Igneous Rock Pahoehoe

Mineral Sciences

Catalog Number:
45746
Specimen Count:
1
Precise Locality:

Kaimu Area eruption, 1989-1991

Locality:
US-Hawaii
Collecting Locality:
North America, United States, Hawaii, Hawaii County, Hawaiian Islands, Hawaiian Windward Islands, Hawaii
Cabinet:
25
Drawer/Shelf:
01
Lava flows at Kilauea volcano in Hawaii
Emerging pahoehoe lava flow at Kilauea volcano in Hawaii
Artist interpretation of volcanism in the Ancestral Cascades, in North West United States.
Map of the western United States showing the inferred extent of Cenozoic ancestral and modern Cascades magmatic arcs in Washington, Oregon, California, and Nevada.
NMNH geologist Ben Andrews stands in front of basalt columns in Washington, USA, which were produced during the time of the Ancestral Cascades.
Pyroclasitc flow at Mayon Volcano, Spet. 15, 1984. Philippines.
The Giant's Causeway, in Northern Ireland, is a large and well-known example of columnar jointing. (Giant's Causeway HDR, 2009 Bill Strong via Flick.)
This dam in Washington, USA, is constructed primarily from basalt from a quarry that mines the Ancestral Cascades.
Volcanic rock quarry in Washington. Dark gray basalt covered by volcanic ash and soil. Location: White River Quarry, Enumclaw, WA.
Volcanic rocks used to protect harbor in Washington, USA.
Artist interpretation of rifting along the Eastern United States, leading to the creating of the Atlantic Ocean and rift basins that filled with sediments and volcanic rock.
Contact between igneous diabase (left) and metamorphic hornfels (right), which used to be sedimentary siltstone. Location: Manassas Quarry, Manassas, VA.

When molten rock (magma) reaches Earth's surface, it solidifies or hardens. Scientists call the resulting solid rocks "extrusive" igneous rocks. Extrusion is the process of pushing material out to the surface of the Earth's crust. At some volcanoes, the extrusive rock flows as lava across the ground before it hardens; the ripples in the lava may freeze in place. Hot, rapidly expanding gases within other volcanoes' vents can force the magma out explosively, forming pumice: low-density rock full of vesicles, or frozen bubbles. Extrusive igneous rocks are easy to find near many volcanoes, such as Mount St. Helens in Washington state. Hawai`i Volcanoes National Park, home of two active volcanoes, contains lava flows that cooled only a few decades, or minutes, ago.

Sea-floor basalt rocks recovered from the Pacific Ocean near the Galapagos Islands
Rhyolite collected from the Leucite Hills of Wyoming, USA
Granite from Livermore Falls, New Hampshire, USA

You can tell a lot about the history of igneous rocks by looking at the size of crystals within them. Rocks that cool quickly contain small crystals, while slow-cooled rocks are filled with large crystals. When magma erupts at the Earth's surface, heat radiates out from the lava allowing it to cool rapidly and the atoms and molecules do not have time to grow into large crystals before the lava solidifies. The resulting rock has such small crystal grains that humans have a difficult time distinguishing them, even with a handheld lens. Geologists describe the texture of these fine-grained igneous rocks as "aphanitic," from the Greek word meaning "unseen." Deep inside the Earth's crust, the magma cools much more slowly because the surrounding rocks insulate the magma from rapid heat loss. This allows the crystals to grow into mineral grains that are easier for humans to see. Geologists describe the resulting coarse-grained rock texture as phaneritic, from the Greek word meaning "visible." Some igneous rocks contain crystals that are much larger than the crystals in the matrix surrounding them. Scientists call these specimens, which resemble a chocolate-chip cookie, porphyritic rocks, and the larger crystals are called phenocrysts. The phenocrysts had started to form within the magma before it later cooled rapidly, probably due to that magma erupting at a volcano.