Lava Tubes and Sea Arches

By Shana Chapel and Jenny Domeier

Sea Arches

Sea arches are formed by wave erosion and are erosional remnants of rocks standing in the surf. Given time they will collapse. Cliffs that extend into the ocean are pounded by the surf due to wave refraction. The waves erode the softer rocks or more fractured rocks first, because they are more easily weathered. At first, sea caves may form as a result of this weathering and erosion. When two sea caves on opposing sides of a cliff meet, a sea arch results, forming a bridge of rock over the water.

After continued weathering, the sea arch will collapse leaving a remnant called a sea stack. The large sea arch pictured was taken along Chain of Craters Road on the island of Hawaii. The picture of the smaller collapsed sea arch and sea stacks was taken on the north shore of the island of Maui.

Lava Tubes

A lava tube is formed by the extraction of molten lava from a hollow space beneath the surface of a solidified lava flow. Lava tubes carry the lava to the flanks of a volcanic shield, which in this case is Kilauea. Lava tubes are formed in Pahoe'hoe' lava, which has a smooth ropy texture and is less viscous than the clinkery a'a flows. Because the lava is encrusted soon after eruption, it is insulated and can travel as far as 20 to 30 miles beneath the surface in the lava tubes. Sometimes the tubes can become blocked by cooled lava clumps. When this occurs, new flows emerge at the surface uphill from the blockage from holes in the tubes called skylights.

While the Geology Club was in Hawaii, we visited one of the largest lava tubes called the Thurston lava tube. The Thurston lava tube carried lava from a forerunner of the present shield of Kilauea, prior to the collapse event which formed the modern caldera. A well graded trail follows a segment of the tube for about 400 feet. It enters through the wall of a small pit crater and exits through a natural tube skylight. Rapid subsidence of Kilauea caldera eventually cut off the source of lava to the lava tube.

The entrance to Thurston Lava Tube is approximately 30' high, and is in a shaded area of the fern forest on the windward side of Kilauea

Inside the lava tube - the ceiling is about 20' above the floor