Zhangye Danxia National Geopark, Gansu province, China

Walking a rocky rainbow. International Rock Day

International Rock Day, on July 13, celebrates these hardy heroes, which have played a big part in human history since at least the Stone Age. They served as humanity's first tools and as nature's journals, protecting ancient fossils and documenting changes caused by geological processes.

Our homepage image shows Zhangye National Geopark's Rainbow Mountains in China, an incredible landscape created over millions of years. Those colorful layers are formed of sedimentary sandstone rock. Iron oxide gave the rock its predominately rusty hue, while other oxides tinted the various layers brown, yellow, and green. Sandstone and minerals that were once layered on top of each other were lifted and distorted by tectonic movement, revealing those colorful stripes. And that is not the end of this geological tale. This unique landscape continues to change, as wind and water erosion sculpts new pillars and ravines. No matter what your favorite category of rock is—metamorphic, sedimentary, or igneous—rocks of every kind helped build the world we live in today.