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 (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”, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth.
Divergent plate boundaries
At the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling and solidifying. The crust is very thin at mid-oceanic ridges due to the pull of the tectonic plates. The release of pressure due to the thinning of the crust leads to adiabatic expansion, and the partial melting of the mantle causing volcanism and creating new oceanic crust.
Most divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, forming new seafloor. Black smokers or deep sea vents are an example of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, Iceland.
Convergent plate boundaries
Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. Water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating magma. This magma tends to be very viscous due to its high silica content, so often does not reach the surface and cools at depth. When it does reach the surface, a volcano is formed. Typical examples for this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire.
There are three types of volcano: shield, composite, and cinder cone. Shield volcanoes are is so named because the gently sloping sides resemble an ancient warrior’s shield. Famous examples of shield volcanoes are Mauna Loa Hawaii, Mt Wrangell Alaska, and Rangitoto New Zealand.
Composite volcanoes have the steepest sides and usually have a crater at the top. Mount Fiji in Japan, Mount Kilimanjaro in Tanzania, and Mount St Helens USA are good examples.
Cinder cone volcanoes are bowl-shaped and rarely rise above 300 meters (1,000 ft.) in elevation. Currie Mountain falls in this category.
Volcanic Debris. Solid pieces of rock that are expelled from volcanoes are known as tephra. Ash is tephra that is smaller than 2 mm in diameter. Pieces larger than 2 mm but less than 64 mm are referred to as lapilli. Blocks and bombs are projectiles greater than 64 mm. Tephra can also combine with superheated gases to form a pyroclastic cloud (or flow) that moves outward or flows downhill from the volcano. These clouds have been known to travel 100 km/hr. Toxic gases, which can suffocate people, can also be emitted from volcanoes. On the other hand, the erupted volcanic material can produce very fertile soil after it has been weathered. Such is the case with Currie Mountain. It is home to great White Pine and several rare plants not found in other parts of the province.
Currie Mountain is volcanic, and it is part of a very old volcanic system. UNB Earth Science Department has classified Currie Mountain as a fissure similar to Krafla volcano in Iceland. It is a vertical dike or volcanic neck, a remnant of the structure that fed a volcano. And it is ancient: it belongs to the Royal Road Basalt of the Mabou Group, which has been dated to the Early Carboniferous, somewhere in the range of 320-330 million years ago. Not only can this dike be seen, but there are lava flows just a little way away in quarries on the Carlisle Road and Royal Road. The summit is covered with lapilli.
To complete this task, answer the following questions in order to log your find:
1) What type of volcano do you think Currie Mountain is?
2) Measure some of the rocks on the summit and tell me how you would classify them?
For posterity you may take a picture of yourself standing on the summit holding your GPSr.
For more information on volcanoes, got to "volcanoes.usgs.gov".
Congrats to a fellow Earthcacher, K57 for FTF!