A cinder cone brimming with a lava lake, on the flank of Hawaii's Kilauea volcano, appeared to drain suddenly over the weekend, its molten rock apparently finding outlet in a new fissure that opened farther down slope.

The cone, named Pu'u 'O'o, saw its "floor" plunge 370 feet as the lava beneath found a weak area in Kilauea's flank. The floor actually a thin crust of cooler material that sits atop the molten rock like skin on pudding.

In addition, lava-lake levels in Halema'uma'u Crater, which sits inside the large caldera at the top of Kilauea, dropped significantly as well, according to scientists at the US Geological Survey's Hawaii Volcano Observatory. The observatory is perched near the caldera's edge.

With Pu'u 'O'o as an outlet, Kilauea has been erupting non-stop since 1983, giving scientists and tourists alike a window on how Earth replenishes the thin skin of rock that makes up the planet's crust.

The opening of the crack on Saturday "is quite an exciting development," says James Deiterich, a geophysicist at the University of California at Riverside who studies Hawaii's volcanoes. "For several years the eruption has been very boring. There's been no change in its pattern at all" – just lava slowly rising and falling as Pu'u 'O'o went through its paces.

Dr. Deiterich explains that while he hasn't seen much of the data yet from the weekend breakout, the event appears to be common to volcanoes of Kilauea's type – a so-called shield volcano.

Shield volcanoes tend to ooze relatively thin, gas-poor lava over the tops of their craters or through fissures that open along their flanks as magma pushes up, causing the slopes to develop cracks. Regions where the cracks appear are known as rift zones. Pu'u 'O'o is the most active outlet in Kilauea's eastern rift zone.

The slope where the fissure appeared has been building a bulge for some time, Deiterich says.

Given a choice, lava would rather take the path of least resistance – like a weak spot lower on the slope – than continue to push its mass vertically to overtop the cinder cone's throat.

Although shield-volcano eruptions can generate spectacular fountains of lava – in the case of this weekend's plumbing change, up to 80 feet high – the eruptions tend to be tame compared with the violent blasts from steeper-sloped stratovolcanoes, such as those found in Indonesia, Chile, or the Pacific Northwest, for instance.

Magmas in these volcanoes tend to be a thicker mix of material and richer in gases, leading to explosive eruptions.

Although lava can overtop a stratovolcano's summit as well, the thicker molten goo doesn't travel as far from the cone as does the thinner lavas from shield volcanoes – a trait that gives the shield volcanoes their wide, low-slung look.

But the look can be deceiving. Kilauea's large sibling, Mauna Loa, rises to just over 13,700 feet above sea level. The mountain's roots lie some 16,400 feet below sea level, making it, as well as its slightly taller sister, Mauna Kea, taller than Mt. Everest.

For its part, Kilauea marks the most active part of a hot spot in Earth's crust that is thought to have formed the Hawaiian island chain during the past 28 million years. Researchers say the hot spot consists of a long-lasting plume of magma beneath a weak patch of undersea crust. Magma breaching the crust formed undersea volcanoes, which slowly grew until they could poke above sea level. Hawai'i's volcanoes sit far from the boundaries of crustal plates, where the bulk of Earth's volcanic activity takes place.

Scientists still argue over whether the chain was created as the Pacific Plate moved over the hot spot to give the chain its follow-the-leader pattern or whether the hot spot itself does some of the moving.