Here's why:
* Extrusive means the rock formed from magma that erupted onto the Earth's surface (as lava) and cooled quickly.
* Basalt is a dark-colored, fine-grained volcanic rock that is very common on Earth's surface. It often forms from the eruptions of shield volcanoes.
Other examples of extrusive igneous rocks include:
* Rhyolite: A light-colored, fine-grained rock with high silica content.
* Andesite: An intermediate-colored rock with a composition between basalt and rhyolite.
* Scoria: A vesicular (full of holes) rock with a rough, dark surface.
* Obsidian: A glassy, dark-colored rock that cools so quickly it doesn't form crystals.
