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Basalt is the most common volcanic rock, constituting nearly all of the oceanic crust and covering parts of the continents. This gallery presents some of basalt's variety, on land and in the ocean.
Go see basalt:
Geology of California, Oregon, Washington, Idaho, Alaska, and Hawaii
Solid basalt, with aphanitic texture, is typical of the great continental flood basalts. This was collected in northern Oregon.01of 17
Fresh and Weathered Massive BasaltPhoto (c) 2006 Andrew Alden, licensed to About.com ( fair use policy)
Basalt may contain the iron mineral magnetite as well as iron-rich pyroxene, which both weather into reddish stains. Expose fresh surfaces with a rock hammer.02of 17
Altered Basalt with Palagonite CrustPhoto (c) 2011 Andrew Alden, licensed to About.com ( fair use policy
When basalt erupts into shallow water, abundant steam chemically alters the fresh glassy rock to palagonite. The typical rust-colored coating can be quite striking in outcrops.
Vesiculated BasaltPhoto (c) 2006 Andrew Alden, licensed to About.com ( fair use policy)
Much basalt has vesicular texture in which vesicles or bubbles of gas (CO2, H2O or both) came out of solution as the magma slowly rose to the surface.04of 17
Porphyritic BasaltPhoto (c) 2006 Andrew Alden, licensed to About.com ( fair use policy)
This Hawaiian basalt contains vesicles and large grains (phenocrysts) of olivine. Rocks with phenocrysts are said to have porphyritic texture.05of 17
Amygdaloidal BasaltPhoto (c) 2008 Andrew Alden, licensed to About.com ( fair use policy).
Vesicles that later become filled with new minerals are called amygdules. Outcrop from the Berkeley Hills, California.
Basalt Flow SurfacePhoto (c) 2008 Andrew Alden, licensed to About.com ( fair use policy)
Once the surface of a lava flow, this basalt specimen shows signs of stretching, tearing and flattening of vesicles while it was still soft lava.07of 17
Pahoehoe and Aa BasaltPhoto courtesy jtu of Flickr under Creative Commons license.
Both of these basalt flows have the same composition, but while they were molten, the smooth pahoehoe lava was hotter than the jagged aa lava. (more below)
Click the photo for the full-size version. This lava flow displays two textures of lava that have the same composition. The ragged, clinkery form on the left is called aa. You pronounce it "ah-ah." Perhaps it has that name because the rough surface of the solidified lava can quickly cut your feet to ribbons, even with heavy boots. In Iceland, this kind of lava is called apalhraun.
The lava on the right is shiny and smooth, and it has its own name, like aa a Hawaiian wordpahoehoe. In Iceland, this kind of lava is called helluhraun. Smooth is a relative termsome forms of pahoehoe can have a surface as wrinkled as an elephant's trunk, but not at all jagged like aa.
What makes the exact same lava produce two different textures, pahoehoe, and aa, is the difference in the way they have flowed. Fresh basalt lava is almost always smooth, liquid pahoehoe, but as it cools and crystallizes it turns stickythat is, more viscous. At some point, the surface can't stretch quickly enough to keep up with the movement of the flow's interior, and it breaks and shreds like the crust of a loaf of bread. This can happen simply from the lava growing cooler, or it can occur as the flow spills down a steep place making it stretch faster.
Profile of Aa Basalt FlowPhoto courtesy Ron Schott of Flickr under Creative Commons license.
Basalt at the top of this lava flow ripped apart into aa while hotter rock below continued to flow smoothly.09of 17
Hexagonal Jointing in BasaltPhoto (c) 2008 Andrew Alden, licensed to About.com ( fair use policy)
As thick flows of basalt cool, they tend to shrink and crack apart into columns with six sides, although five- and seven-sided ones also occur.10of 17
Columnar Jointing in BasaltU.S. Geological Survey photo by S. R. Brantley.
The joints (cracks with no displacement) in this thick basalt flow at Yellowstone form well-developed columns.
Columnar Basalt in Eugene, OregonPhoto (c) 2005 Andrew Alden, licensed to About.com ( fair use policy)
Skinner Butte is a spectacular example of columnar-jointed basalt, popular among Eugene's urban climbers.12of 17
Superimposed Basalt FlowsPhoto (c) 2005 Andrew Alden, licensed to About.com ( fair use policy)
A roadcut north of Maupin, Oregon shows several basalt flows stacked upon earlier ones. They may be separated by thousands of years. (click full size)13of 17
Basalt at Fossil Falls, CaliforniaPhoto (c) 2008 Andrew Alden, licensed to About.com ( fair use policy)
Fossil Falls State Park preserves an ancient riverbed where flowing water once sculpted the vesicular basalt into bizarre shapes.14of 17
Columbia River Basalt in CaliforniaPhoto (c) 2005 Andrew Alden, licensed to About.com ( fair use policy)
The Columbia River basalt plateau is Earth's youngest example of a continental flood basalt. Its south end, in California, is exposed here on the Pit River.15of 17
Columbia River Basalt in WashingtonPhoto (c) 2005 Andrew Alden, licensed to About.com ( fair use policy)
The Columbia River basalt in Washington, across the Columbia River from The Dalles, Oregon, last erupted about 15 million years ago. (click full size)16of 17
Columbia River Basalt in OregonPhoto (c) 2005 Andrew Alden, licensed to About.com ( fair use policy).
Tectonic activity in southern Oregon broke apart the gigantic lava plateau into ranges (like Abert Rim) and basins. See more photos from this region.17of 17
Pillow Basalt, Stark's Knob, New YorkPhoto (c) 2008 Andrew Alden, licensed to About.com ( fair use policy)
Basalt erupting underwater swiftly solidifies into pillow lava or lava pillows. The oceanic crust is largely composed of pillow lava. See more pillow lava