction zone (US Geology Survey). These chains are called Island Arcs where volcanoes rise out of the ocean. An example of which is the Aleutian Peninsula of Alaska.
What makes it different from other volcanoes is that stratovolcano has viscous lava flows that cool and harden before it flows down the slopes. In most cases, lava flows just pile up in the vent forming volcanic domes. The volcanic rocks that composed stratovolcanoes are variations of basalt, rhyolite and andesites but are generally composed of andesites. These types of rocks are high in silica and thus are classified as acidic. These materials come in alternate layers, geologists called “strata” which is the reason why stratovolcanoes are also called “composite cones.”
Although stratovolcanoes infrequently erupt, they are considered to be the deadliest types of volcano have caused the most number of casualties. Because stratovolcanoes are the most common type of volcano, they can be found at the most number of locations where much number of people will settle near the area. Another reason is that these volcanoes have the largest tendency of avalanche and landslides. This is because of the steep piles of ash, domes and lava that results to heavy mudflows during heavy rainfall. There are also cases where stratovolcanoes entirely collapse upon eruption. Among the well-known eruptions which caused thousands of casualties were those of Mt. St. Helens in 1980, Kamchatka in 1956 and Unzen in 1792.
The mudflow called “Lahar” in stratovolcanoes is also considered deadly. Lahars runs down the slopes really fast even with small eruption and rainstorm (volcano.und.edu). The disasters brought by these lahars were experienced by the people of Columbia and the Philippines, where Nevado del Ruiz and Mt. Pinatubo, respectively erupted. Thousands of people were killed in the 1985 eruption of Nevada del Ruiz where the mountain’s ice cap covering its summit melted, carrying vast amount of melted snow and rocks at the speed of 25km/hr. The lahar flow resulted to a 40 meters high water level.
FAULTS
Faults are surfaces along which rocks have fractured and been displaced (M. Glasscoe). It is in the fault line that stress between the rocks or plates concentrates. When the plates or the rocks are pressed together or are pulled apart, earthquakes occur (Hans and Cassidy). The movement of the plates along a fault defines what type of fault. There are basically three types of faults: normal, strike-slip, and reverse faults.
When the earth’s lithosphere is being stretched, normal faults occurs. They are found in deltas and at areas where there are huge gravitation slumps and slides. They are usually found in divergent boundaries. These type of faults show different geometrical patterns. There are normal faults that form a spoon or listric shape which happens when the faults gently dip at the depth. Normal faults can also be in the form of domino faults. These are formed when the plates move in the same direction with that of the listric-shaped faults. However, in this case there exists a rotated fault blocks between the plates. Normal faults also appear in low-angles called “detachments.” Detachments exhibit gentle dips that expose high-grade metamorphic rocks in the footwalls (earth.leeds.ac.uk).
The northern Chilean Coastal Cordillera has several normal faults that exhibit finite extension in the direction of plate convergence. The northernmost segment of the Karakoram fault is an active normal dextral wrench fault. The Kongurshan, which is about 7719 m and Mustaghata (7546 m) mountains are interpreted as twin crustal ramp anticlines formed en echelon on the eastern side of the Karakoram fault by subduction of the Tarim lithosphere under the Pamir (M. Brunel, et. Al).
Strike-slip faults are characterized by the parallel movement of the plates relative to the strike of the vertical fault plane. These faults are the result of side-by-side stress or a pressure in a horizontal direction. Strike-slip faults are commonly found in the sea floor that “create an extensive offsets mapped along the mid-oceanic ridges” (science.jrank.org). They are usually steep and are difficult to identify on cross-section. Among the well-known examples of these types of faults are the North Anatolian Fault and the San Andreas Fault systems. Strike-slip faults are also known as lateral faults, transcurrent faults and wrench faults.
Reverse faults are exactly the opposite of normal faults (C.E. Jones). If the hanging wall rises relative to the footwall, a reverse fault occurs. A reverse fault is caused by compression of two plates or rocks in a horizontal direction wherein the contracts of the earth’s surface are shortened. Thrusts are reverse faults that exhibits staircase trajectories or like that of a ramp. Because the hanging wall moves up relative to the footwall, most of these faults place older rocks over younger rocks. It may also happen that younger rocks are placed over older rocks which occur when previously deformed rocks are thrust faulted. Thrust faults typically dip at low-angles, at about 10-40 degrees. The orientation of thrust faults may vary because of they usually cut through stratigraphic sections as either ramps or flats. Sierra Madre fault, that runs along the San Gabriel Mountains is an example of a thrust fault. The fault along Hebgen Lake, Montana, which exhibits a 20 feet displacement, is also an example of this type of fault.
