Fill in the Blanks Classifying Igneous RocksOnline version A Fill in the Blanks game about the classification of igneous rocks. by Mark Masloski 1 Composition Porphyritic Coarse Granitic Texture fine glassy Igneous Basaltic Classifying Igneous Rocks rocks are classified based on their texture and composition . refers to the size and arrangement of the mineral grains in the rock . - grained rocks have large visible crystals , while - grained rocks have tiny crystals that are hard to see with the naked eye . A rock texture indicates rapid cooling , resulting in a smooth appearance without visible crystals . texture in rocks has large crystals , called phenocrysts , embedded in a finer - grained matrix . refers to the types of minerals present in the rock . Igneous rocks can be classified into two main types : intrusive and extrusive . igneous rocks are usually light in color because they have only about 10 to 25 percent dark minerals . igneous rocks are generally dark and heavy because they have a lot of magnesium and iron .