In the first post in the Hawaiian Evolution series, I wrote a bit about why evolution is so readily apparent on Islands. Today, I'm going to shift away from the biological a little bit, and talk about a different type of evolution - the evolution of the islands themselves. Because of the way that the islands are formed, they go through a distinct lifecycle as they form over the hotspot, then move off to the west and erode. The evolution of the islands is one of the things that shapes the evolution of life on the islands, so let's take a look at it.http://scienceblogs.com/cgi-bin/MT/mt.cgi
The basic plate tectonics involved in the formation of the Hawaiian Islands, or any of the other hot spot volcano chains, are really very simple. The active volcanoes form directly over a hot spot, where magma rises from the mantle. The hot spot is stationary. The plate moves - slowly - over the hot spot. Volcanic islands form over the hot spot. As they move away from the hot spot, the volcanoes go extinct and the islands erode. There are several hotspots in the Pacific, and several chains of islands.
The first stages of formation for one of these hot spot volcanoes are pretty much what you would expect. The volcano begins to form under the ocean, eventually building to the point where it breaks the surface and becomes an island. These islands can reach great heights - Hawaii and Maui both have peaks over 10,000 feet.
After the islands move off the hotspot, volcanism largely stops. There is sometimes a period of secondary volcanism long after the bulk of the volcanism has ended - this is the type of volcanism that produced Diamond Head. In terms of volume, though, the secondary volcanism doesn't contribute much to the growth of the island, so we won't consider it further.
There are a number of things that can happen as the island is developing and moving off the hot spot. The heat from the hot spot tends to push the crust around the active volcano up. This means that the islands are sinking as they move away from the active volcano. This can break up a single island into smaller islands, and split up populations that used to be connected. The early stages of erosion also often play a role in splitting up the island.
Catastrophic landslides are a common feature in the evolution of hot spot volcanic islands. Significant portions of most of the Hawaiian Islands have slid off into the ocean over time, as this picture shows.
At the same time that the early stages of erosion are beginning, the first stages of reef formation are beginning. Fringing reefs begin to form around the edges of the island. As the island continues to erode away, the reefs continue to build upward. Eventually, you are left with nothing but reefs - the classic coral atolls that make up most of the Hawaiian Islands.
Here's how the process relates to the evolution of the Hawaiian biota:
First, the chain-type formation of the islands means that the newest volcano is usually found not all that far to the east of the next oldest volcano in the chain. Sometimes the newest volcano is part of the same island as the older one. Sometimes it is separated by a relatively short stretch of sea, and other times the next island was as much as a couple of hundred kilometers distant. In all cases (after the first island formed, anyway), the closest land to the new volcano is a slightly older volcano in the chain. This is important because, as I mentioned in the last post, islands like Hawaii are lifeless when they first form, and their biota has to come from somewhere else. The closest source is usually the most likely, which means that the newer islands are likely to have similar flora and fauna to the older ones - organisms that evolve on one island can spread to other islands.
Second, after the islands move off the hot spot, they are generally decreasing in height and area. This means that the available habitat decreases in terms of the type of habitats and the available area. Organisms that have evolved in (for example) the high deserts near the peaks on Maui and Hawaii are faced with a limited set of possibilities. They will migrate to a newer island, evolve in response to the total loss of their habitat as their islands erode, or go extinct. Actually, more than one of these might happen to different populations of the species.
Third, the evolution of the islands, particularly during the early stages, is a very dynamic process. Lava flows can destroy patches of habitat, and split populations. The subsidance of the island can also split populations, and it will also result in habitat changes elsewhere in the island, as areas that were once at high altitudes are moved lower. I'm only scratching the surface here - the organisms that live in Hawaii are frequently challenged by changing environments.
Finally, all of these geolgical changes take place very rapidly (in evolutionary terms). The islands don't spend more than 500,000 years or so over the hot spot. All of the high islands are younger than 6 million years or so. That might sound like a lot, but compared to the ages of the continents it's nothing. The dynamic geology of the Hawaiian Islands has been one of the biggest drivers for the evolution of the Hawaiian fauna.
In the next post in the series, we'll start with the oldest island in the chain - Kure.
I really enjoyed your first two posts on Hawaiian Evolution. Keep 'em coming!