Volcano Watch — How'd you like to spend Christmas on Christmas Island?

Sources/Usage: Public Domain. View Media Details

One is a tiny island near Java, while the other is in the South Pacific, about 1,000 miles south of Hawai`i. Both are atolls: ancient seamounts, with the volcanic edifices long since sunken under the sea and fringing coral reefs grown over their tops.

It would be best to tell Santa you'll be in Kiritimati, which is the South Pacific island's name in the local language. The island was not populated when Captain Cook landed there on Christmas Eve, 1777, and named it "Christmas Island," though there is evidence of ancient Polynesian settlement. Currently, somewhere between 5,000 and 10,000 people live on the approximately 125 square miles of land that surrounds about the same acreage of lagoon on Kiritimati. It is the largest coral atoll in the world, but, as with all low-lying islands, the amount of land is decreasing as sea level rises.

Kiritimati is part of the Line Island Group, a linear chain of volcanic islands and seamounts that was, until fairly recently, thought to be analogous to the Hawaiʻian-Emperor chain of hotspot volcanoes. However, unlike Hawai`i, the rocks of the Line Islands do not become progressively older with distance. Instead, there were two major episodes of volcanism, each lasting about 5 million years, with about 8 million years between them. The most recent eruptions occurred about 70 million years ago.

For many years, geologists thought that the island chains of the South Pacific were formed by lava from hot spots deep in the Earth's interior. The Hawaiʻian chain is the classic example of this kind of volcanism. Magma from a hot spot melts a path up through the crust and erupts on the surface, forming a volcano. These hot spots are thought to be stationary, but the Earth's crust is not. As the crust moves over the hot spot, a line of volcanoes forms, one after another, in the direction of the crust's movement.

However, in the South Pacific, it is not uncommon for the lines of volcanoes to be aligned in various directions, or form groups rather than lines, and the ages of the volcanoes do not always decrease in order along chains.

If not formed by the Pacific Plate moving over a hotspot, what causes such linear island chains as the Line Islands? The current thinking is that they are related to the broad upwarping of the crust in that area - called the South Pacific Superswell. The mantle beneath the swell is hotter, perhaps due to upward convection in the region, and melting (and thus volcano formation) occurs at pre-existing, sometimes linear, weak zones in the crust.

Wherever you spend Christmas, the staff and associates of the U.S. Geological Survey's Hawaiʻian Volcano Observatory extend a warm and cheerful holiday greeting to all of the readers of our column. Figure caption: Happy holidays from HVO! Card drawn by HVO alumnus Dr. Ken Hon, currently a professor of geology with UHH.

Volcano Activity Update

During the past week, the number of earthquakes located beneath Kīlauea remains at levels typical of the current eruption. Inflation continues. Earthquakes are still occurring sporadically beneath Lo`ihi.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone. Lava continues to flow through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean, with a few surface flows breaking out of the tube. In the past week, flows were active intermittently on the upper reaches of the tube, about 1 kilometer (0.6 miles) downslope of Pu`u `O`o, and on the steep slopes of Pulama pali, above the coastal plain. Surface flows on the pali are visible at night (weather permitting) from the end of Chain of Craters Road.

As of December 22, lava is entering the ocean at East Lae`apuki, in Hawai`i Volcanoes National Park. The active lava bench continues to regrow following the major collapse of November 28. Access to the ocean entry and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were four earthquakes beneath Hawai`i Island and Lo`ihi reported felt within the past week. A magnitude-3.2 earthquake occurred at 5:22 a.m. on Friday, December 16, and was located 6 km (4 miles) southeast of Ho`okena at a depth of 14 km (9 miles). Just a few minutes later at 5:28 and 5:30 a.m., magnitude-3.3 and a 3.2 earthquakes occurred and were located about 3 km (2 miles) southeast of Lo`ihi; they were felt in a highrise on Bishop Street, O`ahu. A magnitude-2.9 earthquake occurred at 9:37 a.m. on Tuesday, December 20, and was located 5 km (3 miles) south of Volcano; it was felt at Keanakako`i and Mauna Loa Estates.

Mauna Loa is not erupting. During the past week, the count of earthquakes located beneath the volcano remains at low levels. Inflation continues but has slowed over the past several weeks.