Volcanic activity causes magma (molten rock) to be violently ejected at the earth's surface. If magma cools slowly,as in a lava flow,a dense rock known as basalt may be the result; if cooling is extremely rapid,a volcanic glass such as obsidian may result. Gas bubbles trapped in magma are released in the same way as the carbonation in soft drinks when a container is opened. An extreme example of this is scoria,a dark rock created by the rapid cooling of basaltic magma that is filled with expanding gas bubbles | |
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Credit: | Science Photo Library / Arem, Joel |
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