Quando
um relâmpago atinge o topo de uma montanha, ele pode derreter rochas em
um piscar de olhos, deixando uma cicatriz vítrea estreita chamada
fulgurita. Agora, os pesquisadores mostraram que esses raios fossilizados são relógios geológicos que registram a passagem do tempo. A
técnica oferece aos geólogos uma maneira de datar tempestades de
dezenas de milhares de anos atrás e pode dar a eles uma janela para
padrões climáticos antigos.
“É
um método muito interessante e engenhoso”, diz Rafael Navarro González,
químico da Universidade Nacional Autônoma do México, Cidade
Universitária, que não participou do estudo.
Quando
expostos aos elementos, os vidros, como a fulgurita, absorvem a umidade
de forma lenta e constante, “como uma espécie de esponja”, diz Jonathan
Castro, vulcanologista da Universidade Johannes Gutenberg de Mainz. No
passado, os pesquisadores tentaram medir a quantidade de água nas
camadas externas de artefatos arqueológicos vítreos, como pontas de
flechas de obsidiana, como uma forma de datar por quanto tempo eles
ficaram expostos. No entanto, o método era complicado, diz Castro.
Parte do problema é que muitos desses vidros vêm de vulcões e já contêm água da época em que foram forjados. Essa água pode confundir as medições da nova água absorvida perto da superfície do vidro. Mas Castro pensava que, em fulguritas, o raio poderia vaporizar qualquer umidade residual interna.
Para investigar, ele e sua equipe primeiro fizeram seus próprios fulguritos artificiais. Eles chocaram núcleos de rocha vulcânica com um soldador de arco, elevando a temperatura para 10.000 ° C. Zapping as rochas evapora a maior parte da umidade perto das superfícies das amostras. Isso significa que os relâmpagos iniciam um cronômetro geológico no momento em que uma fulgurita é criada.
Um fulgurito vítreo encontrado no topo do vulcão South Sister de Oregon
Castro et al ., Earth and Planetary Science Letters (2020) 10.1016 / j.epsl.2020.116595
“Eles
mostram pela primeira vez que o conteúdo de água diminui completamente”
durante a queda de um raio, deixando para trás uma superfície limpa na
qual a água ambiental pode se acumular, diz o geoquímico Matthew Pasek,
da Universidade do Sul da Flórida.
Em seguida, os pesquisadores tentaram aplicar o sistema de datação aos fulguritos naturais. Eles escalaram picos vulcânicos no Oregon para extrair as amostras pretas brilhantes. As rochas vítreas “destacam-se em relevo absoluto” das rochas circundantes, diz Castro.
Uma
inspeção cuidadosa das cascas externas dos fulguritos revelou níveis
mais altos de umidade que diminuíram drasticamente na rocha. Esses perfis de umidade se comportaram muito “matematicamente”, diz Castro - bons para namorar. Os
cientistas calibraram seu relógio usando modelos de computador para
descobrir qual a taxa de absorção de umidade que melhor representou os
perfis vistos nas amostras.
Eles
descobriram que alguns dos fulguritos (e as tempestades que os
causaram) tinham centenas de anos. Mais importante ainda, o método pode
datar as rochas em décadas ou séculos , Castro e seus colegas relatam esta semana em Earth and Planetary Science Letters
. Isso oferece uma resolução de tempo mais nítida do que as técnicas
que dependem do bombardeio de rochas por raios cósmicos. Além de
esclarecer os padrões históricos de relâmpagos, a equipe acredita que o
novo método pode revelar quando o topo das montanhas se torna vulnerável
a quedas de raios.
Por exemplo, os fulguritos podem mostrar quando as geleiras da era do gelo começaram a recuar. Os
geólogos já sabem onde: Pilhas montanhosas de entulho, chamadas de
morenas terminais, registram o impulso mais distante de uma geleira em
avanço. Em regiões montanhosas, essas morenas devem incluir fulguritos. Ao
datá-los, os cientistas podem definir o momento da retirada, o que pode
ajudar a reconstruir as tendências climáticas do passado, diz Castro. “Eles serão uma maneira muito poderosa de superar as idades das moreias.”
Close view of Stromboli Volcano erupting incandescent molten lava framgents.
Credit: B. Chouet/USGS.
A volcano on Earth is a vent or fissure in the planet's crust through
which lava, ash, rock and gases erupt. A volcano is also a mountain
formed by the accumulation of these eruptive products.
Volcanoes have existed for a long time on Earth, likely causing disasters such as the Permian mass extinction
about 250 million years ago, the greatest mass extinction in Earth's
history, according to a 2015 paper. Volcanoes can and have existed on
other worlds as well: although volcanoes on the moon and Mars have long been dormant, volcanoes are still very active on Jupiter's moon Io.
Researchers are currently striving to find ways to predict when
volcanic eruptions might happen on Earth by analyzing clues such as crystals and gases linked with volcanoes.
Let's take a look at how volcanoes form on Earth:
Earth's crust is 3 to 37 miles (5 to 60 kilometers) thick, according to the U.S. Geological Survey. It is broken up into seven major and 152 smaller pieces called tectonic plates, according to a 2016 paper
by Christopher Harrison at the University of Miami. These plates float
on a layer of magma — semi-liquid rock and dissolved gases. At the
boundaries of these plates — where they move past, are pushed under, or
move away from each other — magma, which is lighter than the surrounding
solid rock, is often able to force its way up through cracks and
fissures. Magma can explode from the vent, or it can flow out of the
volcano like an overflowing cup. Magma that has erupted is called lava. [Related: 50 Amazing Volcano Facts]
Principal types of volcanoes
Cinder cone volcanoes (also called scoria cones) are the most common type of volcano,
according to San Diego State University, and are the symmetrical
cone-shaped volcanoes we typically think of. They may occur as single
volcanoes or as secondary volcanoes known as "parasitic cones" on the
sides of stratovolcanoes or shield volcanoes. Airborne fragments of
lava, called tephra, are ejected from a single vent. The lava cools
rapidly and fall as cinders
that build up around the vent, forming a crater at the summit,
according to the U.S. Geological Survey. Cinder cone volcanoes are
fairly small, generally only about 300 feet (91 meters) tall and not
rising more than 1,200 feet (366 meters). They can build up over short
periods of a few months or years.
Stratovolcanoes are also called composite volcanoes
because they are built of layers of alternating lava flow, ash and
blocks of unmelted stone, according to the U.S. Geological Survey. They
are larger than cinder cones, rising up to 8,000 feet (2,438 meters).
Stratovolcanoes result from a conduit system of vents leading from a
magma reservoir beneath the surface. When dormant, they typically have
steep concave sides that sweep together at the top around a relatively
small crater.
Stratovolcanoes can erupt with great violence.
Pressure builds in the magma chamber as gases, under immense heat and
pressure, are dissolved in the liquid rock. When the magma reaches the
conduits the pressure is released and the gases explode, like soda
spewing out of a soda can that you shook up and opened suddenly,
according to San Diego State University. Because they form in a system
of underground conduits, stratovolcanoes may blow out the sides of the
cone as well as the summit crater.
Stratovolcanoes are considered the most violent. Mount St. Helens,
in Washington state, is a stratovolcano that erupted on May 18, 1980.
Approximately 230 square miles (596 square kilometers) of forest was
completely obliterated and 57 people were killed. Over the course of the
day, winds blew 520 million tons of ash eastward across the United
States and caused complete darkness in Spokane, Washington, 250 miles
(402 kilometers) from the volcano, according to the U.S. Geological
Survey.
Shield volcanoes
are huge, gently sloping volcanoes built of very thin lava spreading
out in all directions from a central vent. They have wide bases several
miles in diameter with steeper middle slopes and a flatter summit. The
gentle convex slopes give them an outline like a medieval knight’s shield.
Eruptions of these volcanoes are not generally explosive, but are more
like liquid overflowing around the edges of a container. The world's
largest volcano, Mauna Loa in Hawaii,
is a shield volcano, according to the U.S. Geological Survey. Mauna Loa
is about 55,770 feet (17,000 meters) from its base beneath the ocean to
the summit, which is 13,681 feet (4,170 meters) above sea level. It is
also one of the Earth's most active volcanoes and is carefully
monitored. The most recent eruption was in 1984.
Besides well-known symmetrical volcanoes such as Mount Fuji in Japan
and Kilimanjaro in Tanzania, volcanic activity is responsible for
several other distinctive landforms.
Calderas: A caldera is a bowl-shaped depression formed when a volcano collapses into the void left when its magma chamber is emptied. There are three types,
according to San Diego State University. The first type is a crater
lake caldera. This is the result of a stratovolcano collapsing into its
magma chamber during a violent eruption. Basaltic calderas have a
concentric ring pattern resulting from a series of gradual collapses
rather than a single event. They are often found at the summit of shield
volcanoes such as the craters at the tops of Mauna Loa and Kilauea.
Resurgent calderas are the largest volcanic structures on Earth. They
are the result of catastrophic eruptions that dwarf any eruptions ever
recorded by human beings. Yellowstone caldera, sometimes called a "super
volcano," is one example.
Volcanic plugs: When magma solidifies in the fissure of a volcano the hard dense rock may form a "neck"
that remains when softer surrounding rock has been eroded away,
according to the U.S. Geological Survey. This can result in dramatic
landmarks such as Ship Rock in New Mexico, and Devil's Tower in Wyoming.
Tuff cones: also known as maars, tuff cones are
shallow, flat-floored craters that scientists think formed as a result
of a violent expansion of magmatic gas or steam, according to the U.S.
Geological Survey. Maars range in size from 200 to 6,500 feet (60 to
1,980 meters) across and from 30 to 650 feet (9 to 198 meters) deep, and
most are commonly filled with water to form natural lakes. Maars occur
geologically young volcanic regions of the world such as the western
United States and the Eifel region of Germany.
Lava plateaus: Shield volcanoes may erupt along lines
of fissures rather than a central vent spilling liquid lava in
successive layers. Over time as these layers form broad plateaus
such as the Columbia Plateau, according to the Encyclopedia Britannica.
These plateaus are often cut by deep canyons that expose the layers of
rock.
A fissure vent opened on Hawaii's Kilauea volcano.
Credit: HVO/USGS
Volcanoes in history
A.D. 79: One of the most famous volcanoes is Mount Vesuvius, which sits along the Bay of Naples in southern Italy. It has erupted dozens of times
in the past 2,000 years, according to the Encyclopedia Britannica. The
A.D. 79 eruption, which buried Pompeii, made Vesuvius famous, but
another eruption in 1631 killed about 3,000 people.
1783: The eruption of Mount Skaptar in Iceland
devastated farming and fishing, causing a famine that killed a quarter
of the country's people, according to Oregon State University.
1815: Whirlwinds and tsunamis from the eruption of Mount Tambora,
on Sumbawa Island in Indonesia, killed at least 10,000 people,
according to the Encylopedia Britannica. The volcano sent a cloud ejecta
into the atmosphere that was more than four times the amount ejected by
Mount Pinatubo in 1991, leading to the "Year Without a Summer" of 1816 in Europe and North America, according to a 2016 paper in Wiley Interdisciplinary Reviews: Climate Change.
1883: Another Indonesian volcano, Krakatoa,
erupted in an explosion heard 3,000 miles away. Seventy-pound boulders
landed on islands 50 miles away, and a 130-foot tsunami devastated
hundreds of villages, including Java and Sumatra, according to San Diego
State University. About 36,000 people died. Dust high in the atmosphere
caused the moon to appear blue, and sometimes green, for two years, according to NASA.
1902: Mount Pelée, on the island of Martinique, smothered the town of Saint-Pierre in deadly gas and hot ash, killing 29,933, according to the Los Angeles Times.
1980: Mount St. Helens in Washington state blew 1,300
feet off its top, killing 57 people and causing a midday darkness in
towns 85 miles away.
Pinatubo's cloud of sulfuric acid, some 20 million tons of it, climbed
to more than 12 miles in the stratosphere. Over the next several weeks,
the cloud encircled the equator and spread to the poles, covering the
entire planet. The particles reflected sunlight and cooled the Earth by
nearly a full degree Fahrenheit.
Other significant U.S. volcanoes
Lassen Peak, California:
Erupted between 1914 and 1917, causing no deaths, according to the
National Park Service. Lassen is considered one of the most likely in
the Cascade Range to erupt again.
Long Valley, California: The Long Valley Caldera
is a 10-by 20-mile (16-by-32 kilometer) depression in the Sierra Nevada
Mountains caused by an eruption 700,000 years ago, according to the
U.S. Geological Survey. A tremendous explosion spit out molten rock from
4 miles under the surface; afterward, the whole mess settled more than a
mile down into the depression where the magma had been.
Magma still feeds hot springs in the caldera. Earthquakes in 1980
marked the beginning of new activity that has included shifts in the
position of hot springs and swarms of other small earthquakes.
Geologists say it probably indicates that magma is again rising from
below, and they suspect the area will erupt again.
Mount Shasta, California: Last known eruption was in 1786. It is believed to erupt every 600 to 800 years,
according to the U.S. Geological Survey. The mountain is significant as
the incredibly dominant visual element in the Northern California
landscape.
Kilauea and Mauna Loa, Hawaii:
Each tends to erupt every two or three years; eruptions are
non-explosive, allowing these two volcanoes to be among the most studied
active volcanoes in the world, according to the U.S. Geological Survey
and the University of Hawaii.
Additional reporting by Charles Q. Choi, Live Science contributor.
