Igneous rocks are among the oldest in the world. They are formed following the solidification of lava, magma or volcanic ash. Learn to identify igneous rocks and distinguish them from other types of rocks - sedimentary or metamorphic.
Steps
Method 1 of 2: Sedimentary or Metamorphic Rocks
Step 1. To distinguish igneous rocks from sedimentary rocks, check for fossils, shells and blunt grains
All igneous rocks have intertwined crystals; in some rocks, these crystals are so large that they are visible even to the naked eye. Other igneous rocks are formed of crystals so small that the rock appears to have a smooth texture. Sedimentary rocks are not crystalline, but somewhat granular (clastic); moreover, it is possible to observe the grains with a magnifying glass.
Step 2. Note the strata in the metamorphic rocks
Igneous rocks have no layers. However, even some metamorphic rocks do not have layers, for example, marble is made up of calcite and quartzite, composed of grains of quartz. By contrast, igneous rocks are never made up solely of calcite or grains of quartz.
Method 2 of 2: Recognizing Igneous Rocks
Step 1. Classify the rocks into two main types:
volcanic or extrusive, which are formed when lava, dust and ashes explode from the volcano; and intrusive or plutonic, which form when magma or molten rocks cool and solidify beneath the Earth's crust.
Divide igneous volcanic rocks into two types: rocks that form from molten rocks (lava); and tephrite or pyroclastic materials that are formed when the volcano erupts ashes and dust which are then deposited on the earth
Step 2. Distinguish the different types of igneous rocks - pegmatitic, phaneritic, aphanitic, porphyritic, vitreous, vesicular, pyroclastic - based on crystal size or texture
Rocks with larger crystals slowly form below the earth's surface; those with smaller crystals form quickly immediately after the eruption of the lava and its consequent cooling. The glassy rocks, on the other hand, are formed so quickly that they do not allow the formation of crystals. In addition, large crystals are visible to the naked eye, while small ones require a microscope.
- Pegmatitic igneous rocks have very large crystals (larger than 2, 54 cm).
- Phaneritic igneous rocks are composed of intertwined crystals, smaller than those of pegmatitic rocks but still visible.
- Afhanitic igneous rocks have a small grain texture and most of the crystals are too small to be visible to the naked eye.
- Porphyritic igneous rocks have crystals of two different sizes.
- Igneous rocks that form too quickly do not have crystals and have a so-called vitreous texture; instead they have a random alignment. Obsidian is the only glassy igneous rock recognizable by its dark color (although in some small sections it is transparent).
- Vesicular igneous rocks, such as pumice, have a bubbly appearance and form before gases are able to escape during lava solidification. These also form when very rapid cooling occurs.
- Pyroclastic igneous rocks have a texture characterized by volcanic fragments that can be very small (ash), thick (lapilli), or very thick (clastic and detrital).
