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Post-shield lava flows and cinder cones have buried the earlier shield stage summit caldera. Although a few flows have traveled long distances and reached the coast, post-shield eruptions form large cinder cones and produce short, thick, and pasty flows. Many Mauna Kea eruptions were explosive and produced widespread ash deposits. Some of these ash-producing eruptions may have been triggered by water-rock interactions as lava encountered glaciers at the summit of Mauna Kea.

Most people don't think about glaciers in Hawai`i, but glacial deposits formed high on Mauna Kea during the ice age. Mauna Kea is the only volcano in the Hawaiian chain known to have been glaciated. Mauna Loa was certainly high enough to have had glaciers, but if glacial deposits ever formed, they have long since been buried by younger flows.

Geologists have identified three periods when mountain glaciers covered the summit region. Glacial moraines formed about 70,000 years ago, and younger ones were deposited approximately 40,000 to 13,000 years ago by a more recent ice cap. It has been estimated that glaciers on Mauna Kea had an average thickness of 100 m (330 ft) and could have attained a thickness of 170 m (560 ft). They covered an area of more than 70 km². (27 sq. mi.).

What evidence exists to support the presence of glaciers on Mauna Kea? The most obvious is the presence of moraines or glacial till. Moraines and till are accumulations of boulders, stones, and other debris carried, bulldozed, and deposited by a glacier. The debris consists of volcanic ash, cinders, chunks of lava, `a`a and pahoehoe. Moraines can be further classified as terminal (found at the distal end of a glacier) and lateral (along the glacier's side). Terminal moraines can be seen and are identified from the Saddle Road by the buff-colored, smooth, gently sloped deposits just above the tree line in the vicinity of the Mauna Kea State Park.

Additional evidence includes striations, groove marks, and polish on the upper surfaces of flows within the Mauna Kea Ice Age Natural Area Reserve. The striations form as debris or rocks are dragged across the lava's surface, under the enormous weight of the moving ice. The groove marks formed in a similar manner and differ only in size. Polish occurs when rocks, pulverized into sand, scour the surface of the flows, leaving it smooth to the touch.

The different shape and color of the Poli`ahu and Pu`u Wekiu cinder cones are additional evidence of glaciers. The sides of the cones are steeper than normal, and the cinders are discolored, because glaciers have carved away the sides and left the interior of the cinder cones exposed.

Other cones and their associated flows exhibit evidence of being erupted under a glacier. The evidence includes pillow-like lavas and hyaloclastites (water-quenched lava debris). Pillow lava is formed when eruptions occur under water and can be found at several places on Mauna Kea. One place is near Lake Waiau. The amount of water in a typical Mauna Kea snow fall is not enough to form pillow lavas or hyaloclastites. However, lots of water in the form of a melting glacier can cause pillows to form.

In fact, one such eruption came into contact with glacial ice and resulted in a very fine-grained rock. This lava was highly sought after by the Hawaiians for adzes, because it is fine-grained and vesicle-(puka) free.

Furthermore, these subglacial eruptions are responsible for the formation of the unusually deep Pu`u Pohakuloa and Waikahalulu gulches. The melt water from the glaciers was dammed up behind terminal moraines until they failed catastrophically, resulting in a torrential flood that deeply incised and sculpted the gulches. Sediments from these events underlie the region in the vicinity of the Mauna Kea State Park.

It may have seemed impossible that a place in the middle of the tropics could have had glaciers, but Mauna Kea bears the unmistakable scars of a period of time when the climate was cooler than it is today.

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Volcano Activity Update

Kīlauea summit and Pu`u `O`o continued to deflate slowly, although the changes have been quite small over the last few days. Seismic tremor levels continued to be low. Earthquakes were located mostly beneath Halema`uma`u Crater and the south flank area.

Episode 58 continued as a continuous discharge of lava into a perched channel near the lower end of eruptive fissure D, 2.3 km (1.4 mi) northeast of Pu`u `O`o. The lava forms a north-northeast-trending molten stream about 1.4 km (0.9 mi) long. The stream widens and narrows irregularly, from 90 m (300 ft) to 20 m (70 ft), along the channel length. At its termination, the open channel has built a distributary apron or fan. From there, pahoehoe and `a`a are oozing northeast for another 1 km. Robust but short-lived `a`a flows seep from the lower northwest flank of the channel to nestle against 23-year-old lava from Pu`u `O`o's early days. On the southeast side of the flow field, pahoehoe and `a`a flows encroach stubbornly upon an area of older lava about 0.5 km (0.3 mi) north of Kalalua cone as they did last week. No forested land has burned in the past two weeks, and active lava flows remain almost entirely within the area covered since July 21.

At Pu`u `O`o, no incandescence has been seen on the Webcam at night for the last several weeks. The heavy fume coming from Pu`u `O`o completely obscures any view into the crater. As in years past, Pu`u `O`o likely is serving as a large chimney, beneath which lava is stored briefly and degassed substantially enroute to the erupting fissure. Sloughing of Pu`u `O`o into its own crater since late August has left numerous fresh cracks on the north rim and south flank of the cone.

Vent areas are hazardous. Access to the eruption site, in the Pu`u Kahauale`a Natural Area Reserve, is closed (http://www.state.hi.us/dlnr/chair/pio/HtmlNR/07-N076.htm).

No earthquakes beneath Hawai`i Island were reported felt within the past week.

Mauna Loa is not erupting. No earthquakes were located beneath the summit. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates.