Tuff Volcanic Cliff

Friday - 16/10/2020 13:02
Hang Cau is a volcanic cliff on the northern side of Thoi Loi’s volcanic tuff ring, East Hamlet, An Hai. Hang Cau literally means the Fishing Cave. In the past, locals used to come here to fish and, when it rained, they hid inside the cliff, hence the name. The cliff is of around 120 meters high, a few kilometers long, making visitors feel tiny when facing it. More interestingly, this site is part of an outer ring of a former active volcano.

Two types of rocks formed the cliff. One is a basaltic rock, a magmatic extrusive rock. It’s grey to greyish black, round and solid, with a few chunks up to a dozen centimeters in diameters. The other formed from volcanic ash and is smaller, with a diameter that can reach a few centimeters. It’s greyish yellow and loose in structure. This volcanic debris, made up of small particles, shot into the air and later became consolidated, making a volcanic slope.

Looking at the cliff carefully, one may identify several layers or flows that have basaltic rocks interfering or breaking the flows. Scientists have shown that Thoi Loi Volcano erupted in a shallow sea.  Volcanic ash and debris fell to the water and then deposited in layers depending on their weight and size. Several basaltic rocks crashed into the very soft layers as they were being formed.

Studies also show that basaltic rock originally exploded to the air hundreds of thousand years to a few million years ago. Most of the volcanic ash debris, however, fell 20-30 thousand years ago, or even less than ten thousand years ago. Then, about 6,000-7,000 years ago, when the sea level was five to seven meters higher than it is today, sea erosion carved out the vaulted caves at the foot of the cliff.
 
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