The complex eruption sequence forming Hakone occurred in stages
separating caldera formation and erosion beginning about 0.4 Ma* (million
years ago). (Hashimoto, 1990) Upon
depletion of volcanic material, the first stage ended when the center of
the main cone collapsed to form a huge caldera. The next stage consisted
of an eruption followed by the collapse of the western portion of the cone
-- forming the second caldera (which overlapped much of the first caldera).
The final stage began with a violent explosive eruption, prompting seven
central cones to form through vents. At the end of this stage, a steam explosion
blew off the northwestern part of the mountain. The debris from this event
was carried down as an avalanche toward the northwestern foot of the volcano.
This blocked the drainage of the caldera, and the result was Lake Ashi.
(Volcano World)
The last eruption of Hakone occurred about 2000 years ago. However,
activity can still be found at four locations on the northern and northeastern
sides of Kami-yam and the northeastern side of Komaga-take, just southeast
of Kami-yama.
| Time
Period |
Volume |
Rock
type |
Associated
Events |
| 0.4 Ma |
130 km3 |
basalt & andesite |
Old caldera formed (12 km
x 8 km) |
| 2 Ka |
14 km3 |
pyroxene, andesite and dacite |
New smaller caldera formed
(7 km diameter) |
| 45 - 2 Ka |
~1-2 km3 |
andesite |
Formation of 7 peaks |
*Geologic Time
| Abbreviation |
Time
Period |
Scientific
notation |
| Ka |
thousands of years ago |
103 |
| Ma |
millions of years ago |
106 |
| Ga |
billions of years ago |
109 |
Members of the geologic community deal with vast amounts of
time. In efforts to use a bit of short-hand, they have devised using
abbreviations. For example, 100,000 years ago can be abbreviated 100
Ka or 0.1 Ma.
References
Hashimoto, M. Geology of Japan. 1990.
Terra Scientific Publishing Company, Tokyo. p. 146.
Volcano World. <http://volcano.und.nodak.edu/vwdocs/volc_images/north_asia/hakone.html>