LL Demos & Labs: Rock Profile: Vesicular Basalt
This is a dark gray rock with lots of large, irregular holes that hail from Central Oregon. Once outside, the hot lava cools very quickly as it comes into contact with either the air or ocean water. This rapid cooling prevents minerals from growing large enough to form crystals that can be seen without magnification, which explains why your specimen appears to be a single, uniform color. A vesicle is a bubble. In this case, very rapid cooling traps tons of small bubbles of air, steam, or other gases. This causes many holes, which is a common characteristic of bubbly basalt, also known as vesicular basalt.