Recent events in Hawaii have been producing some remarkable images. But if you’re not a geologist—or resident of Hawaii—you might be wondering what the heck is going on. We’re here to help: while lava spewing from back yards in a subdivision seems wild, it’s not really surprising.

Why is Kīlauea an active volcano in the first place?

The Hawaiian chain of islands (there are also seamounts that no longer reach above sea level) is the result of a “hotspot” in the Earth’s mantle. Hotspots in the Earth's interior are areas of rising hot rock that can turn to liquid hot magma near the Earth’s surface. These hotspots are basically stationary, while tectonic plates slowly slide around on the surface. That means a hotspot will punch a line of eruptions through the plate a bit like a sewing machine.

The Big Island of Hawaii is the youngest in the chain, still straddling the hotspot that fuels its volcanoes—there are five individual volcanoes that make up the island. Of those five, Kīlauea is the youngest, comprising the southeastern edge of the island. Kīlauea’s summit is home to a collapsed crater called Halemaʻumaʻu Crater. In 2008, a new vent appeared in the floor of the crater, which has hosted a lake of roiling lava ever since.

Volcanic activity also takes place along a straight southwest-northeast line known as the East Rift Zone. In 1983, eruptions produced Pu‘u ‘Ō‘ō Crater, which also hosts a lava lake. Eruptions of lava have continued intermittently in the East Rift Zone ever since.

Is this how it erupts normally?

The short answer is yes, but the long answer is worth exploring. The broad, rounded shape of the Hawaiian islands and the nature of their eruptions are due to basaltic magma. Basaltic magma has a low viscosity, which prevents it from piling up like the thicker magmas responsible for angrier, pointy volcanoes like Mt. St. Helens. Instead of erupting explosively and filling the sky with ash, Hawaiian volcanoes tend to simply barf up lava flows—which can generally be escaped at a brisk walking pace.

Kīlauea’s main magma chamber seems to be a few kilometers below Halemaʻumaʻu Crater at its summit. When the volcano’s guts gurgle with a new injection of magma from below, the lava lake in Halemaʻumaʻu Crater rises. The conduits and crevices that constitute Kīlauea’s “plumbing” extend from the summit region over to the East Rift Zone. So after lava rises in Halemaʻumaʻu Crater, the lake sometimes drains back down as the new magma moves on toward the eastern section of the volcano.

Just a few years ago, the town of Pāhoa—just a couple miles from the current eruptions—was in the news as a lava flow creeped in its direction. That eruption, which started near Pu‘u ‘Ō‘ō Crater, involved lava pouring relatively quietly out of cracks called “fissures” that opened in the ground. This is a common fate for magma piped into the East Rift Zone. The lava from these fissures can sometimes fountain high into the air, driven by pressurized gases that also escape through the fissures, but there is no plume of ash.

Hawaii is occasionally capable of more violent eruptions, though. Cinder cones are piles of rubble you might recognize as porous “lava rock” used for landscaping. They are created by frothier versions of fissure flows, with higher gas content that causes material to sputter higher (and more dangerously) into the air.

More rarely, lava lakes like Halemaʻumaʻu can trigger explosive events when they drain. Normally, the magma beneath the lake maintains a sort of demilitarized zone with the surrounding groundwater—a vapor buffer that separates the two. But if the lava lake level falls below the water table, the uppermost groundwater can seep into the open tube and drop down into the magma, flashing to steam. Because the exposed walls of the lava lake crumble without the lava there, a collection of blocks can clog up the neck of the tube. That can allow the pressure of the steam to build until it blasts through.

That’s exactly what happened in 1924, launching huge boulders like cannonballs and producing a fair amount of ash. The lava lake in Halemaʻumaʻu Crater disappeared afterward until one opened up again in 2008.

What happened now? Were there any warning signs?

Almost four weeks ago, it became clear something could be brewing. The number of small earthquakes within the volcano—produced by the movement of magma—ramped up. And as the lava lake at Halemaʻumaʻu began rising, precise satellite and ground sensor measurements showed that Kīlauea was inflating—clear signs that some fresh magma had been delivered. Eruptions don’t always follow volcano inflation, but it’s a darn good sign that the risk has increased.

#Sentinel1 #InSAR result for Apr 19-May 1 (6 pm local) from #Kilauea shows deformation due to ongoing magmatic intrusion. Magma withdrawn from middle East Rift Zone and intruded beneath lower East Rift Zone. No coincident deflation of summit. Update: https://t.co/sfajMPjR8Z pic.twitter.com/szjuyk2b5D — USGS Volcanoes🌋 (@USGSVolcanoes) May 2, 2018

Next, the summit lava lake level fell more than 30 meters on May 3, Pu‘u ‘Ō‘ō Crater’s lava lake also seemed to be up to something, and the first lava fissures opened up in Leilani Estates. While the East Rift Zone continued inflating, the summit of Kīlauea began to deflate. A helicopter flight around Pu‘u ‘Ō‘ō the next day found… no lava lake to be seen.

On May 5, Hawaii experienced its strongest earthquake since 1975—a magnitude 6.9. The earthquake occurred just offshore and only about 5 kilometers below the surface. This was a significant movement on a fault in the rock rather than some minor volcanic indigestion, but it was still related to the movement of magma through the complex plumbing beneath the volcano. The shaking caused by this and other larger earthquakes knocked some rock into the lava lakes, kicking up momentarily frightening clouds of ash.

Meanwhile, more fissures were opening up in Leilani Estates. Most didn’t produce enough lava to reach far from the fissure itself, although a couple produced significant flows. The greater hazard was actually the volcanic gas audibly gushing out of each one, polluting the local air. That’s not to diminish the impact of the lavas, of course—more than two dozen homes have been destroyed, and the 1,700 evacuated residents have had limited opportunities to rescue possessions.

SInce the eruption of the #Kilauea #volcano, sulfur dioxide has spiked on the Big Island. SO2 is a toxic gas that occurs naturally in magma. https://t.co/zFNi0j3Dw5 pic.twitter.com/Sa0cbcsGG3 — NASA Earth (@NASAEarth) May 8, 2018

What happens next?

The fissure eruptions have quieted in the last couple of days, but the rumblings beneath the surface indicate there’s no reason to think they’re done. Eruptions could resume from the existing 15 fissures or from new ones that open up along the line of the East Rift Zone. At this point, it’s impossible to say how much longer this could continue.

But because the lava lake at Halemaʻumaʻu has continued falling this entire time—a total drop of around 300 meters—concern about a repeat of the explosive 1924 eruption is now increasing. If the lava lake falls much farther, it will reach the water table, potentially setting up a steam blast scenario with very little warning. As a result, Hawaii Volcanoes National Park has been closed to keep people out of reach of projectile boulders.

If you’d like to follow the latest news on these eruptions, frequent updates are available from the USGS Hawaii Volcano Observatory and a special event page.