Article

Two Different Types of Granite

City of Rocks National Reserve contains two different types of granite. They look almost identical from a distance, but they are actually separated by billions of years. Green Creek Complex Granite is 2.5 billion years old and is the oldest rock west of the Mississippi. Almo Pluton Granite is much younger at only 25 million years old.

The two types of granite are easy to tell apart up close by observing the differences in the mineral grain size and the color of the rock. Green Creek Complex is a much darker gray and has larger crystals (potassium feldspar). Almo Pluton is a much lighter white color and has small evenly dispersed crystals.

Identify the type of granite in each picture, then compare them side by side. Slide the arrow on the picture to reveal the answer.