 |
L earning O bjective 17 |
|
Hawaiian volcanoes are called shield-type volcanoes, probably named so for their profile which resembles a war shield on its side. They are broadly rounded, dome-shaped mountains with far-spreading, thin lava flows. For example, Mauna Loa is the most massive single mountain on earth! This type of volcano has very little explosive action above sea level.In contrast, the composite type volcano, like Mount St. Helens, has the cone shape more commonly associated with volcanoes and explosive eruptions. Composite means formed from a combination of lava flows and ash.
The presentation below is adapted with permission from Hawaii: A Natural History by Sherwin Carquist, published by National Tropical Botanical Garden of Kaua'i.
|
Stage one: The island starts forming from opening at ocean floor, builds up, and emerges above the ocean.
|
|
Stage two: After a long eruption stage, lava drains out of the center and summit may collapse, leaving a broad caldera, a depression at the center of the volcano. Examples of calderas are Halemaumau of the Kilauea volcano on the Big Island and Haleakala on Maui. Cracks, or rift zones, may occur on the volcano's sides and continue with considerable lava flows resulting in an elongated oval shaped volcano, rather than round.
|
|
Stage three: During the caldera stage, eruptions ponding within the caldera eventaully fill it up. After a period of no eruption, a period of explosive volcanic activity occurs, resulting in a domed mountain, like Mauna Kea on theBig Island.
|
|
Stage four: From now on, an island decreases in size as a result of erosion and subsidence. Massive landslides have occured throughout the life of a volcano can also take its toll. Sea cliffs orpali form at the coastline like those of Moloka'i and the Na Pali of Kaua'i.
|
|
Stage five: This stage has not been reached by the larger islands of the Hawaiian Island chain, from the Big Island to Kaua'i. However, the much diminished islands above Kaua'i have reached this stage.
They are so worn down that they barely break the surface of the ocean. At the same time as erosion has worn them down, many of the islands have sunk, due to major shifts in the earth's crust. If this goes slowly enough, broad coral reefs develop at the edges of the island. French Frigate Shoals of the upper island chain is an example. Millions of years ago it was probably a large island.
In another 10 million years or so, O'ahu will also be a small remnant of itself. It may be a pinnacle, a sharp peak of land left from the center of the old volcano, or it may be a very small flat area remaining from the original island.
|
|
Stage six: The final stage produces what is called an atoll. It is a small circular, very low island. The landmass above sea level may be very small, although the lagoon may be miles across. It has no piece of the original volcanic structure left above water. It only is composed of the shallow lagoon in the center and the sand island reef making up circular pieces of low-lying land.
Breaks in the reefs are due to the original site of fresh water drainage from the now gone volcanic land mass which prevented coral growth at that site.
|
|
Interestingly enough, Charles Darwin proposed the above explanation for atolls, but only recent scientific studies have shown him to be correct.
| [1 ] | [2 ] | [3 ] | [4 ] | [5 ] | [6 ] | [7 ] | [8 ] | [9 ] | [10 ] |
| [11 ] | [12 ] | [13 ] | [14 ] | [15 ] | [16 ] | [17 ] | [18 ] | [19 ] | [20 ] |
| [21 ] | [22 ] | [23 ] | [24 ] | [25 ] | [26 ] | [27 ] | [28 ] | [29 ] | [30 ] |