In this situation, a zone of magmatic activity —or a hotspot—in the middle of a tectonic plate can push up through the crust to form a volcano. Although the hotspot itself is thought to be largely stationary, the tectonic plates continue their slow march, building a line of volcanoes or islands on the surface. This mechanism is thought to be behind the Hawaii volcanic chain. Some 75 percent of the world's active volcanoes are positioned around the ring of fire , a 25,mile long, horseshoe-shaped zone that stretches from the southern tip of South America across the West Coast of North America, through the Bering Sea to Japan, and on to New Zealand.
This region is where the edges of the Pacific and Nazca plates butt up against an array of other tectonic plates. Importantly, however, the volcanoes of the ring aren't geologically connected. In other words, a volcanic eruption in Indonesia is not related to one in Alaska, and it could not stir the infamous Yellowstone supervolcano. Volcanic eruptions pose many dangers aside from lava flows. It's important to heed local authorities' advice during active eruptions and evacuate regions when necessary.
One particular danger is pyroclastic flows, avalanches of hot rocks, ash, and toxic gas that race down slopes at speeds as high as miles an hour. Such an event was responsible for wiping out the people of Pompeii and Herculaneum after Mount Vesuvius erupted in A. Similarly, volcanic mudflows called lahars can be very destructive. These fast-flowing waves of mud and debris can race down a volcano's flanks, burying entire towns.
Ash is another volcanic danger. Unlike the soft, fluffy bits of charred wood left after a campfire, volcanic ash is made of sharp fragments of rocks and volcanic glass each less than two millimeters across. The ash forms as the gasses within rising magma expand, shattering the cooling rocks as they burst from the volcano's mouth. It's not only dangerous to inhale , it's heavy and builds up quickly. Volcanic ash can collapse weak structures, cause power outages, and is a challenge to shovel away post-eruption.
Volcanoes give some warning of pending eruption, making it vital for scientists to closely monitor any volcanoes near large population centers. Warning signs include small earthquakes, swelling or bulging of the volcano's sides, and increased emission of gasses from its vents. None of those signs necessarily mean an eruption is imminent, but they can help scientists evaluate the state of the volcano when magma is building.
However, it's impossible to say exactly when, or even if, any given volcano will erupt. At subduction zones the less iron-rich intermediate magmas that produce andesite lavas are most commonly erupted, although silicon-rich magmas sometimes erupt there too.
Basaltic magmas are also usually the hottest and the hotter the magma is the less viscous it is as well. Basaltic magmas also tend to contain few crystals, increasing their runniness. When magmas contain a lot of gas this makes them more runny too, although basaltic magmas usually do not contain much gas.
When runny basaltic magma erupts as lava, it pours out of the ground along long surface cracks or through volcanic vents and may be sprayed into the air as spectacular lava fountains. Rivers of lava can flow over the ground or move more slowly as blocky masses bulldozing along. The eruption of intermediate and silicon-rich magma is very different.
It erupts from vents, sometimes as lava, but usually the magma becomes solid within the volcanic vent, giving much more explosive eruptions. At constructive plate boundaries, the tectonic plates are moving away from one another. Volcanoes can sometimes form in these setting; one example is Iceland. Iceland lies on the Mid Atlantic Ridge, a constructive plate boundary, where the North American and Eurasian plates are moving away from each other.
As the plates pull apart, molten rock magma rises up and erupts as lava, creating new ocean crust. The island is covered with more than volcanoes. Some are extinct, but over 30 are still active. The majority of volcanism in Iceland occurs along volcanic rift zones that cut through the centre of the island.
At constructive plate boundaries, also known as divergent boundaries, tectonic plates move away from one another to produce volcanoes. Hot magma rises from the mantle at mid-ocean ridges, pushing the plates apart. In this type of eruption, the magma blasts into the air and breaks apart into pieces called tephra.
Tephra can range in size from tiny particles of ash to house-size boulders. Explosive volcanic eruptions can be dangerous and deadly. They can blast out clouds of hot tephra from the side or top of a volcano. These fiery clouds race down mountainsides destroying almost everything in their path. Ash erupted into the sky falls back to Earth like powdery snow.
If thick enough, blankets of ash can suffocate plants, animals, and humans. When hot volcanic materials mix with water from streams or melted snow and ice, mudflows form. Mudflows have buried entire communities located near erupting volcanoes. Despite their seeming permanence, volcanoes are prone to catastrophic collapse that can affect vast areas in a matter of minutes.
Large collapses begin as gigantic landslides that quickly transform to debris avalanches—chaotically tumbling masses of rock debris that can sweep downslope at extremely high velocities, inundating areas far beyond the Ash, mudflows, and lava flows can devastate communities near volcanoes and cause havoc in areas far downwind, downstream, and downslope. Even when a volcano is quiet, steep volcanic slopes can collapse to become landslides, and large rocks can be hurled by powerful When erupting, all volcanoes pose a degree of risk to people and infrastructure, however, the risks are not equivalent from one volcano to another because of differences in eruptive style and geographic location.
Assessing the relative threats posed by U. At least volcanoes in 12 States and 2 territories have erupted in the past 12, years and have the potential to erupt again. Consequences of eruptions from U. Many aspects of our daily life are vulnerable to volcano hazards, On May 22, , a large explosive eruption at the summit of Lassen Peak, California, the southernmost active volcano in the Cascade Range, devastated nearby areas and rained volcanic ash as far away as miles to the east.
This explosion was the most powerful in a series of eruptions during —17 that were the last to occur in the Viewing an erupting volcano is a memorable experience, one that has inspired fear, superstition, worship, curiosity, and fascination since before the dawn of civilization. In modern times, volcanic phenomena have attracted intense scientific interest, because they provide the key to understanding processes that have created and shaped more than Volcanoes have been erupting in the Cascade Range for over , years.
During the past 4, years eruptions have occurred at an average rate of about 2 per century. This chart shows 13 volcanoes on a map of Washington, Oregon, and northern California and time lines for each showing the ages of their eruptions. Most volcano hazards are associated with eruptions.
However, some hazards, such as lahars and debris avalanches, can occur even when a volcano is not erupting. Our Earth is a dynamic planet, as clearly illustrated on the main map by its topography, over volcanoes, 44, earthquakes, and impact craters.
These features largely reflect the movements of Earth's major tectonic plates and many smaller plates or fragments of plates including microplates. Volcanic eruptions and earthquakes are awe Volcanoes is an interdisciplinary set of materials for grades Through the story of the eruption of Mount St.
Helens, students will answer fundamental questions about volcanoes: "What is a volcano? Volcanoes give rise to numerous geologic and hydrologic hazards. Geological Survey USGS scientists are assessing hazards at many of the almost 70 active and potentially active volcanoes in the United States.
They are closely monitoring activity at the most dangerous of these volcanoes and are prepared to issue warnings of impending In the early s, the emergence of the theory of plate tectonics started a revolution in the earth sciences. Since then, scientists have verified and refined this theory, and now have a much better understanding of how our planet has been shaped by plate-tectonic processes.
We now know that, directly or indirectly, plate tectonics Helens was waking up. Within a week, several eruptions blasted clouds of ash into the atmosphere, and soon after, a new lava dome emerged in the crater. The HVO. Tephra blasted from the summit vent on Saturday night included lithic solid rock fragments from the vent wall as well as spatter molten lava fragments ejected from the lava lake.
The light-colored lithic in the center of this photo is about 20 cm 8 in long—the GPS unit is shown for scale. Tephra, the general term for volcanic rock fragments exploded or carried into. The United States has active volcanoes. The term also includes the cone-shaped landform built by repeated eruptions over time. Teach your students about volcanoes with this collection of engaging material.
A volcano is a feature in Earth's crust where molten rock is squeezed out onto the Earth's surface. Along with molten rock, volcanoes also release gases, ash, and solid rock.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. Background Info Fast Facts Vocabulary.
Worldwide there are 1, active volcanoes that could erupt at any time. Other volcanoes are dormant; they are quiet for now but could become active. There are even extinct volcanoes, which are never expected to erupt again. Volcanoes arent unique to Earth. The largest volcano in our solar system is Olympus Mons on Mars. It is three times taller than Mt. Everest, Earth's highest elevation. Mercury, Venus, and Jupiter's moon Io also have or have had volcanic activity, with Io being the most active of all.
The eruption of Mt.
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