A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from below the surface.
Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust in the interiors of plates, e.g., in the East African Rift, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "Plate hypothesis" volcanism. Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core-mantle boundary, 3,000 km deep in the Earth.
Popocatépetl also known as "Popochowa" by the local population is an active volcano and, at 5,426 m (17,802 ft), the second highest peak in Mexico after the Pico de Orizaba (5,636 m/18,491 ft). Popocatepetl is linked to the Iztaccihuatl volcano to the north by the high saddle known as the Paso de Cortés, and lies in the eastern half of the Trans-Mexican volcanic belt.
Popocatepetl is 70 km (43 mi) southeast of Mexico City, from where it can be seen regularly, depending on atmospheric conditions. The residents of Puebla, a mere 40 km (25 mi) east of the volcano, enjoy the views of the snowy and glacier-clad mountain almost all year long. The volcano is also one of the three tall peaks in Mexico to contain glaciers, the others being Iztaccihuatl and Pico de Orizaba. Magma erupted from Popocatepetl is a mixture of dacite and basaltic andesite.
The United Mexican States (Spanish: Estados Unidos Mexicanos (help·info)), commonly known as Mexico (pronounced i/ˈmɛksɨkoʊ/; Spanish: México, IPA: [ˈmexiko] ( listen)), is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of Mexico. Covering almost two million square kilometres (over 760,000 sq mi), Mexico is the fifth largest country in the Americas by total area and the thirteenth largest independent nation in the world. With an estimated population of over 112 million, it is the eleventh most populous country and the most populous Spanish-speaking country. Mexico is a federation comprising thirty-one states and a Federal District, the capital city.
Volcanic gases include a variety of substances given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating directly from lava or indirectly through ground water heated by volcanic action.
The sources of volcanic gases on Earth include:
Substances that may become gaseous or give off gases when heated are termed volatile substances.
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