Field trip

Lava rock sampling @ Izu Peninsula

On April 14, 2019, we went to the Funabara quarry (Izu City, Shizuoka) to collect lava rock samples for a laboratory experiment conducted by R. Noguchi.

The Funabara quarry is an outcrop of a scoria cone (Figure 1), and suitable place for lava rock sampling because of its good accessibility. The Funabara quarry is a part of the Funabara volcano belt. The Funabara volcano belt was formed by the oldest eruption in the Izu Eastern Volcano Group (150,000 years ago). This volcanic belt consists of three scoria cones, and the Funabara quarry is an exposure of the middle scoria cone. In the Funabara quarry, we can observe an internal structure of the scoria cone; stratigraphic sequences of scoriae including volcanic bombs (Figures 1, 2, 3). Scoria cone was formed by depositing a porous rock called scoria.

    • Fig.1 The Funabara quarry

    • Fig.2 Selecting lava rock sample

    • Fig.3 Volcanic bomb with beautiful crack pattern

    The collected lava will be cut in cubes and be used for a hyper velocity impact experiment which will be performed in this Autumn. These rock samples will be shot and fragmented under various shot speed. Using statistical techniques, we will analyze grain shape of lava fragments and investigate relationship between the shape of volcanic ash particles and the styles of volcanic eruptions.

    The lavas used for fracture experiments are

    ・Less phenocrysts

    ・Less alteration

    ・Homogeneous

    ・Not cracked

    Ten rocks were selected based on the above four points (Figure 4). This basis of selection is determined considering the previous impact experiment.

    After collecting lava rocks, we looked around the columnar joints of Izu Peninsula. Columnar joints are columnar fractures due to volume shrinkage when magma/lava cools.

    First, we visited the Joren waterfall that is famous as a tourist destination (Figure 5). It is also known as one of the 100 most popular waterfalls in Japan because of the beautiful scenery by the rock surface and waterfall. The lava rocks of the Joren waterfall was formed by the lava that flowed out by the eruption of Mt. Hachikubo 17,000 years ago. In the surrounding area, wasabi cultivation is popular using lava plateaus and abundant spring water around this area.

    Next, we visited the Asahi waterfall (Figure 6). The rock surface that looks like an artificially created stone wall, but actually this pattern comes from columnar joints. The Asahi waterfall is a part of a volcanic neck (path of magma) which was once underground. The polygon pattern is vertically aligned was formed by cooling of magma from the lateral direction.

    Finally, we visited the columnar joint of the Shiratori quarry in Mt. Shiratori (Figure 7). Mt. Shiratori is an exposure of a submarine volcano that was formed when Izu Peninsula was on the ocean floor. The entire mountain consists of columnar joints. The huge wall of columnar joints of Mt. Shiratori is a masterpiece of nature.

    We carried out satisfying excursion in Izu!!!!!

    • Fig.4 Lava rock samples selected for the impact experiment

    • Fig.5 The Joren waterfall

    • Fig.6 The Asahi waterfall

    • Fig.7 The Shiratori quarry