The eruption of the Tonga volcano was a unique event in the millennium
From the surface of the water, the islands of Hunga Ha’apai and Hunga Tonga don’t look like much. These are two uninhabited islands, which were nearly connected by a volcanic crater, rising only about 300 feet above the sea. They give little indication of what lies below – a massive volcano of 12 miles wide and 5,900 feet high.
The 15thand In January, the Hunga Tonga-Hunga Ha’apai volcano erupted in what is probably the most powerful volcanic eruption in 30 years. An ash plume swelled 20 km into the atmosphere. From space, satellite imagery has revealed a deadly gray flower of smoke and ash blooming amidst the pristine blue sea. Ashes and stones rained down on the area. Twenty minutes after the eruption, Tonga’s capital, Nuku’alofa, was hit by a 1.2 meter high tsunami. A pressure wave created by the explosion traveled around the world and sonic booms were heard as far away as Alaska.
The full damage is not yet known. The island of Tonga is largely cut off from the rest of the world, as the eruption cut the main cable which carries the majority of communications to and from the island.
What caused the Tonga volcano to erupt?
Eruptions of this magnitude from this volcano are a once-in-a-millennium event. Although we don’t know exactly why this volcano erupted, many factors may contribute. To fully understand, we need to understand the physics of what is happening underground.
There’s a lot going on in the magma chamber under the ocean floor. The Tonga volcano is what is called a subduction volcano, formed when the Pacific plate slips under the Tonga microplate. The water-rich Pacific subduction plate releases some of this water into the mantle. This causes the melting temperature of the rock to decrease, creating liquid magma. Over time, magma slowly fills this huge chamber, a process that takes about 1000 years. Sure enough, radiocarbon dating showed that the last major eruption of this volcano occurred around 1100 AD.
Molten rock is less dense than solid rock. This causes upward buoyancy relative to its surroundings. As magma is added to the reservoir, more pressure builds up on the chamber walls. If the point of least resistance is the top of the magma chamber, the magma will explode outward.
What causes powerful blowouts?
In 2009 and 2014-2015, the volcano experienced much smaller eruptions. In 2016 Professor Shane Cronin, a volcanologist at the University of Auckland, and his colleagues traveled to the Tonga volcano and found that these eruptions originated from the rim of the volcano. In fact, there was a much larger caldera 150 meters below the waves. It is from this caldera that the most recent eruption originated.
If magma gradually enters ocean water, a layer of vapor can build up, which insulates the magma from the water. This works to slowly cool the magma as it exits. However, if magma enters the ocean too quickly, no vapor layer can form. “Hot magma [comes] in direct contact with cold water. Volcano researchers call this the ‘fuel-coolant interaction’ and it’s akin to weapons-grade chemical explosions,” Cronin explains in The conversation. The result looks like a chain reaction in which hot new magma constantly comes into contact with cold seawater.
Gases like water, sulfur dioxide, and carbon dioxide dissolved in magma also play a role. If the magma moves too quickly towards the surface, the the pressure in the gas bubbles increases too quickly. When the bubbles reach the surface, the pressure is released and they expand explosively. This may be a factor that differentiates explosive eruptions from progressive eruptions.
This seemed to be the case for the 2009 eruption. Monash University volcanologist Dr. Heather Handley compared the composition of lava from that earlier eruption. “We could see from the rock chemistry that the magma from this eruption was moving quickly to the surface and was holding its gas too,” she said. ABC Sciences.
The water depth also seems to be perfect for a massive explosion. Deeper, and the ocean would have removed some of the power.
Why do volcanic eruptions cause lightning?
As if a massive eruption wasn’t scary enough, consider this. In three hours during the eruption, there were 400,000 lightning strikes. It’s 100 per second.
These lightning strikes are the result of static electricity. Lower in the volcanic plume, ash particles are rubbing against each other. Higher in the plume, the abundant water from the explosion freezes once it is high enough. (Remember, the plume rose about 20 km, which is fine in the stratosphere, where the temperature is about -50 to -60 C°.) Friction from collisions between ice particles increases the static load.
Will the eruption of the Tonga volcano have an impact on the global climate?
Historically, powerful volcanic eruptions can disrupt the global climate. Sulfur dioxide can cause acid rain and increase the albedo (reflectivity) of clouds. Because of this, more sunlight is reflected back into space, cooling the atmosphere. Even though the Tonga volcano released 400 million kg of sulfur dioxide into the atmosphere, it is not enough to affect the global climate.