What Is Volcanic Eruption?
Answer
Types of volcanic eruptions INTRODUCTION:
"Eruption" redirects here. For other uses, see Eruption (disambiguation).
"Volcanic eruption" and "Volcanic eruptions" redirect here. For the manga, see Volcanic Eruption (manga). For the film company, see Volcanic Eruptions (film company).
There are three different metatypes of eruptions. The most well-observed are magmatic eruptions, which involve the decompression of gas within magma that propels it forward. Phreatomagmatic eruptions are another type of volcanic eruption, driven by the compression of gas within magma, the direct opposite of the process powering magmatic activity. The last eruptive metatype is the phreatic eruption, which is driven by the superheating of steam via contact with magma; these eruptive types often exhibit no magmatic release, instead causing the granulation of existing rock.
Within these wide-defining eruptive types are several subtypes. The weakest are Hawaiian and submarine, then Strombolian, followed by Vulcanian and Surtseyan. The stronger eruptive types are Pelean eruptions, followed by Plinian eruptions; the strongest eruptions are called "Ultra Plinian." Subglacial and phreatic eruptions are defined by their eruptive mechanism, and vary in strength. An important measure of eruptive strength is Volcanic Explosivity Index (VEI), a magnitudic scale ranging from 0 to 8 that often correlates to eruptive types.
Contents
Eruption mechanisms
Volcanic eruptions arise through three main mechanisms:
- Gas release under decompression causing magmatic eruptions.
- Thermal contraction from chilling on contact with water causing phreatomagmatic eruptions.
- Ejection of entrained particles during steam eruptions causing phreatic eruptions.[1]
Volcanic eruptions vary widely in strength. On the one extreme there are effusive Hawaiian eruptions, which are characterized by lava fountains and fluid lava flows, which are typically not very dangerous. On the other extreme, Plinian eruptions are large, violent, and highly dangerous explosive events. Volcanoes are not bound to one eruptive style, and frequently display many different types, both passive and explosive, even the span of a single eruptive cycle.[3] Volcanoes do not always erupt vertically from a single crater near their peak, either. Some volcanoes exhibit lateral and fissure eruptions. Notably, many Hawaiian eruptions start from rift zones,[4] and some of the strongest Surtseyan eruptions develop along fracture zones.[5] Scientists believed that pulses of magma mixed together in the chamber before climbing upward -- a process estimated to take several thousands of years. But Columbia University volcanologists found that the eruption of Costa Rica’s Irazú Volcano in 1963 was likely triggered by magma that took a nonstop route from the mantle over just a few months. Study authors called it the highway from hell.[6]
Volcano explosivity index
See also: List of largest volcanic eruptions
The Volcanic Explosivity Index (commonly shortened VEI) is a scale, from 0 to 8, for measuring the strength of eruptions. It is used by the Smithsonian Institution's Global Volcanism Program in assessing the impact of historic and prehistoric lava flows. It operates in a way similar to the Richter scale for earthquakes, in that each interval in value represents a tenfold increasing in magnitude (it is logarithmic).[7] The vast majority of volcanic eruptions are of VEIs between 0 and 2.[3]| VEI | Plume height | Eruptive volume * | Eruption type | Frequency ** | Example |
|---|---|---|---|---|---|
| 0 | <100 m (330 ft) | 1,000 m3 (35,300 cu ft) | Hawaiian | Continuous | Kilauea |
| 1 | 100–1,000 m (300–3,300 ft) | 10,000 m3 (353,000 cu ft) | Hawaiian/Strombolian | Months | Stromboli |
| 2 | 1–5 km (1–3 mi) | 1,000,000 m3 (35,300,000 cu ft) † | Strombolian/Vulcanian | Months | Galeras (1992) |
| 3 | 3–15 km (2–9 mi) | 10,000,000 m3 (353,000,000 cu ft) | Vulcanian | Yearly | Nevado del Ruiz (1985) |
| 4 | 10–25 km (6–16 mi) | 100,000,000 m3 (0.024 cu mi) | Vulcanian/Peléan | Few years | Eyjafjallajökull (2010) |
| 5 | >25 km (16 mi) | 1 km3 (0.24 cu mi) | Plinian | 5–10 years | Mount St. Helens (1980) |
| 6 | >25 km (16 mi) | 10 km3 (2 cu mi) | Plinian/Ultra Plinian | 1,000 years | Krakatoa (1883) |
| 7 | >25 km (16 mi) | 100 km3 (20 cu mi) | Ultra Plinian | 10,000 years | Tambora (1815) |
| 8 | >25 km (16 mi) | 1,000 km3 (200 cu mi) | Ultra Plinian | 100,000 years | Lake Toba (74 ka) |
| * This is the minimum eruptive volume necessary for the eruption to be considered within the category. ** Values are a rough estimate. Exceptions occur. † There is a discontinuity between the 2nd and 3rd VEI level; instead of increasing by a magnitude of 10, the value increases by a magnitude of 100 (from 10,000 to 1,000,000). | |||||
Magmatic eruptions
Magmatic eruptions produce juvenile clasts during explosive decompression from gas release. They range in intensity from the relatively small lava fountains on Hawaii to catastrophic Ultra Plinian eruption columns more than 30 km (19 mi) high, bigger than the AD 79 eruption that buried Pompeii.[1]Hawaiian
Main article: Hawaiian eruption
Hawaiian eruptions often begin as a line of vent eruptions along a fissure vent, a so-called "curtain of fire." These die down as the lava begins to concentrate at a few of the vents. Central-vent eruptions, meanwhile, often take the form of large lava fountains (both continuous and sporadic), which can reach heights of hundreds of meters or more. The particles from lava fountains usually cool in the air before hitting the ground, resulting in the accumulation of cindery scoria fragments; however, when the air is especially thick with clasts, they cannot cool off fast enough due to the surrounding heat, and hit the ground still hot, the accumulation of which forms spatter cones. If eruptive rates are high enough, they may even form splatter-fed lava flows. Hawaiian eruptions are often extremely long lived; Pu'u O'o, a cinder cone of Kilauea, has been erupting continuously since 1983. Another Hawaiian volcanic feature is the formation of active lava lakes, self-maintaining pools of raw lava with a thin crust of semi-cooled rock; there are currently only 5 such lakes in the world, and the one at Kīlauea's Kupaianaha vent is one of them.[4]
Flows from Hawaiian eruptions are basaltic, and can be divided into two types by their structural characteristics. Pahoehoe lava is a relatively smooth lava flow that can be billowy or ropey. They can move as one sheet, by the advancement of "toes," or as a snaking lava column. A'a lava flows are denser and more viscous then pahoehoe, and tend to move slower. Flows can measure 2 to 20 m (7 to 66 ft) thick. A'a flows are so thick that the outside layers cools into a rubble-like mass, insulating the still-hot interior and preventing it from cooling. A'a lava moves in a peculiar way—the front of the flow steepens due to pressure from behind until it breaks off, after which the general mass behind it moves forward. Pahoehoe lava can sometimes become A'a lava due to increasing viscosity or increasing rate of shear, but A'a lava never turns into pahoehoe flow.[8]
Hawaiian eruptions are responsible for several unique volcanological objects. Small volcanic particles are carried and formed by the wind, chilling quickly into teardrop-shaped glassy fragments known as Pele's tears (after Pele, the Hawaiian volcano deity). During especially high winds these chunks may even take the form of long drawn-out strands, known as Pele's hair. Sometimes basant aerates into reticulite, the lowest density rock type on earth.[4]
Although Hawaiian eruptions are named after the volcanoes of Hawaii, they are not necessarily restricted to them; the largest lava fountain ever recorded formed on the island of Izu Ōshima (on Mount Mihara) in 1986, a 1,600 m (5,249 ft) gusher that was more than twice as high as the mountain itself (which stands at 764 m (2,507 ft)).[4][9]
Volcanoes known to have Hawaiian activity include:
- Pu'u O'o, a parasitic cinder cone located on Kilauea on the island of Hawaiʻi which has been erupting continuously since 1983. The eruptions began with a 6 km (4 mi)-long fissure-based "curtain of fire" on 3 January. These gave way to centralized eruptions on the site of Kilauea's east rift, eventually building up the still active cone.[4]
- For a list of all of the volcanoes of Hawaii, see List of volcanoes in the Hawaiian - Emperor seamount chain.
- Mount Etna, Italy.[4]
- Mount Mihara in 1986 (see above paragraph)[4]
Strombolian
Main article: Strombolian eruption
The term "Strombolian" has been used indiscriminately to describe a wide variety of volcanic eruptions, varying from small volcanic blasts to large eruptive columns. In reality, true Strombolian eruptions are characterized by short-lived and explosive eruptions of lavas with intermediate viscosity, often ejected high into the air. Columns can measure hundreds of meters in height. The lavas formed by Strombolian eruptions are a form of relatively viscous basaltic lava, and its end product is mostly scoria.[10] The relative passivity of Strombolian eruptions, and its non-damaging nature to its source vent allow Strombolian eruptions to continue unabated for thousands of years, and also makes it one of the least dangerous eruptive types.[12]
Strombolian eruptions are similar to Hawaiian eruptions, but there are differences. Strombolian eruptions are noisier, produce no sustained eruptive columns, do not produce some volcanic products associated with Hawaiian volcanism (specifically Pele's tears and Pele's hair), and produce fewer molten lava flows (although the eruptive material does tend to form small rivulets).[10][12]
Volcanoes known to have Strombolian activity include:
- Parícutin, Mexico, which erupted from a fissure in a cornfield in 1943. Two years into its life, pyroclastic activity began to wane, and the outpouring of lava from its base became its primary mode of activity. Eruptions ceased in 1952, and the final height was 424 m (1,391 ft). This was the first time that scientists are able to observe the complete life cycle of a volcano.[10]
- Mount Etna, Italy, which has displayed Strombolian activity in recent eruptions, for example in 1981, 1999,[13] 2002-2003, and 2009.[14]
- Mount Erebus in Antarctica, the southernmost active volcano in the world, having been observed erupting since 1972.[15] Eruptive activity at Erebus consists of frequent Strombolian activity.[16]
- Stromboli itself. The namesake of the mild explosive activity that it possesses has been active throughout historical time; essentially continuous Strombolian eruptions, occasionally accompanied by lava flows, have been recorded at Stromboli for more than a millennium.[17]
Vulcanian
Main article: Vulcanian eruption
Initial Vulcanian activity is characterized by a series of short-lived explosions, lasting a few minutes to a few hours and typified by the ejection of volcanic bombs and blocks. These eruptions wear down the lava dome holding the magma down, and it disintegrates, leading to much more quiet and continuous eruptions. Thus an early sign of future Vulcanian activity is lava dome growth, and its collapse generates an outpouring of pyroclastic material down the volcano's slope.[18]
Volcanoes that have exhibited Vulcanian activity include:
- Sakurajima, Japan has been the site of Vulcanian activity near-continuously since 1955.[20]
- Tavurvur, Papua New Guinea, one of several volcanoes in the Rabaul Caldera.[18]
- Irazú Volcano in Costa Rica exhibited Vulcanian activity in its 1965 eruption.[21]
Peléan
Main article: Peléan eruption
Peléan eruptions are characterized most prominently by the incandescent pyroclastic flows that they drive. The mechanics of a Peléan eruption are very similar to that of a Vulcanian eruption, except that in Peléan eruptions the volcano's structure is able to withstand more pressure, hence the eruption occurs as one large explosion rather than several smaller ones.[25]
Volcanoes known to have Peléan activity include:
- Mount Pelée, Martinique. The 1902 eruption of Mount Pelée completely devastated the island, destroying the town of St. Pierre and leaving only 3 survivors.[26] The eruption was directly preceded by lava dome growth.[18]
- Mayon Volcano, the Philippines most active volcano. It has been the site of many different types of eruptions, Peléan included. Approximately 40 ravines radiate from the summit and provide pathways for frequent pyroclastic flows and mudslides to the lowlands below. Mayon's most violent eruption occurred in 1814 and was responsible for over 1200 deaths.[27]
- The 1951 Peléan eruption of Mount Lamington. Prior to this eruption the peak had not even been recognized as a volcano. Over 3,000 people were killed, and it has become a benchmark for studying large Peléan eruptions.[28]
-
Pyroclastic flows at Mayon Volcano, Philippines, 1984.
-
The lava spine that developed before the 1902 eruption of Mount Pelée
-
Mount Lamington following the devastating 1951 eruption.
Plinian
Main article: Plinian eruption
These massive eruptive columns are the distinctive feature of a Plinian eruption, and reach up 2 to 45 km (1 to 28 mi) into the atmosphere. The densest part of the plume, directly above the volcano, is driven internally by gas expansion. As it reaches higher into the air the plume expands and becomes less dense, convection and thermal expansion of volcanic ash drive it even further up into the stratosphere. At the top of the plume, powerful prevailing winds drive the plume in a direction away from the volcano.[30]
Plinian eruptions are similar to both Vulcanian and Strombolian eruptions, except that rather than creating discrete explosive events, Plinian eruptions form sustained eruptive columns. They are also similar to Hawaiian lava fountains in that both eruptive types produce sustained eruption columns maintained by the growth of bubbles that move up at about the same speed as the magma surrounding them.[29]
Regions affected by Plinian eruptions are subjected to heavy pumice airfall affecting an area 0.5 to 50 km3 (0 to 12 cu mi) in size.[29] The material in the ash plume eventually finds its way back to the ground, covering the landscape in a thick layer of many cubic kilometers of ash.[31]
Major Plinian eruptive events include:
- The historical AD 79 eruption of Mount Vesuvius buried the Roman towns of Pompeii and Herculaneum under a layer of ash and tephra. It is the model Plinian eruption. Mount Vesuvius has erupted multiple times since then, for example in 1822.[29]
- The 1980 eruption of Mount St. Helens in Washington, which ripped apart the volcano's summit, was a Plinian eruption of Volcanic Explosivity Index (VEI) 5.[3]
- The strongest types of eruptions, with a VEI of 8, are so-called "Ultra-Plinian" eruptions, such as the most recent one at Lake Toba 74 thousand years ago, which put out 2800 times the material erupted by Mount St. Helens in 1980.[7][32]
- Hekla in Iceland, an example of basaltic Pilian volcanism being its 1947-48 eruption. The past 800 years have been a pattern of violent initial eruptions of pumice followed by prolonged extrusion of basaltic lava from the lower part of the volcano.[29]
- Pinatubo in the Philippines on 15 June 1991, which produced 5 km3 (1 cu mi) of dacitic magma, a 40 km (25 mi) high eruption column, and released 17 megatons of sulfur dioxide.[33]
Phreatomagmatic eruptions
Main article: Phreatomagmatic eruption
Phreatomagmatic eruptions are eruptions that arise from interactions between water and magma. They are driven from thermal contraction
(as opposed to magmatic eruptions, which are driven by thermal
expansion) of magma when it comes in contact with water. This
temperature difference between the two causes violent water-lava
interactions that make up the eruption. The products of phreatomagmatic
eruptions are believed to be more regular in shape and finer grained than the products of magmatic eruptions because of the differences in eruptive mechanisms.[1][34]There is debate about the exact nature of phreatomagmatic eruptions, and some scientists believe that fuel-coolant reactions may be more critical to the explosive nature than thermal contraction.[34] Fuel coolant reactions may fragment the volcanic material by propagating stress waves, widening cracks and increasing surface area that ultimetly lead to rapid cooling and explosive contraction-driven eruptions.[1]
Surtseyan
Main article: Surtseyan eruption
A distinct defining feature of a Surtseyan eruption is the formation of a pyroclastic surge (or base surge), a ground hugging radial cloud that develops along with the eruption column. Base surges are caused by the gravitational collapse of a vaporous eruptive column, one that is denser overall then a regular volcanic column. The densest part of the cloud is nearest to the vent, resulting a wedge shape. Associated with these laterally moving rings are dune-shaped depositions of rock left behind by the lateral movement. These are occasionally disrupted by bomb sags, rock that was flung out by the explosive eruption and followed a ballistic path to the ground. Accumulations of wet, spherical ash known as accretionary lapilli is another common surge indicator.[5]
Over time Surtseyan eruptions tend to form maars, broad low-relief volcanic craters dug into the ground, and tuff rings, circular structures built of rapidly quenched lava. These structures are associated with a single vent eruption, however if eruptions arise along fracture zones a rift zone may be dug out; these eruptions tend to be more violent then the ones forming a tuff ring or maars, an example being the 1886 eruption of Mount Tarawera.[5][35] Littoral cones are another hydrovolcanic feature, generated by the explosive deposition of basaltic tephra (although they are not truly volcanic vents). They form when lava accumulates within cracks in lava, superheats and explodes in a steam explosion, breaking the rock apart and depositing it on the volcano's flank. Consecutive explosions of this type eventually generate the cone.[5]
Volcanoes known to have Surtseyan activity include:
- Surtsey, Iceland. The volcano built itself up from depth and emerged above the Atlantic Ocean off the coast of Iceland in 1963. Initial hydrovolcanics were highly explosive, but as the volcano grew out rising lava started to interact less with the water and more with the air, until finally Surtseyan activity waned and became more Strombolian in character.[5]
- Ukinrek Maars in Alaska, 1977, and Capelinhos in the Azores, 1957, both examples of above-water Surtseyan activity.[5]
- Mount Tarawera in New Zealand erupted along a rift zone in 1886, killing 150 people.[5]
- Ferdinandea, a seamount in the Mediterranean Sea, breached sea level in July 1831 and was the source of a dispute over sovereignty between Italy, France, and Great Britain. The volcano did not build tuff cones strongly enough to withstand erosion, and disappeared back below the waves soon after it appeared.[36]
- The underwater volcano Hunga Tonga in Tonga breached sea level in 2009. Both of its vents exhibited Surtseyan activity for much of the time. It was also the site of an earlier eruption in May 1988.[37]
-
The fissure formed by the 1886 eruption of Mount Tarawera, an example of a fracture zone eruption.
Submarine
Main article: Submarine eruption
Submarine eruptions may produce seamounts which may break the surface to form volcanic islands and island chains.
Submarine volcanism is driven by various processes. Volcanoes near plate boundaries and mid-ocean ridges are built by the decompression melting of mantle rock that rises on an upwelling portion of a convection cell to the crustal surface. Eruptions associated with subducting zones, meanwhile, are driven by subducting plates that add volatiles to the rising plate, lowering its melting point. Each process generates different rock; mid-ocean ridge volcanics are primarily basaltic, whereas subduction flows are mostly calc-alkaline, and more explosive and viscous.[39]
Spreading rates along mid-ocean ridges vary widely, from 2 cm (0.8 in) per year at the Mid-Atlantic Ridge, to up to 16 cm (6 in) along the East Pacific Rise. Higher spreading rates are a probably cause for higher levels of volcanism. The technology for studying seamount eruptions did not exist until advancements in hydrophone technology made it possible to "listen" to acoustic waves, known as T-waves, released by submarine earthquakes associated with submarine volcanic eruptions. The reason for this is that land-based seismometers cannot detect sea-based earthquakes below a magnitude of 4, but acoustic waves travel well in water and long periods of time. A system in the North Pacific, maintained by the United States Navy and originally intended for the detection of submarines, has detected an event on average every 2 to 3 years.[38]
The most common underwater flow is pillow lava, a circular lava flow named after its unusual shape. Less common are glassy, marginal sheet flows, indicative of larger-scale flows. Volcaniclastic sedimentary rocks are common in shallow-water environments. As plate movement starts to carry the volcanoes away from their eruptive source, eruption rates start to die down, and water erosion grinds the volcano down. The final stages of eruption caps the seamount in alkalic flows.[39] There are about 100,000 deepwater volcanoes in the world,[40] although most are beyond the active stage of their life.[39] Some exemplary seamounts are Loihi Seamount, Bowie Seamount, Davidson Seamount, and Axial Seamount.
Subglacial
Main article: Subglacial eruption
The study of glaciovolcanism is still a relatively new field. Early accounts described the unusual flat-topped steep-sided volcanoes (called tuyas) in Iceland that were suggested to have formed from eruptions below ice. The first English-language paper on the subject was published in 1947 by William Henry Mathews, describing the Tuya Butte field in northwest British Columbia, Canada. The eruptive process that builds these structures, originally inferred in the paper,[41] begins with volcanic growth below the glacier. At first the eruptions resemble those that occur in the deep sea, forming piles of pillow lava at the base of the volcanic structure. Some of the lava shatters when it comes in contact with the cold ice, forming a glassy breccia called hyaloclastite. After a while the ice finally melts into a lake, and the more explosive eruptions of Surtseyan activity begins, building up flanks made up of mostly hyaloclastite. Eventually the lake boils off from continued volcanism, and the lava flows become more effusive and thicken as the lava cools much more slowly, often forming columnar jointing. Well-preserved tuyas show all of these stages, for example Hjorleifshofdi in Iceland.[43]
Products of volcano-ice interactions stand as various structures, whose shape is dependent on complex eruptive and environmental interactions. Glacial volcanism is a good indicator of past ice distribution, making it an important climatic marker. Since they are imbedded in ice, as ice retracts worldwide there are concerns that tuyas and other structures may destabalize, resulting in mass landslides. Evidence of volcanic-glacial interactions are evident in Iceland and parts of British Columbia, and it's even possible that they play a role in deglaciation.[41]
- Mauna Kea in tropical Hawaii. There is evidence of past subglacial eruptive activity on the volcano in the form of a subglacial deposit on its summit. The eruptions originated about 10,000 years ago, during the last ice age, when the summit of Mauna Kea was covered in ice.[44]
- In 2008, the British Antarctic Survey reported a volcanic eruption under the Antarctica ice sheet 2,200 years ago. It is believed to be that this was the biggest eruption in Antarctica in the last 10,000 years. Volcanic ash deposits from the volcano were identified through an airborne radar survey, buried under later snowfalls in the Hudson Mountains, close to Pine Island Glacier.[42]
- Iceland, well known for both glaciers and volcanoes, is often a site of subglacial eruptions. An example an eruption under the Vatnajökull ice cap in 1996, which occurred under an estimated 2,500 ft (762 m) of ice.[45]
- As part of the search for life on Mars, scientists have suggested that there may be subglacial volcanoes on the red planet. Several potential sites of such volcanism have been reviewed, and compared extensively with similar features in Iceland:[46]
"Viable microbial communities have been found living in deep (2800 m) geothermal groundwater at 349 K and pressures over 300 bar..Furthermore, microbes have been postulated to exist in basaltic rocks in rinds of altered volcanic glass. All of these conditions could exist in polar regions of Mars today where subglacial volcanism has occurred."
—Jack Farmer, Arizona State University[46]
Phreatic eruptions
Main article: Phreatic eruption
Often a precursor of future volcanic activity,[50] phreatic eruptions are generally weak, although there have been exceptions.[49] Some phreatic events may be triggered by earthquake activity, another volcanic precursor, and they may also travel along dike lines.[47] Phreatic eruptions form base surges, lahars, avalanches, and volcanic block "rain." They may also release deadly toxic gas able to suffocate anyone in range of the eruption.[50]
Volcanoes known to exhibit phreatic activity include:
- Mount St. Helens, which exhibited Phreatic activity just prior to its catastrophic 1980 eruption (which was itself Plinian).[48]
- Taal Volcano, Philippines, 1965.[49]
- La Soufrière of Guadeloupe (Lesser Antilles), 1975-1976 activity.[49]
- Soufrière Hills volcano on Montserrat, West Indies, 1995–2012
See also
- Timetable of major worldwide volcanic eruptions
- List of currently erupting volcanoes
- List of Quaternary volcanic eruptions
External links
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Wikimedia Commons has media related to Diagrams of volcanic eruptions. |
- USGS Hawaiian Volcano Observatory (HVO) homepage. USGS.
- Distinguishing eruptive types.
- How Volcanoes Work. San Diego State University.
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An earthquake is what happens when two blocks of the earth suddenly
slip past one another. The surface where they slip is called the
fault or fault plane. The location below the earth’s
surface where the earthquake starts is called the hypocenter, and
the location directly above it on the surface of the earth is called the
epicenter.
The earth has four major layers: the inner core, outer core, mantle
and crust. (figure 2) The crust and the top of the mantle make up a
thin skin on the surface of our planet. But this skin is not all in one piece
– it is made up of many pieces like a puzzle covering the surface of
the earth. (figure 3) Not only that, but these puzzle pieces keep slowly
moving around, sliding past one another and bumping into each other. We call
these puzzle pieces tectonic plates, and the edges of the plates are
called the plate boundaries. The plate boundaries are made up of
many faults, and most of the earthquakes around the world occur on these
faults. Since the edges of the plates are rough, they get stuck while the
rest of the plate keeps moving. Finally, when the plate has moved far enough,
the edges unstick on one of the faults and there is an earthquake.
While the edges of faults are stuck together, and the rest of the block is
moving, the energy that would normally cause the blocks to slide past one
another is being stored up. When the force of the moving blocks finally
overcomes the friction of the jagged edges of the fault and it
unsticks, all that stored up energy is released. The energy radiates outward
from the fault in all directions in the form of seismic waves like
ripples on a pond. The seismic waves shake the earth as they move through it,
and when the waves reach the earth’s surface, they shake the ground and
anything on it, like our houses and us! (see P&S Wave inset)
Earthquakes are recorded by instruments called seismographs. The
recording they make is called a seismogram. (figure 4) The
seismograph has a base that sets firmly in the ground, and a heavy weight
that hangs free. When an earthquake causes the ground to shake, the base of
the seismograph shakes too, but the hanging weight does not. Instead the
spring or string that it is hanging from absorbs all the movement. The
difference in position between the shaking part of the seismograph and the
motionless part is what is recorded.
The size of the earthquake is called its magnitude. There is one
magnitude for each earthquake. Scientists also talk about the
intensity of shaking from an earthquake, and this varies depending
on where you are during the earthquake.
Seismograms come in handy for locating earthquakes too, and being able
to see the P wave and the S wave is important. You learned
how P & S waves each shake the ground in different ways as they travel
through it. P waves are also faster than S waves, and this fact is what
allows us to tell where an earthquake was. To understand how this works,
let’s compare P and S waves to lightning and thunder. Light travels
faster than sound, so during a thunderstorm you will first see the lightning
and then you will hear the thunder. If you are close to the lightning, the
thunder will boom right after the lightning, but if you are far away from the
lightning, you can count several seconds before you hear the thunder. The
further you are from the storm, the longer it will take between the lightning
and the thunder.
Scientists then use a method called triangulation to determine
exactly where the earthquake was (figure 6). It is called triangulation
because a triangle has three sides, and it takes three seismographs to
locate an earthquake. If you draw a circle on a map around three different
seismographs where the radius of each is the distance from that
station to the earthquake, the intersection of those three circles is the
epicenter!