If you want to know how a natural arch is formed, here is a quick explanation. Below is an easy natural arch definition that will help you understand how these beautiful rock formations occur.
A natural arch is formed by erosion. Brittle rock is gradually worn down and washed away over a long period of time. The stronger rock is left behind. Sometimes a natural arch is formed by a river or stream running through a canyon for many years. Other times, rain runs down deep vertical cracks that cause chunks of rock to break off and fall away over time.
There are several different kinds of natural rock arches. Some are found by the ocean while others are found in the desert. Not all arches are created equal.
What are natural arches made of?
- Loose sandstone (soft, wears away)
- Limestone
- Calcium carbonate
- Quartz
- Calcite
- Iron oxide
- Shale
A natural arch can contain many different materials. The above list is just an example of some of the most common elements you’ll find in nature.
Natural Arches Start Out as Plateaus
Natural arch erosion often takes place on a plateau first. You can see vertical lines and deep cracks in the photo above. Over time, the elements run along these points of weakness and cause chunks of rock to fall away. When rock freezes, it causes expansion that makes it fall off faster.
Wind, sun, and rain contribute to the rock wearing away as well. Sometimes the rock is made up of a mix of materials. The softer material is the stuff that gets washed away first. The harder material is the stuff that sticks around longer and can even hold the shape of an arch.
Plateaus Wear Down and Form a Natural Arch
It takes many many years for a natural arch to form. The process happens slowly and gradually. You’ll often see a large rock formation progressively turn into a plateau as the rain washes the sides down into a steep cliff.
Once it becomes a plateau, large cracks start to vertically form down the sides of the cliff. The wind, frost, freeze, sun, and pouring rain all contribute to the weathering and breaking away of these cracks. Fragments of rock naturally fall off and can create all kinds of interesting shapes in the process.
One of those popular shapes is the natural arch bridge as we know it. The weakest points in the cliff walls often break away in the middle, leaving a beautiful window or hole below. It’s only a matter of time before the bridge shape collapses as well.
Cracks in Cliff Walls Break to Form Natural Arches
The formation of natural arches is caused by the breaking of joints in the cracks of the cliff walls. When the weather wears these joints down, they snap as pieces of the rock fall to the ground below.
The arch portion of the bridge is formed by many localized breaks in the rock’s joints. The more breaks, the more that rock fragments will fall away. When the joints line up in a parallel fashion, they are more likely to drop off the wall. Haphazard layering of the rock’s joints tends to give the structure more stability and prevents breakage.
Shale, which is a type of sedimentary rock, helps to support the shape of the natural arch. It assists in the prevention of structural collapse. Multiple layers of different kinds of rock also help to hold the natural arch in place.
Weather and Elements Form Unique Shapes
It is widely known that the elements, wind, and water cause the formation of natural bridge arches through erosion.
Wind, in particular, provides a uniform force that causes the rock to wear away at a steady rate. Most of the rock formation is eroded by wind evenly. When the wind beats against the rock over many years, the majority of the rock will be affected equally. The whole thing wears down at the same pace.
This is not the case for water erosion. Water is the main cause of the unique shapes we often see in natural arches. It tends to run in grooves and cracks along the rock’s walls. Because the water flows towards the cracks, it makes these areas wear away faster.
Water washes away the softer rock material first, which often leaves an interesting shape behind.
Natural Arch Formations Along the Coast
Some natural arches are formed along the coast. You’ll often see a lot of rock landscaping, cliffs, and large boulders by the sea. The waves of the ocean push in and pull back from the stony coastline. This repetitive process slowly eats away at the rock as it flows in and out repeatedly.
The crashing in and out of the waves erodes the rock formations, creating unique shapes in the process. Sometimes the water will gain more power at certain points along the rock. The waves can slam the rock hard enough to create a hole or passageway over many years. Through friction and pounding, the waves slowly drill a pathway through the rock, creating a natural arch bridge formation.
Some Natural Arches are Formed by Cave Remnants
Imagine looking at the open entrance of a cave. The opening contains a huge hole in the rock. Sometimes there are sinkholes inside the cave that cause the interior to collapse and break away. This leaves the cave opening standing strong while everything inside crumbles and drops deep into the earth.
Sinkholes can contribute to the formation of natural arch bridges in this way. What was once a cave now appears as 2 large rock pillars with a connecting natural arch bridge overhead. The rocks that fell away into the sinkhole are no longer visible because they dropped into the depths of the earth.
Do natural arches eventually collapse?
All natural bridges are formed through the process of erosion. What you see is the result of many years of weathering elements gradually wearing the structure down. Eventually, the entire natural arch bridge will collapse as part of the natural process of erosion.
It is not recommended to walk across or build anything significant on the top of natural arch bridges for this reason. The arch could break and collapse at any given moment. This is a natural and inevitable process that should be respected when considering any kind of activity around these structures.
What is the world’s largest natural arch?
The world’s largest natural arch bridge is the Fairy Bridge in China. It was formed naturally by the current of the Buliu River. In Chinese, it’s called the XianRen Bridge. It measures about 400 feet across, according to the Natural Arch and Bridge Society (NABS).
Where are natural arches found?
Natural arches are often found along the coastline, near cliffs, around plateau regions, through rivers, canyons, and in some cave landscapes.
Natural arches can be found in a variety of areas and in a variety of styles. Here is a list of some of the most popular natural arches in the world:
- Utah, United States (there are many famous bridges in this state!)
- Shipton’s Arch, China (near Artux) – one of the tallest natural arches
- Pont d’Arc, France – natural bridge along the Ardèche River
- Durdle Door, England – beautiful limestone arch on the sea
- Arco Naturale, Capri – stunning ocean views in pristine nature
- The Azure Window, Malta – a breathtaking natural arch sea window
- The Green Bridge of Wales, Wales – a natural bridge in a magical setting
What is the difference between a natural bridge and an arch?
The table below compares a natural bridge vs arch:
| NATURAL BRIDGE | NATURAL ARCH |
| A type of arch | A broad category that includes natural bridges |
| All natural bridges are a type of natural arch. | All natural arches are not necessarily natural bridges. |
| Natural bridges often have water running under/through them, or were created by a river or stream. | Not all natural arches are created by a river or stream. |
| Natural bridges can be used to travel across or walk on. | A natural arch is not always used to travel across or walk on. |
Hopefully you found this article super helpful and that it answered all of your questions regarding how a natural arch is formed.
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