Is this igneous rock intrusive or extrusive?
You can't really tell from this view. A close up view would show small crystals in a dark colored rock. This indicates rapid cooling from a basalt lava. The most interesting feature you can see are the hexagonal "posts." For them to form, cooling must not have been too rapid. This amazing feature is Devil's Tower in Wyoming.
Igneous Rock Classification
Igneous rocks are classified by composition and texture. The composition can be felsic, intermediate, mafic, or ultramafic. The composition depends on the minerals the rock includes. A felsic rock will contain felsic minerals.
Texture indicates how the magma cooled. Magma that cools rapidly has small or no visible crystals. Magma that cools slowly has large crystals; reference for the type of rock, the amount of silica it has, and examples.
[Figure 1]
Is this an intrusive or an extrusive igneous rock?
[Figure 2]
Is this an intrusive or an extrusive igneous rock?
Pictured above are two pairs of igneous rocks (Figures above and above). The first pair is two rocks that formed from a felsic magma. The rhyolite cooled rapidly, and the granite cooled slowly. The second pair is two rocks that formed from mafic magma. The basalt cooled rapidly, and the gabbro cooled slowly. The color of the rocks and sizes of the crystals are your clues. There are other rock pairs that you could find listed below (Table below).
Summary
Igneous rocks are classified first by composition. Categories go from from felsic to ultramafic. Color can indicate composition.
Texture is second when classifying igneous rocks. Texture indicates how a rock cooled.
Igneous rocks are categorized in pairs. The two rocks in a pair have the same composition but different textures: gabbro-basalt, diorite-andesite, and granite-rhyolite.
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