How Are Natural Arches Formed?

Fin Formation

• Before an arch can form out of the rock, the fins must be formed. These fins are thinner strips of rock that jut out of the ground. These fins occur in areas where cracks in the Earth's crust are exposed to the elements. Over time, these cracks expand and deepen due to its exposure. Once these fins are formed, the elements transfer their forces to the walls of the fins, forming the natural arches little by little.

Temperature Changes

• One of the contributing factors to the formation of natural arches is extreme temperature changes in a short period of time. This is why natural arches are most common in desert areas. Desert areas are known for their hot temperatures during the day and their colder temperatures at night. These temperature fluctuations take place throughout the year, which allows for the creation of the natural arches over a shorter period of time. Arches that are formed in this manner in other areas can take longer.

Frost Expansion

• While water is not abundant in the desert, when it does rain, the water collects in the crevices of the rocks. As the temperature drops during the night, the water in these crevices freezes and expands. In the daytime, the frost melts again and the process repeats. Each time the water freezes in the crevices, the crevices become wider. Eventually, the crevice breaks through the rock and a chunk of the rock falls out. This forms a small arch. As more chunks of rock fall out, the arch grows.

Chemical Exfoliation

• Some arches form in other areas of the United States as well. Because these arches are formed from different types of rocks and without the quick, extreme temperature changes, they require different types of erosion. Chemical exfoliation takes place when the water dissolves the rock into smaller pieces. As these pieces dissolve, the opening that will become a natural arch gets bigger. In some cases, these arches begin as potholes in the surface of the rock before the chemical exfoliation affects the side of the hole and creates an opening.

Other Factors

• Many other factors can contribute to the formation of arches depending on the location and type of rock. Tectonic plate movement can help to create the ridges in which arches may later appear. Water flow and glacial movement can also contribute to the formation of the natural arches. Rain and wind can play a factor in some types of erosion that form natural arches, especially where the rock is soft. For each type of arch, specific conditions must be met to result in an arch.