Volcanic pipe in Dalsan area occurs as 1.7km × 1.3km in plan. Rock units in the volcanic pipe can be roughly divided into rhyolitic pyroclastic rocks, andesitic rocks and rhyolitic intrusive rocks. The rhyolitic pyroclastic rocks can be classified into 7 lithofaices: (1) massive tuff breccia, (2) massive ash tuff, (3) massive lapilli tuff, (4) reverse graded lapillistone, (5) normal graded lapilli tuff, (6) accretionary lapilli tuff, (7) lithic breccia. The andesitic rocks are subdivided into 3 lithofaices: (1) andesitic hyaloclastite, (2) porphyritic andesite, (3) peperite. And the rhyolitic intrusive rocks are subdivided into (1) flow-banded rhyolite, (2) porphyritic rhyolite, (3) rhyolitic breccia.
The rhyolitic pyroclastic rocks can be associated with lower, middle and upper rock units by eruption mechanisms. The lower rock unit, composed of massive tuff breccia, lithic breccia, was formed by lower wet pyroclastic surges due to phreatomagmatic eruption and rock slides of basement rocks of vent wall. The middle rock unit, composed of massive tuff breccia, reverse graded lapillistone, normal graded lapilli tuff, accretionary lapilli tuff, is made up from lower to higher wet pyroclastic surges by phreatomagmatic eruption due to much influx of groundwater into magma. And the upper rock unit, including massive lapilli tuff and massive tuff breccia, is produced from higher wet pyroclastic surge by phreatomagmatic eruption. The area began with intermediate volcanism elapsing a volcanic quiescence, andesitic lavas effused from cental vent and ring fractures, formed hyaloclastites from quench fragmentation in contact with surface water in crater lake and peperites by chaotic mixing with quench debris at the base under water. And lavas far away from water and magma feeding through conduit could be remained as massive andesites. Afterward, the area began with rhyolitic volcanism again. rising magma along southwestern ring fracture formed rhyolitic breccia owing to contact with groundwater. Intruding magma along cental vent formed flow-banded and porphyritic rhyolites because of no contact with groundwater. Therefore, It can be seen that rhyolitic pyroclastic rocks were formed in the vent, andesitic rocks were produced in the crater and rhyolitic intrusive rocks were formed by the intrusion in the conduit.
As results, it could be thought that lithofacies in the volcanic pipe are variously formed by extent of contact of magma and external water, which controlled by location of their magma and lava.