Visiting Hawai'i...for the Limestone Caverns? (Part one)

People visit Hawai'i for a great many reasons. There are the stereotypical reasons: beautiful beaches, surfing, palm trees, luaus and so on. There are other reasons: a wish to learn about the culture of the many people groups who call the island home, some of them for upwards of a thousand years. There are others who find fascination with volcanoes and lava flows. And the biology! The isolation of the islands has resulted in the evolution of hundreds, even thousands of species that exist nowhere else in the world.

I'm willing to bet that very few people come to the islands to tour the limestone caverns...

Limestone? Caverns? On the Hawaiian Islands?

Outcrop of limestone (!?) on the Mahaulepu Coast of Kauai.

There are actually a great many caves on the islands, but they are not the kind of caves that most people find familiar. They are called lava tubes, and they form when a lava river forms a ceiling of congealed basalt. When the lava flow ends, it drains out the tube leaving behind a long cylindrical cave. Hawaii has one of the longest such caves in the world, Kazimura, with a total length of 40 miles. The much shorter Nahuku Cave (formerly Thurston Cave) in Hawai'i Volcanoes National Park is well-known to park visitors.

Nahuku Lava Tube in Hawaii Volcanoes National Park

But limestone caverns are not a familiar sight on the islands. Before my last trip I didn't know that there were any such caves in Hawai'i. For one, a limestone cavern requires limestone and the islands are composed largely of volcanic basalt. There are coral reefs around the islands in many places and they are composed of calcium carbonate (calcite), the mineral that makes up limestone. But coral forms at sea level, and caverns form from the dissolving action of fresh water, usually above sea level. And the islands are sinking, not rising, so on the face of it there seems to be no opportunity for the development of large bodies of limestone, much less caverns in Hawaii. 

Photo courtesy of Allie Brown

And yet it happened here, on the island of Kauai (and also, upon researching the subject, in downtown Honolulu; check out Moiliili underground caverns if you dare). It was a unique situation, a cavern formed in a sand dune environment. The cavern has been known as Warrior's Cave, Grove Farm Cave, Limestone Quarry Cave and others, but some extensive research revealed the ancient name (from an 1885 student essay) to be Makauwahi, or "source of the smoke".

Sand dunes hardly seem like a place for cavern formation because most of the time sand is composed of quartz and other silicate minerals. Hawai'i's sands are not. They are composed sometimes of basalt fragments (the black sand beaches), but the white or yellow sandy beaches of the islands are mostly composed of bits and pieces of coral reefs, in other words, calcite, the ingredient making up limestone. On the south side of Kauai near the extensive resort complexes of Poipo Beach there are some extensive coral sand beaches. Over the millennia the constant trade winds have blown sand off the beach inland, forming sand dunes above the shoreline. This happened in stages starting about 435,000 years ago during an interglacial period when sea level was higher than today.

Cliff of eolian limestone showing the tilted layers of cross-bedded dune sands at Makauwahi.

When the dunes were stabilized by vegetation, thick layers of soil developed on the surface. Beneath the surface the sand was lithified (glued together) by dissolved calcite and silica derived from the overlying soil. Then the climate changed and dunes moved in again. Over time thick layers of solid limestone resulted from the petrified dunes.

Makauwahi Cave, courtesy of Tylor Ghaffari

The soil layers provide the last part of the process, carbonic acid. Mixed with groundwater, the acid ate away at the lower layers eventually producing the caverns themselves. Later on, dripping and flowing water produced a variety of cavern formations (speleothems) like stalactites, stalagmites, and flowstone. Makauwahi Cave has around 500 feet of passageways, accessed through a small passageway. And so we paid a visit during our field studies exploration of Kauai.

The thing is, if Makauwahi were simply a cave it would merit some attention, but that is not what makes it extraordinary. Makauwahi was the site of three catastrophes, one of which was local, and two others that were island-wide. And from these catastrophes, an amazing story unfolds about life on Kauai, past, present and future. That will be the subject of part two, coming soon (I hope)!

Landslides and Slope Mitigation in California's Great Valley...Wait...What?

 

I used to talk to my students about the geological hazards that we face as inhabitants of California's Great Valley (or Central Valley, for those who don't appreciate its actual greatness). I would go down the list of things to worry about: earthquakes, droughts, wildfires, volcanic eruptions, flooding, and so on. But then I somewhat jokingly described things we didn't have to worry about such as hurricanes (Florida's problem), tsunamis (a problem for coastal cities), tornadoes (Oklahoma's problem), and mass wasting (also known by the generic term 'landsliding').

Unfortunately, over the years I've become aware that some of those unlikely hazards actually can be a factor in living in the valley. We've had a fair number of tornadoes in recent years, including two that came within a few miles of my house (they weren't anything like the monsters of Tornado Alley in the Midwest, but still a bit scary). A powerful tropical storm hit Southern California last summer that came up just short of being a hurricane, and the heavy downpours were statewide. And then there is mass wasting (slope failures and landslides). I know of at least two fatalities caused by mass wasting in the last few years. One was a homeless person who had dug a tunnel into a river embankment that later collapsed, and another was a person who was driving along a freeway in heavy rains when the freeway embankment collapsed as a mudflow and spread across the lanes causing a fatal accident.

The Great Valley is famously flat so mass wasting doesn't seem to be much of a danger to those who live here, since landslides and other slope failures require, well, a slope to happen. But the valley is not quite so flat as people may think. The valley is 400 miles long, and most of it is close to sea level. Much of it is low-lying river floodplains, but other sections sit at slightly higher elevations because of complex history of climate change and glacial ice ages over the last 1,000,000 years. These bluffs and terraces protect my city and others nearby because even the worst floods are contained within the floodplains and do not spill over onto the terrace surfaces where cities like Modesto and Turlock have been built near the Tuolumne River.

During the ice ages glaciers never reached the valley floor, but meltwater from the Sierra Nevada glaciers swelled the rivers to several times their average flow, and they carried tremendous amounts of muddy sediment that spread widely across the valley floor building up alluvial fans. When the glaciers receded, the rivers flowed less, but carried clear water that was more capable of eroding the soft sediments of the alluvial fans, forming channels and floodplains several tens of feet deep. Once these channels developed, floods never covered the terraces again. It's the bluffs that form the boundaries of these terraces that provide the conditions that can result in slope failure.

The heavy rains of 2022-23 led to widespread flooding across many parts of California including some real problems on the Tuolumne River Parkway Trail when I regularly go birdwatching. I wrote about these in January of 2023 in the aftermath of one of the biggest storms. The most serious problem was the access road to our town's water treatment plant. It's on the river floodplain about 60 feet below the river terrace. Slumping had caused major cracks to form in and near the pavement.

Eventually the rains subsided and the soil dried up. The slide seemed to stabilize, but the threat to the roadway remained and would eventually have to be dealt with. That is what was new this week: the cranes and were in place to start the slope mitigation process.

The main problem is that the access road traverses unstable debris and soil that slumped in the 2023 event. They would need to re-engineer the slope by rebuilding it from scratch. Their strategy was complicated by the fact that all the equipment and materials had to traverse the very road they were trying to repair. Truckloads of heavy boulders were going down the road every few minutes. Meanwhile a huge long-reach excavator was digging away at the slope below the road!

After digging away and smoothing off the slope they covered it with felt matting and then started piling many tons of boulders on the slope. The boulders are intended to buttress the slope and hopefully keep it stable during future weather events.

It's a lot of work being done to keep a single paved road open, but it's a pretty important road since it provides the only access to the water-treatment plant for the city of Waterford. And thus we are dealing with slope mitigation in what is supposedly the flattest place in the country!