It's hard to imagine something so unchanging as a rock. Stones appear to be immovable objects that will stay the way they are now for eternity. But while it's true that Stonehenge will probably not erode away within the next few years and that a precious gem can be handed down for generations, rocks, like everything else on the earth, evolve and change over time. Over millions of years, the shape, make-up, and qualities of rocks can change. This process of evolution and change is called the rock cycle, and it is part of the nature of Earth's constantly changing surface.

Although there is no real "beginning" to the rock cycle, as it is indeed a constant process, with all elements happening simultaneously in different locations across the earth, the first stage is often said to be igneous rock. To understand this first step, we must first understand the nature of the earth's surface. Until relatively recently in human history, we've understood the earth as a constant thing. Now, we know that the crust of the earth is made up of plates, which are metaphorically "floating" on top of the super-hot mantle, occasionally moving and bumping into each other. Plate tectonics explores this theory. One may have heard about the theory that South America may have once been a part of Africa or about the mega-continent of Pangaea. Because the plates are moving and shifting, some sections are pushed into one another, and in other cases, sections slide apart. These exposed areas often lead to volcanic activity, where magma rises from the mantle to the surface. This can happen in volcanic places (like Hawaii), or underwater in vast oceanic trenches. When the magma bubbles up to the surface and then cools and crystallizes, it forms igneous rocks, the first step of the rock cycle. Common igneous rocks are basalt, obsidian, pumice, and granite.

The next stage of the cycle involves the change from igneous rocks to sedimentary rocks. This stage often takes many hundreds or thousands of years and has to do with weathering and erosion. Many kinds of environmental changes slowly degrade rocks, from frost, to glaciers, to vegetation, to wind, to rivers. Often, little bits of rocks are pulled away due to one of these factors, and these bits are called sediments. Due to these environmental changes, the sediments are often transported to a new location (like a river bed), where they collect. Lithification, the process in which the sediments turn back into stone, often is caused by slow compaction and pressure. This process is evident in the popular tourist attraction the Grand Canyon, where a river has deposited and eroded sediment over time. Sedimentary rocks are common, and some names you may recognize are limestone, shale, salt, and coal.

The final stage happens when heat and pressure increase more and more over time and the sedimentary rock is often pushed deeper into the earth's surface. This process can form metamorphic rocks, which are sometimes valuable materials like quartzite and marble. As this heat and pressure continue to increase, even the metamorphic rocks melt, returning them to magma. Hence, we are at the beginning of the rock cycle again.

Note that Earth as a whole is not going through this change and that every stage of the rock cycle is represented across the globe almost constantly. You might find lava cooling into extrusive igneous rocks in Hawaii, sedimentary rocks forming in the Grand Canyon, and the slow compacting of sedimentary rocks in the Himalayas. As the earth's surface moves and changes, humans can take advantage of many different materials and minerals, harvesting them for gems, building materials, and industry.

Read up on some rock cycle resources below:


Content written and provided by Vanessa LeBeau