Identify the Three Kinds of Places Where Volcanoes Can Occur
Where Does Volcanic Activity Occur?
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Myth: Most of the Earth'due south igneous rocks are produced at the "Band of Fire."
Fact: 70% of the Earth's surface is ocean flooring, which is made of basalt, an igneous stone produced at mid-ocean ridges. The ridges are the almost volcanically active features on the planet.
Plate boundary volcanism
Volcanic action occurs at two types of plate boundaries: mid-ocean ridges and subduction zones. At mid-ocean ridges, basaltic eruptions produce new sea-flooring crust. These underwater eruptions don't produce large mountainous volcanoes, which is why they are often overlooked as the most volcanically agile features on Earth. Usually, basalt is erupted at mid-ocean ridges every bit blob-shaped "pillows." These pillows form when basalt is suddenly quenched as it comes into contact with body of water water. If you cut a pillow in half, you'll detect a glassy rind around the outside, where the lava cooled and then fast that it couldn't form whatever crystals. Inside the pillow volition exist a crystalline matrix of cooled basaltic lava.
Watch this!
Hither's a short video clip taken from the Alvin, a submersible oceanographic vehicle, every bit scientists tried to collect some pillow basalts underwater in the Gulf of Alaska.
- NOAA: Ocean Explorer - Undersea Rock Chase
At subduction zones, volcanoes are created on the overriding plate as cook from the subducting plate rises upwards through the mantle and chaff. See the map below.
Tectonic map showing active volcanoes, plate boundaries, and the Band of Fire.
volcanoes.usgs.gov
Recall most this...
Recall that there are three types of plate boundaries: convergent, divergent, and transform. Volcanism occurs at convergent boundaries (subduction zones) and at divergent boundaries (mid-ocean ridges, continental rifts), only not normally at transform boundaries. Why not?
Hot spots (or not)
Hot spot volcanoes occur somewhat randomly around the globe. Their relationship (or lack of one) to the plate tectonic cycle is nevertheless existence debated. The map below shows several hot spots, only not all the existing ones. In fact, there are over 100 hot spots that have been active sometime during the final 10 meg years or so. Notice on the map beneath that out of the 25 hot spots shown, almost 10 occur on top of a mid-bounding main ridge. Whether this is a coincidence or not is a current topic of debate amongst scientists.
World map showing the locations of selected prominent hot spots.
usgs.gov
Mantle plumes (hot jets of cloth that well upwards from deep in the mantle at a speed of centimeters per year) were proposed as the source of hot-spot volcanoes at about the time of the plate tectonics revolution. Until recently, the prevailing wisdom held that hot spots have a deep source (mayhap as deep every bit the core-mantle boundary) and that they are nearly stationary with respect to the plates. Geologists, therefore, accept used hot spots as an absolute reference frame from which to derive plate motions, and they take studied the geochemical signatures of the lava that has erupted at hot-spot volcanoes as a way to learn something about the composition of the lower mantle. Recent observations of some pocket-sized young body of water mounts east of Nippon have initiated a vigorous debate about whether the standard plume model needs to exist revised, or maybe fifty-fifty thrown out completely. This particular chain of seamounts occurs away from a plate purlieus and the melt is probably coming from a source deeper than 100 km, but researchers who studied the geochemical signature of the lava concluded that the melt cannot have a very deep source, such as the lower drape or core-mantle boundary. Their hypothesis is that a crevice in the plate immune some partial melt that was present in the upper curtain to rise to the surface and form the sea-mount volcanoes. The schematic diagram beneath shows their model, which they call "petit-spot" volcanism.
A conceptual model of petit-spot volcanism. Magmas from the asthenosphere escape to the shallow depths because of the extensional surroundings of the lower lithosphere and migrate up through the brittle compressed upper lithosphere by exploiting fractures created by flexure of the plate.
Hirano et al., 2006.
Source: https://www.e-education.psu.edu/earth520/content/l6_p2.html
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