Erosion and deposition — the Hawaiian cliffs
When most people think of Hawaii, they probably don't realize cataclysmic landslide events created many of the stunning cliffs that characterize these volcanic islands. Yet the giant landslides that have occurred in Hawaii are recognized by geologists as the largest of their kind in the world. Large chunks of the island have literally fallen into the Pacific Ocean, forming extensive underwater rubble zones that cover several thousands of kilometres. These enormous erosional events occur on average just once every 100,000 years.
These catastrophic landslides weren't only unique to Hawaii — similar events have happened on other volcanic islands in the Pacific. Before we were able to map the sea floor, geologists puzzled over the dramatic nature of the cliffs because normal rates of marine island erosion don’t allow enough time for the formation of such steep cliff faces. Geologists only uncovered the answer after pairing the discovery of the large underwater rubble zones on sea floor maps with the locations of the steep cliffs. Just one landslide equaled a giant rubble zone.
Weathering, erosion and deposition— what's the difference?
Weathering is a destructive process that happens near the earth’s surface, whereby earth materials (such as rock) are changed in colour, texture, form or composition by exposure to atmospheric agents — which in the case of Oahu, is water. Erosion is the process where the same materials are loosened, worn away and moved to another place — normally a dynamic process. Deposition is the laying down, placing, or accumulation of any material!
But what about weathering? Well, Hawaii goes “big” on that one too. On the island of Oahu, the well-populated and third largest Hawaiian island, extreme rainfall events are known to cause shallow landslides and danger for residential areas. The large volume of water from an intense rainfall can trigger a landslide originating on steep hillslopes. The landslide material can become a fast-moving debris flow travelling downhill and/or in stream channels. The debris flows carry anything loose including soil, vegetation, and loose bedrock — creating significant hazards for communities. The flow hazard has impacted residential development in Oahu. Measures to prevent future danger (and to avoid community evacuation) involve mechanical slope reinforcement. Steel mesh bolted over unstable slopes is one stabilizing method being applied in Oahu.