MASSIVE COLLAPSE OF CERRO QUEMADO 1200 YEARS AGO

About 1,200 radiocarbon years B.P., Cerro Quemado erupted explosively to form a debris avalanche, a laterally-directed pyroclastic blast, and pyroclastic flows. The sequence of events during this episode of volcanism was: shallow intrusion or extrusion and cooling of magma, disruption and failure of the edifice, laterally-directed explosions due to disruption and fragmentation of hot dome rock, pyroclastic flows derived from secondary flow of hot lateral blast deposits down steep slopes, and subsequent rain-induced lahars. The abundance of massive prismatically-jointed dome rock and vitreous charcoal in the pyroclastic blast and flow deposits suggests that the source rock was hot. The lack of vesicularity in juvenile rock suggests that the magma had cooled at very shallow depths before eruption. An alternate hypothesis consistent with these observations is that the blast was formed of hot rock that had already been emplaced at the surface as a dome.

The age of the edifice collapse is constrained by 5 radiocarbon ages from charcoal contained in the genetically-related pyroclastic blast and flow deposits. The radiocarbon ages range from 980 yr. B.P. to 1,440 yr. B.P. and cluster around 1,200 yr. B.P. (Conway et al., 1992). The 1,200 yr. B.P. age corrected for temporal variations in atmospheric CO2 is about 1,150 yr. B.P. (Stuiver and Pearson, 1986).

(Vallance et al., in press)

Geology of Cerro Quemado and environs.

Debris Avalanche

The debris avalanche underlies 13 km² of the Llano del Pinal valley, extends about 6 km north of its source, is 5-7 m thick at its terminus (average thickness is 10 m), and can be traced to the scarp on Cerro Quemado. The avalanche deposit is characterized by a hummocky surface, with more than 150 hummocks, closed depressions, and boulder-strewn fileds. Hummocks are generally circular to oval in plan view but may be elongate and oriented transverse of longitudinally to the axis of the scarp; they range from 4 to 15 m high and from 16 to 500 m in diameter.

Quarrying of hummocks for road aggregate has exposed numerous cross-sections. Hummock deposits are unstratified, unsorted and massive, and comprised of subangular boulders up to 4 m in diamter in matrix of pebble-size clasts and sand. Both block and matrix include more than 95% Cerro Quemado rocks. Other inclusions are diatomite adn ripped up soil fragments. Diatomite clasts measure up to 1-2 m across and apparently were bulldozed from a dry lake bed on the floor of Llano del Pinal.

The volume of the avalanche deposit is 0.13 km³, identical to that enclosed by the scarp on Cerro Quemado (which averages 1.0 km wide, 1.5 km long, and 0.087 km deep). The distribution, composition, and morphology of the deposit are also consistent with collapse of Cerro Quemado. However, the relative timing and cause of dome collapse is uncertain. The avalanche caldera is located in stage 1 material but the fresh, uneroded appearance of the scarp zone and the absence of dense vegetation or tephra on the post-collapse dome (Qcq3dm) suggests a post stage 2 event. Moreover, the proximity of the avalanche caldera to the stage 4 event (Qcq4) indicates post stage 2 migration of the volcanism from the south to the central part of the dome complex. A possible mechanism for dome failure is shallow intrusion of magma and consequent dilation of the edifice causing oversteepening and ultimately failure of the west flank of the dome. This, however, suggests a somewhat greater role for endogenous growth at Cerro Quemado than is apparent from dome deposits.
(Conway et al., 1992)

Lateral Blast

The pyroclastic blast deposit covers about 40 km² in a discontinuous sheet extending west from the vent and more than 500 m up onto the opposing slopes of Siete Orejas. The distribution of the deposit subtends an arc of 110 degrees, is symmetrical about an axis of N70W that also matches that of the center of the avalanche scarp, and is thus consistent with a source in the scarp area. The sharp to transitional contact, incorporation of avalanche material, and the lack of a weathering profile is consistent with contemporaneous deposition of the blast and avalanche.

The deposit is typically fines-deficient and contains carbonized twigs and stems and ripped-up soil. It is gray, unstratified and unsorted, and contains friable, fines deficient, openwork rock debris with angular and sub-angular blocks to 1 m in diameter. Vesicular blocks and lapilli and sparse breadcrust bombs suggests a juvenile component.(Vallance et al., in press)

These vesicular juvenile rocks and charred stems indicate that the fines-poor pyroclastic flow was hot, and its wide distribution and the absence of fine ash suggests it was energetic and highly inflated. All of this evidence suggests that the fines-poor deposit resulted from a lateral blast. (Conway et al., 1992)

The deposit is normally graded near its top, especially on hillsides further from its source. On hillsides about Llano del Pinal, the deposit generally becomes finer-grained away from the source, but nevertheless contains sparse coarser-grained bombs. In its distribution and texture, the blast deposit strongly resembles the deposit of the Sugar Bowl lateral blast at Mount St. Helens (Crandell and Hoblitt, 1986). (Vallance et al., in press)

In the valley and on opposing hillsides, the blast deposit is thin or absent on stoss sides of objects like hummocks or on ridges and thick (up to 2 m) on the lee sides of hummocks or in depressions. A thin veneer (<10 cm thick) mantles the east flank of Siete Orejas, to 3100 m. If the deposit has an average thickness of from 10 to 25 cm, than its approximate volume is 0.004 to 0.010 km³ (Conway et al., 1992).

Secondary Pyroclastic Flows

A pyroclastic flow deposit overlies the lateral blast deposit in the Llano del Pinal valley (Qcq3pfa). It is light gray to brown and composed of angular to sub-angular blocks (10-25 cm in diameter) supported in a matrix of fine-grained ash. The deposit is poorly sorted and generally massive and ungraded, except near its base where it is inversely graded. Fines-poor, openwork cylindrical pipes, which operated as channels for escaping gases, are common. Like the underlying fines-poor lateral blast unit, this deposit contains over 95% Cerro Quemado andesite and dacite. Vesicular and prismatically jointed blocks and lapilli are common, as are charred wood fragments.

The pyroclastic flow deposit forms a discontinuous sheet, extending from the south end of the valley to the lower north-central end, and is thickest where it abuts Siete Orejas. Thickness ranges from 22 cm to 6 m and is greatest in depressions. Llano del Pinal valley slopes northward but the pyroclastic flow is more widely distributed and thicker in the higher part of the valley. This distribution suggests a source other than the scarp zone on Cerro Quemado, because a pyroclastic flow from the scarp zone should have inundated the central Llano del Pinal and flowed north toward Quezaltenengo. We suggest that the fines-rich pyroclastic flow was derived from secondary flowage of the laterally directed blast deposit from surrounding hillslopes. (Conway et al., 1992)

A fines-rich pyroclastic flow deposit that overlies the lateral blast deposit in the Llano del Pinal valley has a distribution suggesting a source other than the scarp area. If the pyroclastic flow emanated from the crater, it should have inundated the central Llano del Pinal and then moved north toward Quezaltenango, but the deposits are principally found up valley to the south and also at the base of Siete Orejas. This distribution implies a different origin. We suggest that subsequent gravitational flow of the hot, lateral-blast debris from the surrounding steep hill slopes caused the fines-rich pyroclastic flow -- an origin similar to that of the valley-ponded pyroclastic flows associated with the 1980 lateral blast at Mount St. Helens (Hoblitt et al., 1981). (Vallance et al., in press)

Debris Flows

Debris flow deposits are sparse at Cerro Quemado. However, debris flow are common on many volcanoes and are produced in a number of ways. At Cerro Quemado, debris flow are most likely to occur when unconsolidated clastic deposits are mobilized by rainfall. Debris flow typically threaten drainages and valley bottoms and the primary danger to people is from burial, whereas buildings and bridges could be crushed, buried, or swept away by viscous debris flows. Rain-induced debris flows are more likely to occur during the rainy season, from March to November, but rainfall events large enough to remobilize unconsolidated debris could occur during the dry season.

Deposits of small-volume debris flows crop out of the base of the avalanche scarp and in the Llano del Pinal valley near the base of the Siete Orejas. The deposits are typically unstratified, poorly-sorted mixtures of boulders and pebbles in a matrix of silt- to clay-sized particles. The deposits probably originated when loose debris from stage III eruptions moved downslope during torrential rains.
(Conway et al., 1992)