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Volcanoes are among the most powerful and destructive forces of nature on Earth. A volcano is an opening, or rupture, in the planet’s surface or crust, which allows hot, molten rock (lava), ash, and gases to escape from deep below the surface. Volcanic activity involving the extrusion of rock tends to form mountains or features like mountains over a period of time.
Volcanoes generally form where
(1) tectonic plates pull apart (diverge)[Mid-Atlantic Ridge] or come together (converge or collide)[Pacific Ring of Fire] rather than sliding past one another. Common to both is a melting of the crust causing the volcanism;
They can also form where
(2) there is stretching of the Earth’s crust and where the crust grows thin (“non hotspot” intra-plate volcanism)[African Rift Valley]; or
(3) by “mantle plumes”, so-called “hotspots”, which can form far from plate boundaries [Hawaiian Islands].
Volcanic Features
The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater in its top (Figure 1). However, the features of volcanoes are much more complicated. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater, whereas others present landscape features such as massive plateaus. Vents that issue volcanic material (lava, which is what magma is called once it has broken the surface, and ash) and gases (mainly steam and magmatic gases) can be located anywhere on the landform. Many of these vents give rise to smaller cones.
The structure and behavior of volcanoes depends on a number of factors, most notably the composition of lava.
If the erupted magma contains a high percentage (>63%) of silica, the lava is called felsic. These lavas (or rhyolites) tend to be highly viscous and are erupted as domes or short, stubby flows. Viscous lavas tend to form stratovolcanoes (tall, conical mountains composed of lava flows and other ejecta in alternate layers or strata) or lava domes. Because siliceous magmas are so viscous, they tend to trap volatile gases that are present, causing the magma to erupt catastrophically. Pyroclastic flows (ignimbrites) are highly hazardous products of such volcanoes, since they are composed of molten volcanic ash too heavy to go up into the atmosphere, so they hug the volcano’s slopes and travel far from their vents during large eruptions. Temperatures as high as 1200 degrees C are known to occur in pyroclastic flows, which will incinerate everything flammable in their path, and thick layers of hot pyroclastic flow deposits can be laid down many meters thick.
Lower silica-content lavas are generally less viscous than felsic or rhyolitic lavas, and tend to be hotter, thus contributing to longer, broader flows. These lavas, called mafic (because of higher amounts of magnesium and iron) or basaltic, gradually build wide mountains with a shield-like profile (shield volcanoes). Smaller versions include lava cones and lava mounds.
Volcanic cones or cinder cones result from eruptions that throw out small pieces of pyroclastics resembling cinders that build up around the vent. These tend to erupt only once, producing a cone-shaped hill (30-400m high) that can occur on its own or as flank vents on larger volcanoes.
In addition to the above terrestrial volcanoes, submarine volcanoes are common features on the ocean floor. Depending on their depth, even large submarine eruptions may not disturb the ocean surface, whereas shallow ones may break the surface as new islands.
Volcanic Activity and Effects
The lifespan of a volcano can vary from months to several million years.
Volcanologists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic times
Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again. Extinct volcanoes are those that scientists consider unlikely to ever erupt again. Whether a volcano is truly extinct is often difficult to determine. Since “supervolcano” calderas can have eruptive lifespans in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct.
There are many different kinds of volcanic activity and eruptions: phreatic eruptions (steam-generated eruptions), explosive eruption of high-silica lava (e.g, rhyolite), effusive eruption of low-silica lava (e.g., basalt), pyroclastic flows, lahars (debris flow) and carbon dioxide emission. All of these activities can pose a hazard to man. Earthquakes, hot springs, fumaroles, mud pots, and geysers often accompany volcanic activity.
The concentrations of different volcanic gases can vary considerably from one volcano to the next. Water vapor is typically the most abundant volcanic gas, followed by carbon dioxide and sulfur dioxide. Other principal volcanic gases include hydrogen sulfide, hydrochloric acid and hydrogen fluoride. Large, explosive volcanic eruptions eject these gases and ash (pulverized rock and pumice) into the stratosphere to heights of 16 to 32 km (Figure 2). Sulfur dioxide is converted to sulfuric acid, which forms fine sulfate aerosols. These aerosols reflect more radiation from the Sun back into space, thus cooling the Earth’s lower atmosphere (trophosphere); however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere. Several eruptions during the past century have caused a decline in the average temperature at the Earth’s surface of up to half a degree F for periods of 1 to 3 years. The sulfate aerosols also promote complex chemical reactions that ultimately destroy the ozone layer and produce acid rain. Volcanic activity also releases between 145 to 225 million short tons of carbon dioxide each year.
On the beneficial side, volcanic eruptions add nutrients to soil through the weathering process of volcanic rocks, and can also create new islands.
Volcan Masaya
Masaya is one of 18 distinct volcanic centers that make up the Nicaraguan portion of the Central American Volcanic Belt. It was formed by the subduction of the Cocos Plate beneath the Caribbean Plate, along the Mesoamerican trench, running from Guatemala to Costa Rica.
Volcan Masaya (Popogatepe, “Mountain that Burns”),is the most active volcano in the region. Since first described by the Spanish in 1524 it has erupted at least 19 times (most recent 1993). Masaya is an unusual basaltic volcano because it has had explosive eruptions. The eruption in 4550 BC was one of the largest on Earth in the last 10,000 years.
Masaya is a caldera (6 by 11.5 km) that contains 13 vents. Most activity at these vents consists of effusion of basaltic lava. Pyroclastic eruptions have constructed 3 main cones: Masaya, Nindiri, and Santiago. Unlike the stratovolcanoes that characterize subduction zones, Masaya has a shield-like morphology.
Masaya is the only volcano in the Western Hemisphere where you are able to drive to the rim. In the park is an underground tunnel which was formed by lava flows, you can find bats, and look inside and observe the flowing lava in the dark crater mouth of the volcano.
To claim this EarthCache:
1. Take and post a picture of you and your GPSr at the posted coordinates and e-mail me the answers to the following:
2. What are 3 general differences between felsic and basaltic lavas?
3. What is the name of the crater found at N11.58.987 W086.09.785?
Do NOT post your answers on your log, encrypted or otherwise.
DO NOT LOG AS A FIND UNTIL YOU HAVE A PICTURE READY TO POST AT THE TIME OF LOGGING A FIND FOR THIS EC. Logs with no photo of the actual cacher logging the find or failure to answer required questions through e-mail will result in a log deletion without notice. Exceptions will be considered if you contact me first.
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