A volcano is a mountain that opens downward to a pool of molten rock below the surface of the earth. When pressure builds up, eruptions occur. Gases and rock shoot up through the opening and spill over or fill the air with lava fragments. Eruptions can cause lateral blasts, lava flows, hot ash flows, mudslides, avalanches, falling ash and floods. Volcano eruptions have been known to knock down entire forests. An erupting volcano can trigger tsunamis, flash floods, earthquakes, mudflows and rockfalls.
Scientists have categorized volcanoes into three main categories: active, dormant, and extinct. An active volcano is one which has recently erupted and there is a possibility that it may erupt soon. A dormant volcano is one which has not erupted in a long time but there is a possibility it can erupt in the future. An extinct volcano is one which has erupted thousands of years ago and there’s no possibility of eruption.
The volcano which is long since extinct began erupting around 16 million years ago and had 3 main eruptive phases. Between phases the slopes of the volcano were forested, and this is known due to the discovery of the preserved forest between lava flows in swamp and lake deposits. The last eruption was dated at about 10 million years ago.
Geologists believe that the volcano reached around 1,000m high but has been eroded down over millions of years. The hills around Dunedin and the Peninsula (wpts2-6; Mt Cargill, Flagstaff, Saddle Hill, Signal Hill and Harbour Cone) are remnants of one side of the volcano, with the highest point today being Mt Cargill at 700m.
At GZ you can look our over where the vent of the volcano once stood (1000m above you, it was approximately 15 square kilometers, the area between Port Chalmers and Portobello. The erosion of the slopes over the years has allowed the sea to enter and frown the centre and lower slopes of the former volcano.
Volcanoes are grouped into four types: cinder cones, composite volcanoes, shield volcanoes and lava volcanoes.
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Cinder Cones
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Cinder cones are circular or oval cones made up of small fragments of lava from a single vent that have been blown into the air, cooled and fallen around the vent. They are generally small, not rising more than 1,200 feet and build up over short periods of time and are the symmetrical cone-shaped volcanoes we typically think of.
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Composite Volcanoes
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Composite volcanoes are steep-sided volcanoes which can reach up to 2,400m tall (their slopes length has under 2:1 ratio with thier height). Whilst they are Stratovolcanoses, it is easier to know them by composite volcanoes because they are built of many layers of volcanic rocks, usually made from high-viscosity lava, ash and rock debris. Mt. Rainier and Mount St. Helens are examples of this type of volcano. Stratovolcanoes can erupt with great violence. Pressure builds in the magma chamber as gases, under immense heat and pressure, are dissolved in the liquid rock.
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Shield Volcanoes
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Shield volcanoes are huge volcanoes shaped like a bowl or shield in the middle with long gentle slopes made by basaltic lava flows spreading out in all directions from a central vent. They have wide bases several miles in diameter. Their slopes are easily over five times the length of their hieght. Basalt lava flows from these volcanoes are called flood basalts. The world's largest active volcano, is a shield volcano, according to the U.S. Geological Survey. Mauna Loa is about 55,770 feet (17,000 meters) from its base beneath the ocean to the summit, which is 13,681 feet (4,170 meters) above sea level. It is also one of the Earth's most active volcanoes and is carefully monitored.
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Lava Volcanoes
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Lava domes are formed when erupting lava is too thick to flow and makes a steep-sided mound as the lava piles up near the volcanic vent. The eruption of Mount St. Helens in 1980 was caused in part by a lava dome shifting to allow explosive gas and steam to escape from inside the mountain.
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