What you need to know about the Tonga volcano and the Pacific's 'Ring of Fire'

The powerful eruption of an undersea volcano near the Tonga islands in the Pacific Ocean sent tsunami waves around the world on Saturday, closing beaches, flooding marinas and activating emergency plans from Japan to California.

Experts say the Tongan volcano is capable of producing an explosion that massive roughly every 1,000 years, akin to a weapons-grade chemical explosion.

The Hunga Tonga-Hunga Ha'apai volcano, which has erupted several times in recent years, is part of the Pacific Ring of Fire, home to some of the world's most active volcanoes.

These events may have just been a warm-up. Volcanologists and officials say there could be further activity to come, with several weeks or even years of major volcanic unrest.

How dangerous is the Hunga Tonga-Hunga Ha'apai volcano?

The official magnitude of the eruption has yet to be finalized, but the height of the ash and gas plume - about 19 miles - indicates it was "very powerful," said Heather Handley, a volcanologist at Australia's Monash University. The associated shock wave and tsunami were reported as far away as Alaska.

Shane Cronin, a volcanologist at the University of Auckland who has researched the Tongan volcano for years, said "we could be in for several weeks or even years of major volcanic unrest" following Friday's eruption. All the signs, he said, suggest the volcano's caldera - a large hollow formed when a volcano erupts and collapses - "has awoken."

"Looking at the chemistry of past eruptions, we now think the small eruptions represent the magma system slowly recharging itself to prepare for a big event," Cronin wrote in the Conversation on Sunday. "It remains unclear if this is the climax of the eruption."

Geological deposits from historical eruptions indicate these episodes once every 1,000 years "involved many separate explosion events," he added.

What was the Tonga eruption's impact?

The blast was powerful enough to generate a tsunami of several feet in Tonga and prompted tsunami advisories across Hawaii, Alaska, British Columbia and much of North America's West Coast, including the states of Washington, Oregon and California.

A four-foot spike in water levels was observed in Port San Luis, Calif., while Arena Cove, Calif., reported a 3.5-foot jump. Crescent City, Calif., got a 2.7-foot spike, and a tsunami of 2.8 feet was seen in King Cove, Alaska.

The eruption was also heard as far away as Alaska. Experts at the National Weather Service in Anchorage and the University of Alaska Fairbanks confirmed that audible booms early Saturday morning local time originated from the volcano. That means the sound traveled more than 5,800 miles.

How does a volcano create tsunami waves?

Volcanic eruptions can generate tsunami waves because of the atmospheric and ocean shock caused during the eruption. They can also be triggered by underwater landslides, and when the volcano's magma chamber empties and the sides and top of the volcano collapse inward.

About 5 percent of the Earth's recorded tsunamis have been produced by volcanoes, according to Andrew Gissing, an expert who was involved in developing Australia's tsunami warning systems after the 2004 Indian Ocean tsunami. That devastating tsunami, triggered by a 9.1 magnitude quake off the coast of Indonesia's Aceh province, killed around 230,000 people in Indonesia, Sri Lanka, India, Thailand and nine other countries.

One of the deadliest examples in modern history of a volcano triggering a tsunami was the Krakatau disaster in Indonesia in 1883. The volcano collapsed into the sea, generating a tsunami more than 120 feet high - tall enough to submerge a six-story building. It destroyed 300 towns and villages, and killed more than 36,000 people.

What is the Ring of Fire?

The Pacific Ring of Fire extends in the shape of a horseshoe from the southern tip of South America, along the west coast of North America, across the Bering Strait, down through Japan and into New Zealand. It is where the Pacific Plate meets many surrounding tectonic plates.

The United States Geological Survey describes the ring as "the most seismically and volcanically active zone in the world."

The Ring of Fire contains about 75 percent of the world's active volcanoes and is also the source of about 90 percent of the world's earthquakes, according to the USGS.

Among the most notable Ring of Fire eruptions of the 20th century was the 1991 eruption of Mount Pinatubo in the Philippines, which sent fine ash and gases high into the stratosphere, forming a large volcanic cloud that drifted around the world. Scientists attributed cooler-than-normal temperatures recorded worldwide and brilliant sunsets and sunrises to this eruption.

Andrew Tupper, a natural disaster consultant who previously led Australia's Darwin Volcanic Ash Advisory Center and tsunami warning center, said satellite data from NASA indicates the Tonga eruption reached nearly 20 miles in altitude on Saturday, making it one of the highest eruption clouds observed, just shy of an estimated 25 miles from the larger Pinatubo eruption in 1991.

The most powerful earthquake in recorded history - a magnitude 9.5 quake in Valdivia, Chile, on May 22, 1960 - was also in the Ring of Fire. More than 1,600 people were killed, many because of large tsunamis that swept from Chile to the Pacific and the United States. A tsunami caused by the earthquake killed 61 people and caused severe damage in Hawaii.

How many underwater volcanoes are there in the world?

Hunga Tonga-Hunga Ha'apai is technically classified as an underwater volcano, but it consists of two small, uninhabited islands, Hunga-Ha'apai and Hunga-Tonga. Before the eruption, its cone poked about 300 feet above the water - but hiding beneath the waves is a massive volcano, about 5,900 feet high and 12 miles wide.

The most productive volcanic systems on Earth are underwater, according to Oregon State University. The magma and lava they produce create the edges of new oceanic plates and supply heat and chemicals to some of the Earth's most unusual and rare ecosystems.

Most submarine explosions occur deep underwater and don't disturb the ocean surface, resulting in lava flows along the ocean floor. Still, hundreds of islands around the world - from the Asia-Pacific to Iceland - have been created by volcanoes.

"If an estimate of 4,000 volcanoes per million square kilometers on the floor of the Pacific Ocean is extrapolated for all the oceans," then there are more than a million underwater volcanoes, the Oregon State researchers say. "Perhaps as many as 75,000 of these volcanoes rise over half a mile (1 kilometer) above the ocean floor." (Many of these volcanoes are extinct.)

How many active volcanoes are there in the world?

There are about 1,350 potentially active volcanoes worldwide, according to the USGS, including 161 potentially active volcanoes in the United States and its territories.

Recent eruptions include a volcano on La Palma, one of Spain's Canary Islands, that sent streams of red molten lava downhill toward villages and a column of thick, dark smoke into the sky in September, forcing some 5,000 residents to flee.

The Washington Post's Matthew Cappucci contributed to this article.