Water has a greater opportunity to penetrate into the cracks of a'a, where it expands explosively and fragments the lava into airborne particles. Littoral cones can develop at the water's edge from the accumulation of these airborne fragments. These are bulbous bodies with quenched, glassy rinds. They are often spherical, with diameters of cm, although many are lobate.
During formation, the outer surface of each lobe chills instantaneously. However, the constant injection of lava may cause a single lobe to swell, thus cracking the glassy rind, allowing a new lobe bud forth. Individual pillows can break off the budding lobe and cool as a single mass. As the interior of a pillow cools, it forms crude cooling joints that radiate inward, perpendicular to the cooling surface.
Each pillow will settle into gaps between the pillows below it. In continental lakes, pillow accumulations can be several tens of meters thick, whereas those that develop on the ocean floors can be hundreds of meters thick. Basalt fragmentation can occur by the explosive eruption process, or by an essentially nonexplosive process associated with the spalling of pillow basalt rinds by thermal shock or chill shattering.
The hyaloclastite fragments are equant, angular shards of black glass, generally between. This water-quenched basalt glass is called sideromelane , a pure variety of glass that that is transparent, and lacks the very small iron-oxide crystals found in the more common opaque variety of basalt glass called tachylite.
In hyaloclastite, these glassy fragments are typically surrounded by a matrix of yellow-to-brown palagonite , a wax-like substance that forms from the hydration and alteration of the sideromelane. A related hydrovolcanic deposit, called palagonite tuff , contains a mixture of sideromelane shards and coarser-grained fragments of basaltic rock in a palagonite matrix. Hyaloclastite Bedded palagonite tuff Limu o Pele - Recent hydrovolcanic basalt deposits may preserve paper-thin glassy fragments known as Limu o Pele seawead of Pele.
What gives basalt its color? Basalt has a lower percentage of silica and a higher percentage of iron and magnesium than other volcanic rocks. These characteristics give it a very dark, almost black color. Hot, mineral-rich seawater flowed through much of the basalt that is part of the Franciscan Complex, changing some of its minerals into chlorite and other green minerals. This altered basalt is called greenstone. What makes basalt sometimes appear speckled?
Sometimes basalt in the Franciscan Complex will have small white or pink speckles or even small round holes in it. These are the result of gas bubbles that were captured in the lava when it cooled. Geology Wiki Explore. Wiki Content. Explore Wikis Community Central. Register Don't have an account? Pillow basalt. Edit source History Talk 0.
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