Classroom-Ready Igneous Rock Characteristics Quiz

Scoria is a dark, mafic volcanic rock that is glassy and extremely vesicular. It commonly contains minerals like pyroxene, olivine, magnesium, plagioclase feldspar, and biotite mica. Unlike granite, basalt, and quartz, scoria forms as a result of explosive volcanic activity that leads to the rapid cooling and solidification of lava with high gas content, resulting in its vesicular texture.

Pumice is characterized by its felsic mineral composition, light color, glassy strands, and vesicular texture, which are in stark contrast to the characteristics listed in the incorrect answers.

Obsidian is typically black in color, has a glassy appearance with no visible crystals, and is classified as felsic in terms of composition.

Basalt is a mafic rock with a dark color, typically aphanitic, meaning crystals are present but not visible to the naked eye. It contains minerals such as plagioclase feldspar, pyroxene, olivine, magnetite, and ilmenite.

Andesite is classified as an intermediate rock with medium to light grey color, commonly containing phenocrysts such as feldspar, quartz, pyroxene, and biotite mica. Basalt, rhyolite, and granite are different types of rocks with distinct compositions and characteristics compared to andesite.

Rhyolite typically has a light color, felsic composition, and no visible crystals. It may contain phenocrysts like feldspar, quartz, and biotite mica in a glassy matrix. The incorrect answers describe characteristics of other types of rocks such as mafic, intermediate, and compositions not typically found in Rhyolite.

Diorite has nearly equal proportions of light, felsic minerals and dark, mafic minerals, making it an intermediate composition.

GranitePhaneritic is abundant in granite and is characterized by its felsic composition. The correct answer lists the four main minerals found in GranitePhaneritic, while the incorrect answers offer descriptions of other types of rock compositions to create a diverse array of options for the multiple-choice question.

Gabbro Phaneritic is a mafic, phaneritic rock composed of pyroxene, plagioclase feldspar, amphibole, and olivine.

Aphanitic texture forms when lava cools rapidly at or near Earth’s surface, preventing crystals from growing large enough to be visible without magnification. This fine-grained texture is typical of extrusive igneous rocks, such as basalt, rhyolite, or andesite. The rapid cooling rate contrasts sharply with phaneritic textures, where slow underground cooling allows large crystals to form. Aphanitic rocks are identified primarily by color and mineral patterns rather than crystal size, making texture an essential classification feature.