Hawaii's Rocks From'A'a to (almost) Z
An amazing variety of rocks appear in Hawaii from a single type of molten rock called basalt. Here's a guide to some of the weird and wonderful stones that result from the active lava flows in Hawaii, as well as a description of different types of volcanic rocks.
This rough, rubbly lava, pronounced "ah-ah," is the Hawaiian term for a thick, pasty lava flow with a fragmented surface. The distinctive, blocky flow advances like a tractor tread, with a jumbled mass steepening at the front until it tumbles over.
Andesite, named for the Andes Mountains, is a gray to black volcanic rock that usually contains crystals of a mineral called plagioclase. While rare in Hawaii, andesite is common in subduction zone volcanoes, such as along South Americaand Alaska.But there is an andesite lava, called hawaiite, that is found only on Mauna Kea and Haleakala.
Though ash particles are less than a tenth of an inch (about 2 millimeters) in size, these tiny fragments can clog car filters, destroy jet engines and pile up on buildings, collapsing roofs. Ash can cover a wide area, drifting on winds after an eruption. Researchers are working on ways to predict the direction and size of ashfall after a volcanic blast.
The Hawaiian Islands are almost entirely built from basalt lava, the most common rock on Earth.
Volcanic bombs are big rocks formed from pieces of molten lava thrown into the air during eruptions. They often have rounded shapes from flying through the air.
Lava spluttering up through an opening above a tube or the cooled surface of an active flow creates a hornito, or "little oven." The spatter builds a tall, steep-sided cone.
Small, rounded balls created in a cloud of volcanic ash during an eruption, lapilli means "little stones" in Italian.
Pahoehoe lava flowing down Hawaii's gentle volcanic slopes form natural tubes. As the lava surface chills in the air, the molten rock underneath continues to flow, creating its own conduit. Big flows are often a series of tubes.
Pele's seaweed, limu, also known as limu o pele, is thin basaltic glass created by waves of frothing pahoehoe lava pouring into the ocean. As the water steams, thinly-walled lava bubbles filled with steam shatter in the waves.
Obsidian is quickly-chilled lava, cooled so fast its crystals don't have time to grow. Obsidian's shiny appearance and curved, keen edge make the rock a favorite for knife blades and arrowheads in traditional cultures. Most obsidian forms from silica-rich rhyolite, an uncommon lava in Hawaii. The only obsidian in Hawaii is found at Pu'u Wa'awa'a, a cone on the north flank of Hualalai Volcano.
Pahoehoe is the Hawaiian word for a ropy, wrinkled-looking lava flow. The smooth surface makes a sharp contrast to blocky 'a'aflows, Hawaii's other common lava flow type. Often, a single lava flow can switch back and forth between pahoehoe and 'a'a as conditions such as slope steepness and loss of gas and heat change.
As winds stretching strands of lava cool the rock to glass, Pele's hair forms. Pele's hair can appear at the front of 'a'a flows or as pahoehoe lava plunges over a cliff, according to the U.S. Geological Survey. The long strands (up to 6 feet, or 2 meters) are also found around Kilauea volcano's Halema'uma'u crater, home of an active lava lake.
These droplet-shaped rocks are made of glass, or lava cooled so quickly it couldn't form crystals. The shiny black pebbles form in lava fountains or on the ends of Pele's hair, as winds pull out strands of molten rock from active lava flows and lakes.
The distinctive pillowy appearance of lava flows cooled underwater gives pillow basalts their name. Whether at Hawaii's islands or at underwater volcanoes, the lava forms a quickly cooled crust that looks like a rounded pillow. The oldest pillow lavas on Earth are more than 3 billion years old, indicating Earth had large bodies of water in its early history.
Some volcanic eruptions are like opening a bottle of soda — gas rushes out, frothing the lava into foam. Pumice is the solidified foam. The rock is so light and porous it can float for miles in the ocean, forming pumice rafts.
Another common lava type, but one that's rare in Hawaii. Rhyolite lava carries a lot of silica, making it viscous and sticky. It tends to create explosive eruptions because the magma has trouble getting rid of its gas. Some spectacular calderas created by rhyolite magmas in the U.S. include Yellowstone in Wyoming, Valles in New Mexico and Long Valley in California.