Earth is rumbling and volcanoes are erupting around the world this week.
On May 21, Mount Etna in Italy began spewing ash, shutting down a nearby airport. In Mexico, officials raised the alert level for Popocatépetl, a volcano 50 miles southeast of the capital, Mexico City, that began to spray smoke and ash over the weekend. Disaster managers are preparing shelters and clearing evacuation routes in case the commotion picks up further.
People in the Democratic Republic of Congo are now warily eying Nyamulagira volcano, which started to extrude lava in its crater last week. They’re worried that a larger eruption could threaten nearby cities including Goma, a provincial capital. And in Hawaii, Kilauea volcano, one of the state’s youngest and rowdiest, is showing signs of “heightened unrest,” according to the US Geological Service.
For the more than 200,000 residents of Hawaii’s Big Island, the latest rumblings may stir memories of the 2018 Kilauea eruption that cracked open 22 fissures, launched ash 11,000 feet into the air, triggered the largest earthquake on the island in 40 years, swallowed cars, and destroyed 700 homes. One of the lava flows reached the ocean and created at least 250 acres of new coastline. But prior to that upswell, Kilauea had been erupting at a low level since 1983 and was a popular sight in Hawai’i Volcanoes National Park.
And Hawaii isn’t the only place acting up. As of May 23, USGS has issued alerts for eight volcanoes in the US, six of which are in Alaska.
Despite the danger, many choose to stay and build their lives near these temperamental giants. Approximately 800 million people worldwide live within 60 miles of active volcanoes.
There are few things in the world more awesome than a volcanic eruption. The earth itself comes apart, and from its depths blast fiery molten rock, acrid gas, and towering plumes of ash.
Eruptions create new lands and destroy civilizations. They release more energy than nuclear weapons. They are visible from space and can change the temperature of the whole planet. The Krakatoa volcano in Indonesia produced the loudest sound ever heard. Even mild eruptions, like the ashy burp from Iceland’s Eyjafjallajökull in 2010, can force the global economy to grind to a halt.
And we keep learning more about their immense power. Scientists reported that the Hunga Tonga-Hunga Ha’apai volcano that erupted in Tonga on January 15, 2022, sent an extraordinary amount of water into the atmosphere, enough to temporarily cool the planet. It also triggered tsunamis and created a sonic boom that circled the planet twice.
A volcano is considered potentially active if it has erupted in the past 10,000 years. America has 169 active volcanoes, mainly clustered in the West — in Hawaii, Alaska, Wyoming, Washington, and California. Around the world, there are 1,500 potentially active volcanoes, 500 of which have erupted since humans have been around, according to the USGS. Some of these potentially active volcanoes may never erupt, some may ooze lava slowly for years, and some may one day have a massive ejection that wreaks havoc.
To study volcanoes, scientists brave the rippling heat of lava and acrid gases, climbing up slopes and sometimes into craters to place instruments and take measurements. Here are some of the coolest things they’ve learned — from the most likely way a volcano would kill you to whether it’s okay to throw your garbage into one.
1) How volcanoes form, and what makes them erupt
A volcano is what happens when the earth’s crust leaks and molten rock squirts through. The crust is made of giant blocks called tectonic plates that slide on top of the mantle, which can reach temperatures of 3,700 degrees Celsius.
The mantle makes up 84 percent of Earth’s volume, and though it’s solid rock, over the course of millions of years, it behaves like a liquid. This leads the tectonic plates on top to slowly jostle one another. The buildup and sudden release of friction from this movement can cause earthquakes.
The movement also creates gaps in tectonic plates, which reduces pressure on the mantle beneath it, allowing it to melt and push through. These rift zones are where new land is created as magma bubbles up to the surface and cools off, forming basalt rocks. Plates also slide on top of one another, pushing the edge of one plate up and the edge of the other plate down in a process called subduction. The Mariana Trench, the deepest point in the world, occurs in a subduction zone.
At plate boundaries in the ocean, subduction moves water into the mantle, lowering the melting point of rock underneath the edges of continental plates. The molten rock can then push its way up to the surface of the Earth.
This is the mechanism behind the geologic activity in the Ring of Fire, the 25,000-mile perimeter of the Pacific tectonic plate, running from Southeast Asia toward Russia, Alaska, and down toward South America. The region is home to 90 percent of the world’s earthquakes and 75 percent of all volcanoes.
Volcanoes can form in the middle of tectonic plates as well. Hot spots can emerge in the mantle, creating a conduit for molten rock in the plate toward the surface.
The Hawaiian islands were formed over such a hot spot as molten rock breached the earth’s surface and cooled down, layering with each eruption until islands cropped up from the bottom of the ocean.
In fact, Hawaii’s Mauna Loa, the largest active volcano on earth, can also be considered the largest mountain since the distance from its base beneath the ocean to its peak is 30,085 feet, higher than Mount Everest’s 29,032 feet. Above the water, Mauna Loa is 13,678 feet tall.
Molten rock that reaches the surface of the earth is, of course, called lava. How syrupy or watery the lava is depends on the kind of rocks being melted. Goopier magma and lava tends to cause more explosive eruptions and form steep-sided volcanoes, while runnier rocks tend to ooze out and create gradually sloping volcanoes.
2) It’s really hard to predict when volcanoes will erupt. But there are warning signs.
The forces that create volcanoes act over hundreds of thousands, if not millions, of years. Humans have only been around for a measly 200,000 years.
That mismatch is the key reason geologic activity in general and volcanoes in particular are so hard to predict. We’ve only experienced a narrow slice of their existence.
In general, an eruption occurs when the pressure of magma, underground molten rock, exceeds that of the rocks on top holding it in place, though earthquakes can also trigger eruptions.
“All of us ... have an experience with a toilet backing up,” said Tracy Gregg, a professor of geology at the University at Buffalo. “What causes the eruption is that the pressure inside the pipe builds up until the pipes burst, and earthquakes help open up pathways.”
Though the past can’t perfectly predict the future, the history of eruptions does show what is possible and where. The Kilauea volcano, the most active volcano in the world and one that has been slowly dribbling out lava off and on since 1983, is a case in point.
It’s one of the best-studied volcanoes in the world, and geologists knew that a larger eruption was possible. In 2018, they were able to read some warning signs ahead of that surge in activity and anticipate how it would play out.
“Cracks appeared within a few kilometers of where we expected them to,” Gregg said. “From a scientific standpoint, it was a fantastic coup.”
Unlike earthquakes, volcanic eruptions are often heralded by rumblings, fissures, and releases of gases like sulfur dioxide in the weeks, days, or hours ahead of an eruption.
“Volcanoes show precursory signs of eruptive activity — what we call ‘unrest,’” a spokesperson for the US Geological Survey said in an email. “One of the ways we detect signs of unrest is actually using earthquakes. At volcanoes, earthquakes can tell us that the ground surface is fracturing as a result of magma pushing against and through rocks.”
However, scientists are uncertain whether large earthquakes occurring farther away could trigger an eruption.
With better instruments and monitoring, geologists are aiming to build more robust forecasts to get people out of harm’s way. But the amount of warning these signals can give varies depending on the volcano. Emily Brodsky, a professor of earth and planetary sciences at the University of California Santa Cruz, noted that volcano monitoring has a high false positive rate since earthquakes and fissures don’t always mean a big eruption is imminent. The trick is to develop warnings for people in volcanic areas that instill caution without complacency.
3) There are lots of ways volcanoes can kill you
Some of the most haunting relics of history are the plaster casts made of the people of Pompeii, Italy, who died during the massive eruption of Mount Vesuvius in CE 79 and left behind hollows in the solidified ash.
Excavations of the ash there have revealed a civilization frozen in time as a roaring torrent of extreme heat, ash, and toxic gas suddenly swept over it, killing at least 13,000 people.
History is filled with many accounts of devastating and deadly volcanic eruptions, though the death tolls are usually more modest. Mount Kusatsu-Shirane, 100 miles northwest of Tokyo, erupted in January 2018, killing one soldier in an avalanche.
But we don’t have to go too far back to find an utterly catastrophic eruption. The Nevado del Ruiz volcano killed more than 20,000 people in Colombia when it erupted in 1985.
From rockslides to earthquakes to lava, volcanic eruptions pose many hazards. One common threat is the release of sulfur dioxide gas from deep underground. It’s colorless but has a pungent burnt odor. Sulfur dioxide can irritate the airways, and in high enough concentrations, it can suffocate. When mixed with water vapor and carbon dioxide, it creates a hazy volcanic fog, a.k.a. vog.
The next, more visible risk is ash. It’s rarely an immediate danger to health, though it can damage aircraft and hamper visibility for drivers, leading to accidents. Falling ash can accumulate on roofs and collapse structures as well. Jagged ash particles irritate the lungs and over the long term can lead to a disease known as silicosis.
And ash can destroy crops. The deadliest eruption on record, the eruption of the Tambora volcano in Indonesia in 1815, killed 92,000 people, largely by starvation.
Earthquakes associated with eruptions can also knock over buildings and trigger deadly landslides.
Then there’s lava. While some lava flows inch forward, others can reach speeds up to 40 mph. This molten rock reaching temperatures of 1,250°C is nearly unstoppable, though people have certainly tried to redirect it. A study reported that aerial bombing “has a substantial probability of success for diversion of lava” based on US Air Force bombing experiments on Mauna Loa in the 1970s.
Perhaps the scariest consequence of an eruption is a combination of all of the above in a phenomenon known as pyroclastic flow, a fast-moving mix of lava, rock, ash, and toxic gas.
This cascading torrent of earth can destroy anything in its path at speeds up to 300 mph, which was observed when Mount St. Helens erupted in 1980. “If you witness a pyroclastic flow, run in the opposite direction as quickly as possible,” the US Geological Survey notes on its website.
4) A single eruption can alter the whole planet
When Mount Pinatubo in the Philippines erupted in 1991 in the second-largest volcanic eruption in the 20th century, it injected 20 million tons of sulfur dioxide and ash 12 miles up into the stratosphere. These particles and aerosols spread all over the world and scattered incoming sunlight.
Scientists found that global temperatures cooled by 0.6°C on average in the 15 months following the eruption.
The eruption is a vivid example of how a single event can ripple throughout the planet. But not every volcano can nudge the thermostat.
“You need a very high plume and a lot of sulfur,” said NASA climate scientist Chris Colose in an email. “If there’s no material injected into the high atmosphere (the stratosphere) then there will be minimal climate impact, since the sulfur has a low lifetime in the lower parts of the atmosphere closer to the surface (the troposphere).”
The cooling effects of a volcano, if there are any, only linger for one to three years, so we can’t count on erupting our way to a cooler planet as the climate changes. But scientists are studying volcanoes in case humanity does need to deliberately cool the planet in the face of catastrophic climate change, an emerging field known as geoengineering.
5) A “high threat” American volcano might be closer than you realize
About half of the 169 volcanoes in the United States are considered dangerous because of the manner in which they may erupt and the communities nearby that are in harm’s way, according to the USGS. Though most of the volcanoes are in sparsely populated areas, their ash plumes and toxic gas emissions can spread for hundreds of miles.
Clear Lake Volcanic Field, for example, has a “high” threat potential and is 90 miles north of the San Francisco Bay Area, home to 7 million people. The last eruption at the site was 11,000 years ago around Mount Konocti, but “the numerous hot springs and volcanic gas seeps in the area point to its potential to erupt again.”
Despite the potential hazards, researchers say we know alarmingly little about many threatening volcanoes in the United States. “Currently, many of these volcanoes have insufficient monitoring systems, and others have obsolete equipment,” the USGS said.
Lawmakers have proposed creating a National Volcano Early Warning and Monitoring System to fill in these gaps, but the bill did not pass.
6) The Yellowstone supervolcano is real, but it might never erupt
There’s a massive volcano beneath Yellowstone National Park that has sparked speculation that its eruption could one day lead to devastation unlike anything humanity has ever witnessed, scattering ash over the entire continental United States.
Scientists say the volcano is capable of an eruption so powerful, it could eject more than 1,000 cubic kilometers of earth. That’s more than double the volume of Lake Erie, yielding a score of 8 on the Volcanic Explosivity Index, the highest on the scale. This “super-eruption” would be one of the most powerful explosions in natural history. Cerro Galán in Argentina and Toba in Indonesia are other examples of supervolcanoes.
“Yellowstone has erupted before. And it can erupt again. And if it erupts again, it can be catastrophic to our way of life,” said Gregg.
Its last eruption was 664,000 years ago, before humans walked the earth, and was thousands of times more powerful than an average volcanic eruption, based on the geologic record. It formed a caldera larger than Rhode Island. Were it to erupt like that again, it would coat parts of the US in more than 5 feet of ash.
But the odds of such an event are very low, there are no indications one is coming, and the earth would likely provide us with some advance warning in the form of tremors. “It’s not something that’s going to take us by surprise,” Gregg said.
7) There are many foolish things to do around a volcano. Don’t do any of them.
The fearsome power of volcanoes have inspired awe and reverence for centuries, but they’ve also brought out some of humanity’s worst impulses.
The trope of virgins being tossed into a volcanic crater to appease gods is a Hollywood myth, but there is evidence of human sacrifice on volcanoes in antiquity.
Mummified remains of people, including children, have been found on the slopes of mountains in the Andes. Evidence shows that these people were ritually sacrificed by the Inca civilization.
“The high peaks of the Andes were sacred to the Inca,” researchers wrote in a 2007 study of frozen child mummies. “[S]ome are active volcanoes, from which smoke, noise, and fire could be at times observed, phenomena easily interpretable within superstitious or religious frameworks. Human sacrifice at significant peaks reinforced reverence for locally sacred mountains.”
People have also sought to weaponize volcanoes. During World War II, there were serious proposals to bomb Japanese volcanoes as both a material and a psychological tactic.
“Fear of volcanoes is so thoroughly ingrained in the minds of the Japanese that they have made gods of them,” wrote geology professor Harold Whitnall in Popular Science in 1944. “I believe that explosives dropped down their throats may cause such a vomiting of lava and ash as to hasten the day of unconditional surrender.”
Thankfully, such gonzo proposals never came to fruition. But people may have accidentally triggered eruptions.
In 2006, a magnitude 6.3 earthquake struck the Indonesian island of Java. A company drilling for natural gas 2 miles below the surface on the island saw a borehole suddenly lose pressure before filling with liquid. Drillers sealed the well, but soon hot, steaming mud began bubbling up around the site. This mud volcano, now known as the Lusi mudflow, soon sprayed steaming mud over nearly 3 square miles, killing 20 people and forcing 40,000 to evacuate.
Scientists are still debating whether the eruption was caused by humans or by natural activity.
Another bad idea is throwing garbage into volcanoes. Aside from being horrendously offensive (many volcanoes are considered sacred), it’s expensive to truck waste to lava flows, it’s often not hot enough to properly incinerate waste, it can release hazardous chemicals into the air, and the trash itself can trigger an explosive reaction, as you can see in this clip of a trash bag being thrown into Ethiopia’s Erta Ale volcano:
So while the awesome fury of volcanoes continues to make thrill seekers (idiots) tempt fate, the most prudent course of action is to keep a safe distance.
Update, May 23, 2023, 2:20 pm: This story, originally published in May 2018, has been updated throughout several times to reflect new information, most recently to include the 2023 eruptions of additional volcanoes.