American Samoa Territorial Stone
Basalt is a gray to black, typically fine-grained volcanic rock composed primarily of feldspar (AlSiO2 with potassium, sodium, or calcium) and pyroxene (Al2O6 or Si2O6 with sodium, calcium, iron, or magnesium, among others). Additional components of basalt may include olivine (particularly in the Samoa Islands) and iron oxide. Because basaltic lava is relatively low in silicates, it is more fluid and erupts more gently, often forming smooth flows.
A TERRITORIAL REPRESENTATIVE
While American Samoa has not designated an official territorial rock, basalt is an obvious choice because it underlies most of the islands’ terrain, excepting the coral atolls. American Samoa’s geologic map reads like a catalogue of forms that igneous rock can take: dike complex, olivine basalt, pahoehoe flow, pumice deposits, lithic-vitric tuff, breccia cone, and so on. Non-basaltic materials such as beach sand, marsh, and coral, as important as they are in providing a diversity of environments, are in the minority.
The Samoa Islands are the peaks of a chain of undersea volcanoes. The chain began forming less than two million years ago, quite recently in geologic terms. Geologists disagree about whether the chain formed over a "hot spot" in the Earth’s mantle, as did the Hawaiian Islands, or as a result of the subduction of the Pacific Plate under the Australian Plate, similar to the formation of the Marianas.
Subduction is an unusual process for the formation of basalt, which is more normally associated with divergent tectonic plates. In either case, basaltic materials from the oceanic crust were heated and welled up as mountains. Some of the rock making up Samoa is extrusive (erupted from a volcano), and some is intrusive (from magma that cooled without erupting).
Basalt makes up a small percentage of continental landforms, but it covers most of the ocean floor and is thus the most common rock of Earth’s crust. Where basalt does occur onshore, it often takes the form of colossal volcanic flows. In India, the vast Deccan Traps testify to a lava-flooding event 65 million years ago. The largest flow is the Columbia River Basalt underlying much of Washington State (for more, see the article on Washington’s proposed state rock).
From prehistoric times, Samoans made tools of the best-quality basalt quarried in at least 10 locations on Tutuila. The archaeological record shows that production of stone tools was a major industry for hundreds of years. In other parts of the world where basalt is the prevalent rock, such as the Middle East, Armenia, Greece, and Turkey, entire ancient cities were built out of it. The impressive black gloss of finished basalt has also made it popular for sculpture.
In modern times, basalt is quarried for production of crushed stone to be used in concrete, riprap (shoreline protection), railroad ballast, road base, and gravel for unpaved roads. Since much of American Samoa is now protected parkland, the island’s needs for stone are generally filled by imported materials rather than quarried on the islands.
Basalt has been used to create some of the iconic artifacts of ancient cultures. The remarkable jet-black stone statues and statuettes of pharaonic Egypt are carved of fine-grained basalt. The renowned huge statues of Easter Island are carved of basaltic tuff.
Chemical Formula: SiO2
Color: Usually grey or sometimes brown, depending on the iron content
|Author: World Trade Press|