Shield Volcanoes
1. Shield Volcanoes (notes)
a. Largest type, has gentle slopes
b. Most occur at hotspots or diverging plate boundaries c. Flows easily from vent, forms thin layers d. Made of dense basalt e. Not very explosive i. Not much gas f. Ex. Mauna Loa i. ~30,000 feet above ocean floor |
Shield volcanoes
_Shield volcanoes are built almost entirely
of low viscosity (very thin, liquidity) fluid lava flows. Flow after flow pours out in all directions from a
central summit vent, or group of vents, building a broad, gently sloping
cone of flat dome-like shape. They are built up slowly by thousands of highly
fluid lava flows called basalt lava that spread widely over great distances,
and then cool as thin, gently dipping sheets.
Some of the largest volcanoes in the world are shield volcanoes. The Hawaiian Islands are composed of linear chains of these volcanoes including Kilauea and Mauna Loa on the island of Hawaii-- two of the world's most active volcanoes. The floor of the ocean is more than 15,000 feet deep at the bases of the islands. As Mauna Loa, the largest of the shield volcanoes (and also the world's largest active volcano), projects 13,677 feet above sea level, its top is over 28,000 feet above the deep ocean floor, making it taller than Mt. Everest. |
How the Hawaiian Islands were formed
The Hawaiian Islands were formed in the middle of the Pacific tectonic plate, rather than at its boundaries. They were formed over a hot spot
(see picture above) where
magma breaks through the lithosphere. Scientists are still debating hotspots, but one explanation is that as the tectonic plate (in this case, the Pacific Plate) moves
or drifts,
the location of the hot spot remains in the same place. So as the
seafloor moves over the hot spot, new volcanic mountains are built.
As the plate moves past the hot spot, the volcano eventually becomes extinct. That's why Hawaii is made of a chain of volcanic islands. [Did you know
that the mountain that makes up the Big Island of Hawaii is taller than
Mt. Everest in the Himalayas?]
Eventually, the big island of Hawaii will drift past the hot spot and its volcanoes will go extinct. Already, a new island has already begun to form about 15 miles off the southeast coast of the Big Island. Named Loihi, it has already risen about 2 miles above the ocean floor, and within 1 mile of the ocean surface. In another 10 to 100 thousand years, a new island will exist where the Big Island of Hawaii currently rests. From the picture above, can you figure out where Loihi might be forming?
Eventually, the big island of Hawaii will drift past the hot spot and its volcanoes will go extinct. Already, a new island has already begun to form about 15 miles off the southeast coast of the Big Island. Named Loihi, it has already risen about 2 miles above the ocean floor, and within 1 mile of the ocean surface. In another 10 to 100 thousand years, a new island will exist where the Big Island of Hawaii currently rests. From the picture above, can you figure out where Loihi might be forming?
Hotspot LinksClick on this website for another explanation of hot spots.
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2011, 2018 & 2019 Eruptions in HawaiiKilauea is a very active volcano in Hawaii. opened a new vent and began pouring out hot lava. Notice the lava is spewing out at the fissure (opening) and is flowing. This kind of magma is made of basalt, which is less viscous (more runny, like honey) and does not carry a lot of dissolved gases. Therefore, the lava flows rather than explodes out of the vent. It is called a shield volcano.
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Types of hardened lava
The very fluid lava that looks like coils or thick rope is called pahoehoe (pa-hoy-hoy).
The rouch jagged lava is called clinkers or aa (ah-ah). The pahoehoe that cools under water forms pillow-like rocks, called pillows! |