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Masaya Volcano
Masaya is a caldera located in the department of Masaya, 20 km south of Managua, Nicaragua. It is Nicaragua's first and largest National Park, and one of 78 protected areas of Nicaragua. The complex volcano is composed of a nested set of calderas and craters, the largest of which is Las Sierras shield volcano and caldera. Within this caldera lies a sub-vent, which is Masaya Volcano ''sensu stricto''. The vent is a shield type composing of basaltic lavas and tephras and includes a summit crater. This hosts Masaya caldera, formed 2500 years ago by an 8-km³ basaltic ignimbrite eruption. Inside this caldera a new basaltic complex has grown from eruptions mainly on a semi-circular set of vents that include the Masaya and Nindiri cones. The latter host the pit craters of Masaya, Santiago, Nindiri and San Pedro. Observations in the walls of the pit craters indicate that there have been several episodes of cone and pit crater formation.
Masaya continually emits large amounts of sulfur dioxide gas (from the active Santiago crater) and volcanologists study this (amongst other signs) to better understand the behavior of the volcano and also evaluate the impact of acid rain and the potential for health problems.
==History==
The floor of Masaya caldera is mainly covered by poorly vegetated ʻaʻā lava, indicating resurfacing within the past 1000 or so years, but only two lava flows have erupted since the sixteenth century. The first, in 1670, was an overflow from the Nindiri crater, which at that time hosted a 1-km-wide lava lake. The other, in 1772, issued from a fissure on the flank of the Masaya cone. Since 1772, lava has appeared at the surface only in the Santiago pit crater (presently active and persistently degassing) and possibly within Nindiri crater in 1852. A lake occupies the far eastern end of the caldera.
Although the recent activity of Masaya has largely been dominated by continuous degassing from an occasionally lava-filled pit crater, a number of discrete explosive events have occurred in the last 50 years.〔 One such event occurred on November 22, 1999, which was recognised from satellite data. A hot spot appeared on satellite imagery, and there was a possible explosion. On April 23, 2001 the crater exploded and formed a new vent in the bottom of the crater. The explosion sent rocks with diameters up to 60 cm which travelled up to 500 m from the crater. Vehicles in the visitors area were damaged and one person was injured. On October 4, 2003 an eruption cloud was reported at Masaya. The plume rose to a height of ~4.6 km. In 2008, the mountain erupted spewing ash and steam.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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