Volcano Watch — A walk on the wild side—of the road, that is

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Many readers have visited Halema`uma`u, but how many have turned the other way and walked across Crater Rim Drive? Within 800 m (half a mile) roundtrip, you can see some very different features from those near Halema`uma`u, generally without the sulfurous gas.

Cross the road at the southeast end of the Halema`uma`u parking lot. The black pahoehoe flow, erupted out of Halema`uma`u in 1919, looks strange until you realize that it is peppered with rocks that rained from the sky during the last explosive activity of Halema`uma`u in May 1924.

Across a crack, you will find a stone-lined trail built on the 1919 flow. Follow it a short distance west to a place where a big falling rock broke through the top of the flow, creating an impact crater. The rock is still in the hole, next to the trail.

Now turn south and amble through a field of rocks dotting the 1919 flow. Note the wide range in size, down to the sand that fills some shallow depressions. You won't find scoria, spatter, or cinder; all the material exploded out in 1924 is made of older lava flows, not fresh lava.

Eventually you reach the edge of the 1919 flow. Look beneath the flow, where a small stream has undercut it. There are more rocks there (cemented by opal), but they can't be from 1924, since the 1919 flow overlies them. This is the first indication that you have stepped back in time to an older explosion-the one that killed Keoua's warriors in 1790.

Continue south across the rocky ground surface made of explosion debris from both the 1790 and 1924 explosions. It is hard to tell the vintage of many blocks, but you can, for some. For example, walk east along the edge of the 1919 flow to two places where the lava laps against large rocks. It is hard to imagine that these rocks fell from the sky in 1924, just missing the edge of the flow. They must be rocks from the 1790 explosion.

Another hint is how deeply the rocks are buried. The sandy ash on top of the 1919 flow is very thin. But here you will see many large blocks that are mostly buried. They probably date from 1790, whereas those that rest right on the surface are most likely of 1924 vintage.

The largest block, weighing at least 13 metric tonnes (14 tons), is partly buried and has no impact crater. In contrast, nearby you can find several shallow craters, dating from 1924, in various states of preservation. Some have raised rims on the side away from Halema`uma`u, formed as rocks fell obliquely and kicked up loose debris. You can even guess what rocks did the damage.

Continue southward past the benchmark on a rock to the edge of the cliff, which is made of explosion debris from the 1790 eruption, here 4-5 m (13-18 feet) thick. The fresh flow below was erupted in 1982.

Go a little east and examine the strange mounds carefully. They look like impact pits, but some have raised rims on the side nearest Halema`uma`u. Were rocks falling from the south? Nope. Think about Pearl Harbor and what was done afterward to keep enemy planes from landing on bare flat ground. These mounds were bulldozed up to destroy potential landing sites.

Walking back to the parking lot, you cross a smooth area almost free of rocks, the remains of an airstrip. Planes landed here before the 1924 explosion closed it. Rocks were removed, and the strip was once again used. Then it was bulldozed in 1942 and again opened for landings after the war.

That's a lot to see in a short distance. Take the walk on the wild side of the road for pleasure and education.

The Volcano Watch column has received a Customer Service Excellence Award from the U.S. Geological Survey. Mahalo nui loa to our readers for their valued suggestions, which have led to improvements and topics for future columns. Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Pu`u `O`o vent during the past week. The surface flow activity on Pulama pali and in the coastal flats was observed all week, but lava stopped entering the ocean on January 30. The lava is filling low areas back of the sea coast and will eventually start flowing into the ocean again.

One earthquake was reported felt during the week ending on February 1. A resident of Pahala felt an earthquake at 8:38 p.m. on January 27. The magnitude-2.0 earthquake was located 6 km (3.6 mi) northwest of Pahala at a depth of 8.6 km (5.2 mi).