Tephra and Deposits
The deposits at Mount Mayon are from airfall, pyroclastric flows, rain triggered debris flows and lava flows. The resulting material in these deposits are unconsolidated tephra, sized from ash to small boulders. The majority of deposited tephra is a result of pyroclastic flows and lahars. This includes juvenile material composed of uniform porpheritic Olivine- Pyroxene and hypersthene-augite andesites. Phenocrysts of Plagioclase and Pyroxene within these rocks constitute more than half the rock and range from 1-4mm.
Figure 6 (above).
A handful of the juvenile material found at the base of Mount Mayon
Fallout ash was deposited over a few hundred square kilometers with a thickness of up to 5cm. Even at a distance of 4 km from the crater, the maximum diameter of airfall material is 3 mm and most is less than half a millimeter. At 1km from the crater, the pyroclastic flow deposits are a unconsolidated mixture of crystal-rich ash, lapilli, and blocks. Some blocks are angular and some have a characteristic rounded breadcrust surface . The texture of the deposits indicates that the ejected material was plastic and welded into a carapace capping the volcano. This material formed short lobes as it flowed down the slopes. One block more than 25 m long was observed at an elevation of 600 m on the southwest slope. The median size was 28 cm; 42 were breadcrust blocks, 12 were angular blocks. The median size of six samples of ash flow material containing lapilli less than 16 mm was found to range from 0.4 to 1.I mm. These data indicate that the deposits are particularly coarse and poorly sorted as compared to other pyroclastic flows. Such coarseness probably results from the high crystal-toglass ratio, the short transport distance, and the extremely steep slopes of the volcano (This summary is from Moore et al).
Figure 6 (above).
A handful of the juvenile material found at the base of Mount Mayon
Fallout ash was deposited over a few hundred square kilometers with a thickness of up to 5cm. Even at a distance of 4 km from the crater, the maximum diameter of airfall material is 3 mm and most is less than half a millimeter. At 1km from the crater, the pyroclastic flow deposits are a unconsolidated mixture of crystal-rich ash, lapilli, and blocks. Some blocks are angular and some have a characteristic rounded breadcrust surface . The texture of the deposits indicates that the ejected material was plastic and welded into a carapace capping the volcano. This material formed short lobes as it flowed down the slopes. One block more than 25 m long was observed at an elevation of 600 m on the southwest slope. The median size was 28 cm; 42 were breadcrust blocks, 12 were angular blocks. The median size of six samples of ash flow material containing lapilli less than 16 mm was found to range from 0.4 to 1.I mm. These data indicate that the deposits are particularly coarse and poorly sorted as compared to other pyroclastic flows. Such coarseness probably results from the high crystal-toglass ratio, the short transport distance, and the extremely steep slopes of the volcano (This summary is from Moore et al).
Geochemistry and Gases
TABLE 1- Chemical, modal, and mineral analyses of rocks from Mayon Volcano.
What is it? Chemical Analysis (Weight %) Model analyses (Volume %)
What is it? Chemical Analysis (Weight %) Model analyses (Volume %)
SiOz
AI20~ Fe20~ FeO MnO MgO CaO Na20 K~O H20 + H20- TiOz P~O~ Total: |
55.09
18.90 3.40 4.62 0.15 4.03 8.56 3.42 1.11 0.00 0.04 0.63 0.26 100.21 |
54.90
19.04 Z96 4.81 0.15 4.05 8.62 3.40 1.13 0.00 0.04 0.64 0.26 100.00 |
50.25
18.95 4.90 4.19 0.14 4.00 9.60 5.19 1.38 n.d. n.d. 0.89 0.34 99.83 |
Petrology
Eruption History
The eruptions of Mayon are usually "Vulcanian" or explosive in nature, although it is sometimes necessary to categorize the eruption as "Strombolian" because the majority of material is ejected scoria from a pulsing lava fountain. The symmetry of the volcano indicates that the eruptions have always occured at the central vent and that these have never been violent enough to destroy Mayon's form.
TABLE 2. Eruption history of Mount Mayon in the past 100 years.
TABLE 2. Eruption history of Mount Mayon in the past 100 years.
Thorough descriptions of Mayon eruptions:
"The first indication of a revival of activity on Mayon Volcano
after the eruption of January-February, 1947, was the appearance of
a glow from the summit crater at 2200 hours, April, 20, 1968, local
time. At 0900 April 21, the seismometer operated by the Commission
of Volcanology 8 km east of the summit began recording harmonic
tremor, and a small eruption cloud appeared above the surnmit
crater. Four explosive eruptions occurred o11 April 21 and 22, and
early on April 23 the tempo of these explosions increased one
occurring every few hours. The explosions were characterized by
deep rumb]ing from the volcano followed by vertical ejection of ash
and blocks commonly 300 and sometimes 6(10 m into the air, indicating
their maximum velocity was about 100 m per sec (Figs. 3 and
4). Much of ~he ejected material was faintly incandescent, though its
red glow could only be seen at night (Plates I and 2). The ejection was
accompanied by development of a vertical ash-laden explosion
cloud which rapidly reached a height of 3 to 10 km. From the base
of this vertical cloud, tongue-like clouds emerged and swept down
the flank of the mountain, generally within the deeper ravines. These
downward-moving hot clouds or nu6es ardentes continued to billow
hot gases and ash all along their path. Lightning was common within
both the vertical explosion column and the downward-moving tonguelike
clouds. The larger nu6es ardentes also appeared to generate
torrential rainstorms which eroded the loose ash and caused destructive
mudflows on the lower flanks." (Moore et al)
"The first indication of a revival of activity on Mayon Volcano
after the eruption of January-February, 1947, was the appearance of
a glow from the summit crater at 2200 hours, April, 20, 1968, local
time. At 0900 April 21, the seismometer operated by the Commission
of Volcanology 8 km east of the summit began recording harmonic
tremor, and a small eruption cloud appeared above the surnmit
crater. Four explosive eruptions occurred o11 April 21 and 22, and
early on April 23 the tempo of these explosions increased one
occurring every few hours. The explosions were characterized by
deep rumb]ing from the volcano followed by vertical ejection of ash
and blocks commonly 300 and sometimes 6(10 m into the air, indicating
their maximum velocity was about 100 m per sec (Figs. 3 and
4). Much of ~he ejected material was faintly incandescent, though its
red glow could only be seen at night (Plates I and 2). The ejection was
accompanied by development of a vertical ash-laden explosion
cloud which rapidly reached a height of 3 to 10 km. From the base
of this vertical cloud, tongue-like clouds emerged and swept down
the flank of the mountain, generally within the deeper ravines. These
downward-moving hot clouds or nu6es ardentes continued to billow
hot gases and ash all along their path. Lightning was common within
both the vertical explosion column and the downward-moving tonguelike
clouds. The larger nu6es ardentes also appeared to generate
torrential rainstorms which eroded the loose ash and caused destructive
mudflows on the lower flanks." (Moore et al)