Rocks of Hakone volcano and adjacent areas cover a wide range from basalt to dacite approximate to soda-rhyolite. They were erupted from various different centers during the period from older Miocene to Recent.

Two different rock series are recognized, one characterized by the occurrence of monoclinic-pyroxene (a solid solution series from augite to iron-rich pigeonite) as the only pyroxenic phase of the groundmass; the other by the presence of hypersthene (sometimes together with augite) in the groundmass. They are designated as “pigeonitic rock series” and “hypersthenic rock series” respectively. The pigeonitic rock series was derived from a parental olivine-basalt magma through simple fractional crystallization; the hypersthenic rock series was formed from the same magma through contamination of acid igneous rocks (probably granitic).

In the pigeonitic rock series, the crystallization of the groundmass pyroxenes took place over the clino- and orthopyroxene inversion temperatures, resulting in pigeonite. In the hypersthenic rock series, the crystallization temperatures were lowered by concentration of volatile substances due to the contamination; its crystallization took place always below the inversion temperature, resulting in hypersthene. Porphyritic hornblende appeared in the later stage of the crystallization of the latter series.

The chemical compositions of the groundmasses of these rocks resemble those of basalts and dolerites of the tholeiitic magma-type of the world.

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