Geology - the Alaska Volcano Observatory:

In support of public land-use planning, development of emergency response plans, and general public awareness of the nature of volcanic activity in Alaska, AVO is responsible for assessing the full range of potential hazards at specific volcanic centers. Hazard assessments include a description of the eruptive history of a given volcano, explanations of likely eruption scenarios, and determination of probable impact zones for the range of expected hazards.

These assessments are based on helicopter-supported field investigations of one to several seasons and on laboratory investigations of the composition and chemistry of the deposits. A suite of preliminary hazard reports for the volcanoes which AVO actively monitors have been published, or are in various pre-publication stages. AVO also conducts basic volcanological investigations to determine the conditions of origin, rise, and eruption of magmas. Geologic investigations also provide a framework within which to interpret seismic and satellite data.

AVO Geology in the field:

The following is a daily project log from the 1996 summer mapping of Makushin Volcano, Unalaska Island, Alaska.

Day 1: Sunday July 7 - V. McConnell, C. Nye, K. Bean, A. Lockhart, A. Roach arrive in Unalaska / Dutch Harbor to broken clouds, blue sky, warm temperatures and a light breeze. Began to move into the apartments and get set up. The helicopter and pilot - Gary Brogdan - arrive at about 11:30 pm after a twelve hour trip from Homer.

Day 2: Monday July 8 - Weather begins to change at midnight, by morning there are 30 to 50 knot winds and horizontal rain. Wind drops and rain continues through the day. B Hammond, M Garces, J Beget arrive in the afternoon, along with A Lockhart's lost luggage and equipment.

Day 3: Tuesday July 9 - Worked on final assembling of seismic equiptment enclosures. Worked on power hookups at the FAA downlink site. Prepare the first field location. Brief recon flight by geologists and geophysicists.

Day 4: Wednesday July 10 - Finished site preparation for, and cemented in first enclosure. Tried to recon the long repeater lines, worked on station assembly in the warehouse. Geology crew spent wet afternoon getting acquainted with the thick Holocene volcanic deposits in upper Makushin Valley.

Day 5: Thursday July 11 - Enclosures, batteries, and cement for three more stations slung to near their final sites. Station preparation continues. Geologists continue to work in upper Makushin Valley - the surprise for the day is finding deposits from 28 large mid- to late-Holocene eruptions. Low clouds and uncertain weather prevent start of work on the most poorly known, and probably youngest, part of the volcano, which is in the northwest sector.

Day 6: Friday July 12 - Most of the heavy slinging is done. Installation of seismic components remains. The crew is feeling pretty much on top of it.

Day 7: Saturday July 13 - Enclosures at the northwesternmost site and a repeater site on Table Top were installed. Geologists began work on a package of young deposits on the northern flank of Makushin. These deposits appear to record a number of different major Holocene eruptions of varied types and ages. This is also the most poorly known sector of the volcano.

Day 8: Sunday July 14 - Enclosures on the southern side of the volcano were installed. Geologists began reevalulating upper Glacier Valley deposits. Lahars and debris avalanches of mid to late-Holocene age (as young as a few thousand years) occur in the upper valley. Initial inspection of the hot springs and fumaroles in the area show some changes since the last systematic survey over ten years ago.

Day 9: Sunday July 15 - The Nateekin station is transmitting now. Geologists continued to map a debris avalanche down Glacier Valley, on the west side of the volcano.

Day 10: July 16 - Geologists working in an unnamed valley draining the southwest corner of Makushin found previously undescribed Holocene valley-filling lava flows and a complicated package of lahars, tephras, and another debris avalanche. This further reinforces the newfound appreciation of the vigorous nature of Makushin volcanism in the last few to several thousand years. Seismic station installation continues.

Day 11: July 17 - This valley which comes to the coast at Bishop Point has long been known to contain Holocene volcanic fragmental debris. Today's reinvestigation found lava flows of Holocene age sandwiched between probable Pleisotocene deposits and the pyroclastic debris. We were also able torecognize several different ages of mid to late Pleistocene flows.

Day 12: July 18 - The last two days provided more new information about the Holocene history of Makushin. A package of previously unrecognized post-glacial lava flows was found at the head of the valley ending at Bishop Point. Two very young vents - one of which fed a substantial lava flow, were found high above Pt. Kadin. A lateral blast and debris avalanche package of Holocene age was found at Koriga Point. The antenna mast at the receive site was erected. Seismologists continue with in-town wiring of seismic components. Field installation is slowed, at least in part because Andy has the crud.

Day 14: July 20 - Bright sunny and moderately windy day. Patchy low clouds prevented recovery of tools and parts from the southern repeater station on both attempts to reach the station. Geologic work concentrated on the Wide Bay Cone, a prominent cinder cone easily visible from Dutch Harbor.

Day 15: July 21 - Our first day of dense fog in town. Jim Beget and Kirby Bean found some exciting tephra sections in town. The seismology crew worked on the receive site.

Day 16: July 22 - Finally the southern repeater site cleared and the seismologists recovered tools and parts and got a lot of work done. Five of the six seismic stations are fully functional. The sixth needs some (hopefully minor) re-engineering of the telemetry. Geologists had a hot, sunny, glorious day chasing postglacial deposits among the flowers of various unnamed valleys on the volcano's northwest flank.

Day 19: July 25 - All eight tones from the six seismic stations are now being received in Dutch Harbor (YES!!!!). Phone link is still not operational. Three fumarole gas samples and two sets of thermal water samples have been collected. The fumaroles are superheated, but just by a degree or two. The 152C fumarole has cooled down. Geologists continue to unravel the complex postglacial history of flows, explosion pits, debris avalanches, and tephra recorded by deposits on the northern side of the volcano.

Day 22: July 28. - Seismic signals from all eight channels of the new sites are now being recorded at the AVO seismology lab in Fairbanks. Data are recorded on masscomps and in triggered and continuous mode on the PC's.

Geologists continue to make good progress on mapping the Pleistocene and Holocene geology of the volcano. One exciting result is the discovery of a large region of very fresh ground cracks which probably formed during the mid-March seismic swarms, when in the course of several days thousands of earthquakes were felt.

Day 24: Monday, July 30 - Angie Roach and John Eichelberger reached the summit caldera of Makushin Volcano. The big surprise was abundant evidence of a recent - probably late last winter - phreatic explosion. A thick, slippery, gray mud surrounds the crater and is 5 mm thick, with clasts to 4 cm, at 1km to the south. On the north side, a well-defined rim of ejecta lies at the crater's edge, atop the intracaldera glacier. In the near-field, ejecta blocks range upward to 1 m in diameter. Some blocks were obviously hot, as they melted their way as much as a meter into the underlying ice. The phreatic mud layer lies within fresh-appearing corn snow, no more than 50 cm below the present surface, and therefore almost certainly within the 1995-96 accumulation.

Although a broad and opaque steam plume fills most of the 300-m-diameter crater, it was occasionally possible to see inside. The crater lies at the west base of an andesitic lava dome, which protrudes through the ice. The west side of the crater rim is ice, although ground, in this case fragmental material, is exposed as an inner rim 10 m to 30 m deeper. This west wall is covered with myriads of vigorous steam vents, most of which have bright yellow sulfur around their orifices. At the bottom of the crater is a steaming 100-m-diameter lime green lake. Irregular lines of clear water within the opaque body attest to vigorous upwelling. There is a strong sulfur smell, accompanied by the unpleasant effects of running eyes and nose and coughing. A blue haze pesists across the caldera, beyond the steam plume. Our pilot reports that it is visible as far as 10 km from the vent. We measured no temperatures above the boiling point, but the most vigorous fumaroles were so energetic that approaching them was out of the question.

The crater has clean walls and is almost devoid of talus. In comparison with air photos taken in the early eighties, it appears to be larger and deeper and to have extended farther south. Judging from the clean layers of snow exposed in crevasses, the recent phreatic explosion is an unusual event that formed the present crater and affected the entire caldera, but not the volcano's flanks.

The caldera itself is an inspiring place. A black, cliff-forming, scoria-agglutinate caps the ridges that compose the rim. To the southwest, the rim of Okmok Caldera rises through Aleutian fog. To the northeast, Akutan, a smaller-scale Makushin, steams endlessly through heavy seas. A dike, exposed as a black fin by wasting of hydrothermally altered rock, cleaves the caldera rim and strikes directly toward Akutan, in ignorance of the direction of regional least-principal stress. The high outer slopes of the volcano are mantled with black scoria, which apparently remobilized and flowed a short distance after deposition. To the south, only the white hydrothermally altered clasts of the recent phreatic explosion salt this blackened skin.

Day 26: Thursday, August 1 - The small summit caldera of Makushin is roughly 3 km in diameter. A rubble-covered dome (?) rises about 150 m above the glacier ice which fills the caldera, and vigorous fumaroles jet from its flanks. A crater in the top of the dome is filled with a lime-green lake about 40 meters across, visible only when steam clouds briefly swirl away from the crater. Surrounding the rim of the dome's crater are large pieces of older lavas and muddy sediment with numerous marble-sized accretionary lapilli, which were explosively ejected sometime within the last year. Northwest of the main dome a smaller crater in the snow also emits sulfurous fumes from several fumaroles. This, and another dry crater to the northeast, are each about one-half kilometer in diameter.

Previous descriptions of the summit area have not mentioned either a crater lake or extensive bare, steaming, ground. The summit region seems to be hotter and more active than in the recent past.

Day 29: Sunday August 4 - The four remaining geologists continue to plug along, using these final days to check contacts and finish collecting samples. Jim Beget leaves this afternoon. He has provided me with a preliminary unit description and breakdown report. He and Kirby spent the last couple days completing their survey of the valleys on the northwest side of the volcano and tracing out a set of hydrothermally altered clays that were deposited as surges on the Lava Ramp.

Angie and I continue our survey of the lavas in the high country, searching for any that appear to be deposited post-glaciation. Our big news is I observed a distinctive gray deposit appearing beneath the rapidly melting snow on the southwest side of the volcano. Upon investigation we found a gray ash deposited in an arc reaching from 4000' to 2700' elev (this is now on bare rock as the snow has melted away) and covering about 3 miles radius. It is not windblown as it obviously mantles the entire area and continues to appear in the snow as the yearly snow melts off. It does concentrate in patches on the bare rock and we collected a bagful. It looks in hand sample identical to the ash and lapilli deposited around the summit dome. This would indicate at least one of the recent phreatic eruptions was of enough force to deposit an ashfall mantle outside of the caldera. It is of an earlier timing than the deposit observed at the summit that is sitting on new snow, as this one is only appearing as the firn melts away. It must be within the last year though as this fine ash would not stay in stratigraphic section as the snow melts and it is carried away by wind and water.

The big questions are, Is this the same material we observe at the summit? Does this represent the hollowing out stage that has allowed a summit lake to appear? And, of course, is the any juvenile material in it? Angie will be addressing the latter question as soon as we get out of the field and can get samples prepared. - VSM

Day 32: Wednesday August 7 - We have successfully completed the Makushin field season. And we finished with a bang! After being skunked yesterday by fog to the deck we were treated to severe clear all day. Angie and I were able to get back to the south side to have a more detailed look at the ash deposit we observed last week. With dogged determination we dug snow pit until we were able to find the ash in stratigraphic section and got good thicknesses from several elevations. I would not consider this an isopach map but we were ascertained that the ash layer thickens toward the volcano and we noted the limits of deposit. We also nabbed a couple more samples of the deposit in the snow, messy. In addition to chasing the gray layer we identified two more landslides and another rock glacier.

Kirby was busy also don't doubt that. He spent his time meticiously sampling the 29 peat site at Sugar Loaf.

All in all a very positive ending to a super field trip. My hat's off to both Angie and Kirby for excellent work and good attitudes. They are both fine geologists.

Signing off, Vicki