The Subsidence of Hawaiian Islands: A Dynamic Geologic Process
The Hawaiian Islands are undergoing a complex and dynamic process of subsidence, a gradual sinking that is influenced by both geological processes and environmental factors. Understanding this process is crucial for assessing the long-term stability and future of these iconic volcanic archipelagoes.
Subsidence Due to Geological Processes
The Hawaiian Islands are experiencing an average subsidence of 1 to 2 millimeters per year, primarily due to the weight of volcanic rock and erosion. This process, known as subsidence, is exacerbated by climate change, leading to rising sea levels. However, it is important to note that the islands are simultaneously being built up by volcanic activity. This dual process results in variable overall elevation changes, with a general trend of slow sinking.
The Case of The Big Island
The Big Island, one of the largest and youngest islands in the Hawaiian archipelago, is an excellent illustration of the dynamic nature of these processes. The weight of the massive Kilauea volcano, combined with subsequent earthquake activity, causes the island to sink. At the same time, new volcanic activity is forming new land on the island.
Recent seismic activity in the Big Island includes 142 quakes in the past week, with 53 being magnitude 2.0 or higher. These earthquakes, while not directly caused by magma movement, are a result of the bending of the ocean lithosphere beneath the island. As the volcanoes grow and their weight becomes greater than the lithosphere can support, the lithosphere bends downward, producing earthquakes primarily near the top, where it is more brittle.
Impact on Sea Levels
One clear indicator of this subsidence is the changes in sea levels recorded at tide gauges around the islands. For example, Hilo is sinking at a rate of nearly one tenth of an inch each year. While this rate may seem small, over 100 years it results in a significant 10 inches of subsidence. This process is ongoing and is exacerbated by sea level rise due to climate change.
Geologic Evidence of Subsidence
Offshore, there is substantial geologic evidence of subsidence, as seen in a series of drowned coral reefs. These reefs mark the historical shorelines of the Big Island, with the deepest reef being approximately 465,000 years old and the shallowest being about 15,000 years old. The ages of these reefs were determined through the dating of corals recovered from each reef.
These reefs were drowned as the sea level rose due to the sinking of the island and the inability of new coral growth to keep up with the rising water. During warm-climate periods, rising sea levels and subsidence drowned these shoreline reefs. Data shows that the Big Island has sunk by at least 4,380 feet, with similar drowned reefs off the eastern coast of Haleakalā on Maui sinking at a much slower rate, with some reaching depths of up to 6,900 feet.
Glacial Cycles and Sea Level Changes
The frequency of these drowning events is linked to glacial cycles. Over the past 2.6 million years, there have been 26 periods of glaciation. Recently, Earth experienced about 85,000 years of glaciation and about 11,000 to 15,000 years of interglacial warming, following a repeating cycle. Despite the complex nature of these cycles, the overall trend is clear: the sinking of the Hawaiian Islands is driven by a combination of volcanic growth and climate-induced sea level rise.
Implications for Future Research and Management
The subsidence of the Hawaiian Islands has significant implications for future research and management. As the islands continue to sink, they present challenges for infrastructure, coastal protection, and ecosystem management. Continued monitoring and research are essential to understand the full extent of the subsidence and its impacts on the archipelago.
In conclusion, the subsidence of the Hawaiian Islands is a fascinating and complex phenomenon that is driven by a combination of geological and environmental factors. This dynamic process highlights the interplay between volcanic growth, subsidence, and sea level rise, providing valuable insights into the long-term stability of these iconic islands.