Mauna Loa is a shield volcano on the island of Hawaii. It last erupted in 1984, and many volcanologists predict that it will erupt again in the near future. Considered the largest active volcano in the world, Mauna Loa makes up nearly half of the big island. Most of the rocks that can be found on the slopes of Mauna Loa are a result of some form of volcanic activity.
The lava from Mauna Loa's various eruptions is basaltic, which is a type of rock found in the ocean floor, and within the Earth's mantle. The basalt from Mauna Loa is predominantly tholeiitic basalt, which has a very small percentage of silica. It is iron and magnesium rich and can occasionally include crystals of olivine, a pale green mineral. Basalt is usually a deep red to dark gray and often appears black. Depending on the properties of the lava flow, basalt can be smooth or cindery.
Types of Lava Rocks
There are two primary types of volcanic flows on the Hawaiian islands. Fast-flowing pahoehoe and slow-moving aa. Pahoehoe tends to be smoother and more dense, while aa has more of a crumbled, airy consistency. Mauna Loa and the older Ninole shield beneath it have erupted with both types of lava. In the Ninole volcanic series formations, around the base of Mauna Loa, you can find thin layers of alternating pahoehoe and aa that have been carved through by river erosion.
Metamorphic and Sedimentary Rocks
While the continental U.S. contains a high percentage of granite and silica-rich rocks, the land mass of Hawaii is almost entirely basaltic lava. But volcanic pressure can metamorphose basalt into schists and some of these can be found in small quantities on the Hawaiian islands, though it is rare. More common are layers of sand and ash that are slowly cementing into rock. Since the Hawaiian islands are young, relative to continents, the sediments are uncommon and thin.
Other Sand and Soil Constituents
Coral and shells, while not rocks, make up a great deal of the sand beaches on Hawaii along with eroded basalt, and some of the composite rocks further inland. Since basalt is such a dark rock, most of the lighter colors that you can find in sediments or sand will be from broken shells and eroded pieces of coral. On some of the beaches, the pieces will be larger, and easier to identify as shells, while others are finely rounded and can easily be mistaken for rock fragments.
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