Natural Arch Formation

As stated in the definition of what natural arches are, they are formed by the natural, selective removal of rock. The natural processes that lead to selective removal of rock from a rock exposure are almost exclusively processes of erosion. Erosion can selectively remove rock both macroscopically and microscopically. Both modes are effective, albeit on different time scales, because of the basic structure of virtually all types of rock.

Rock of any type (with the sole exception of a pure crystal) is a complex matrix of small, interlocking, solid particles. These particles are mostly microscopic fragments of various mineral crystals known as grains. Under high temperatures and pressures, some of the crystalline grains fuse, especially the smaller ones, and act as a cement between the larger grains.

Macroscopic erosion occurs when joints or fractures are first induced in this rock matrix through some (usually catastrophic) process, and then widened through a variety of other processes. This splits the rock into distinct macroscopic pieces that can then move relative to each other under the forces of gravity or water pressure.

Microscopic erosion occurs when certain processes dissolve the crystalline cement, thus destroying the rock matrix and allowing other processes to disperse the remaining loose grains.

Both types of erosion occur separately and in combination on all rock exposures. Only under very special circumstances will a natural arch form. These circumstances include the type, or types, of rock that are present, the shape of the rock exposure (especially in relation to the gravity gradient), and the combination of erosional processes that act upon it. Usually a very specific sequence of erosional processes must operate on a specific shape of rock exposure before a natural arch will form. Since some erosional processes are more effective on certain types of rock than others, the type of rock is also an important factor.

Relevant Processes of Erosion

Several processes of erosion can contribute, usually in combination, to natural arch formation. Each of these process is described separately in the paragraphs below. Different sequences or combinations of these individual processes conspire to form natural arches of different types. Because the type of arch is critically dependent upon them, these combinations are described as part of the natural arch taxonomy included on this site rather than here.

Before delving into the details of these processes, an important observation should be made to dispel what has been a persistent myth about natural arches. Every single process relevant to natural arch formation involves the action of water, gravity, temperature variation, or tectonic pressure on rock. Wind is not a significant agent in natural arch formation. Wind does act to disperse the loose grains that result from microscopic erosion. Further, sandstorms can scour or polish already existing arches. However, wind never creates them.

Finally, it must be acknowledged that most of the material in the paragraphs below is based on more detailed treatments by several other authors available in the literature on geology and physical geography. An excellent summary of this material is found in the chapter in reference 3 on natural arch formation and in its bibliography.

Tectonic movement and uplift. The earth's crust consists of plates that float on a sea of magma. Magma is rock that is liquefied by the tremendous pressures of the earth's interior. As these crustal plates slowly move over the magma, a process known as tectonic movement, they collide in places. Such collisions cause portions of the plates to be raised up. This is one example of what is known as uplift. Tectonic movement can also result in thinner areas of crust gradually becoming repositioned over hot spots in the magma. When this happens, these areas also experience a general uplift due to the increased pressure from below. Uplift generally accelerates erosion. It is especially important in creating certain land features that frequently are the precursors to natural arches, e.g., joints, fins, and incised meanders. As a result, many of the world's natural arches are found in areas currently experiencing uplift.

Glaciation. The advance and retreat of glaciers can result in significant erosion. Advancing glaciers can carve shear-walled valleys and highly sculpted terrain. Such features are likely places for natural arches to form. The run off from retreating glaciers usually causes a temporary increase in local erosion rates. This also may contribute to arch formation if other conditions are right.

Incised meander. A continuous flow of water over rock, e.g., a stream or river, will erode its path into that rock. If the rock is highly sloped, the water will generally cut a fairly straight channel down the slope. However, if the rock is level, the water will snake its way around any slight bump in the terrain. This frequently leads to the water course making wide, curling loops that almost, but not quite, double back on themselves. Such a loop is called a meander. The point where the water course almost closes the loop is called the neck of the meander. If there is uplift in the area, the water will tend to erode its path into the rock to remain at a constant elevation as the rock around it rises. If the uplift is rapid, shear-walled cliffs may form along the banks of the water course. In this way, meanders can become deeply incised into rock. For many such incised meanders, the neck will become a tall, thin wall of rock. Other processes of erosion can then create an opening through the wall to form a natural arch.

Lateral stream piracy. When two water courses, e.g., two streams, are separated at some point by a relatively thin rock barrier, this barrier may be breached, allowing one of the streams to shorten its path. In a sense, the water of one of the streams is 'stolen' by the other. This is known as lateral stream piracy. It can occur in two similar situations. One is at the neck of an incised meander. The other is where two tributaries run closely parallel to each other for a distance upstream of their juncture. The breach in the separating barrier may be caused by any of several processes, but most of these do not lead to arch formation. The process of interest here is wall collapse, which can lead to the formation of a natural arch. The opening created by wall collapse grows down to a level where water can flow through the opening when the stream is in flood. This clears out any debris in the opening and accelerates the growth of the opening. Eventually, the stream channel is re-routed through the opening, completing the process of lateral stream piracy.

Subterranean stream piracy. Water flowing over rock in a channel, e.g., a stream, will, of course, seep into any cracks or joints in that rock. In most cases, seeping water will cause chemical exfoliation and freeze expansion, enlarging the crack or joint. This allows a greater flow of water into the crack or joint which accelerates erosion. When cracks and/or joints combine to create a pathway through the rock through which the water can travel and rejoin the stream (or a different nearby stream), subterranean stream piracy can occur. Basically, the pathway is enlarged until most, if not all the water in the stream flows through it rather than the original channel. It has 'stolen' the water from the original stream. When this occurs at the lip of a waterfall, a waterfall natural bridge may form. In other situations, subterranean stream piracy can create long and extensive underground passageways. These may become caverns (a type of natural arch) or, if roof collapse occurs above the passageway, a variety of waterfall natural bridge.

Vertical joint expansion. Water seeping into a crack or joint in a rock exposure will, over time, act to enlarge the joint, creating a gap in the rock. Chemical exfoliation and freeze expansion frequently combine to cause this to happen. The expansion of joints that are roughly vertical may contribute to natural arch formation in several ways. Three examples follow:

• When a series of parallel vertical joints are present in a rock exposure, e.g., as a result of uplift or tectonic movement, some or all may expand into sizeable gaps. This results in a field of parallel rock walls or fins. Wall collapse and other mechanisms can then cause a natural arch to form in one or more of the fins.
  
  
• When a vertical joint is present near, and parallel to, a cliff, e.g., as a result of stress relief exfoliation, its expansion may couple with other processes, e.g., wall collapse or cavity merger, to form various types of natural arches.
  
  
• When a vertical joint is present in, and perpendicular to, an exposed wall, fin, or narrow projection of rock, it may expand preferentially near the bottom or middle. In certain cases, this can result in a natural arch being formed.

Bedding plane expansion. Sedimentary rock is deposited in layers. The boundaries between these layers, known as bedding planes, are similar to joints or cracks. Water seeping between the bedding planes will cause chemical exfoliation and freeze expansion. This often leads to the growth of a horizontal air gap between the layers of rock. In this way, the expansion of a bedding plane in a rock exposure can contribute to the formation of a natural arch.

Cavity merger. Differential erosion and chemical exfoliation acting on the surfaces of a rock exposure frequently cause concave recesses in the rock. As these grow into cavities, some may become connected. Cavities can become connected, or merge, by growing into and expanding a joint that was already present in the rock, or simply by growing into each other. This can happen in simple and complex ways. When a lintel is left as a remnant of the barrier that once separated the cavities, a natural arch is formed.

Roof collapse. When the roof of rock that is over a subterranean passage or a cave becomes too thin for the tensile strength of the rock to hold it together against the force of gravity, it will fracture catastrophically and collapse, i.e., sections of rock will fall out of the roof. The sections of roof that remain suspended may be left as the lintels of natural arches.

Wall collapse. Wall collapse is a complex, cyclic process that can occur as a result of gravity and thermal flexing acting upon a tall, thin exposure of rock. This process first causes the formation and growth of an arched shape recess (an alcove) above the base of the wall. This alcove eventually grows into a semicircular aperture through the wall. Wall collapse does not require water to occur, but the presence of water can accelerate it. It is one of the most important erosion processes that can lead to the formation of a natural arch. For this reason, and because of its complexity, the reader may choose to link to this more detailed description of wall collapse.

Wave action. The waves that batter the shoreline of a large body of water, such as an ocean, sea, or great lake, are a major force of erosion on any coastal rock exposures that are present there. Waves trigger and accelerate several erosional processes, especially chemical exfoliation, differential erosion, cavity merger, and wall collapse. In addition, particles carried in the waves (e.g., sand) act as an abrasive on the rock. As a result, coastal rock exposures experience erosion rates ten to a thousand times higher than those inland. Therefore, coastal natural arches are formed and destroyed relatively quickly and frequently. They are short-lived compared to most inland natural arches. Furthermore, combinations of erosional processes occur on coastal rock exposures that are seldom, if ever, encountered inland. This often results in natural arches of unusual shape.

Lava flow. Flowing lava cools from the outside in. At first, the crust of hardened, solid rock that forms on the outer layers of a lava flow gets carried along with it. But as this crust cools even more, it eventually thickens and stabilizes. Nevertheless, the lava inside this stable crust is still hot enough to flow. Indeed, the crust acts as an insulator, keeping the interior parts of the flow viscous for a long time. The 'inside' lava may even drain out of the stable, outer rock crust, emerging 'down-flow' to cool and become rock as well. This sequence of events frequently leaves behind long chambers or "tubes" in the interior of the newly cooled rock - "tubes" that were evacuated by the last of the hot, flowing lava. If roof collapse subsequently occurs above such a "tube," one or more natural arch may form.

Compression strengthening. The weight of rock is, of course, due to the force of gravity. This force acts to compress any rock that resists it. Normally, this force acts in the vertical direction. Rock underneath other rock is compressed by the weight of the rock above it, i.e., the rock it supports. However, when rock is supported over an opening or hole, the lines of force are diverted from the vertical into a pattern the shape of an inverted catenary. A catenary is the shape a rope takes when suspended freely from its two ends. An inverted catenary is that shape turned upside-down. It's the shape of an arch. Thus, the weight of rock above an opening compresses the rock that supports it along force lines that are arch-shaped. Regardless of whether the compression is vertical or arch-shaped, it strengthens the rock that gets compressed. This is because compression acts to fuse more grains, including larger grains, in the rock matrix. In effect, it adds cementing and increases the bonding force of the cement that is there. The rock becomes harder and more resistant to erosion. Natural arch lintels that take the shape of an inverted catenary often experience compression strengthening. Compression strengthening makes a lintel more resistant to erosion and, therefore, increases the lifespan of a natural arch.

Stress relief exfoliation. Rock is subjected to many forces. Tectonic movement, uplift, and gravity can each put stress on a rock exposure. Rock will eventually fracture as more and more stress is placed upon it. The specific point and pattern of the fracture is dependent upon a complex set of variables. When stress-related fracturing leads to macroscopic fragments of rock separating from a rock exposure, this is called stress relief exfoliation. Stress relief exfoliation contributes in many different ways to natural arch formation.

Chemical exfoliation. Water that is in contact with rock will, over time, dissolve the lattice of fine crystalline grains that cement the larger grains of the rock together. In effect, the water dissolves the rock into grains which can then be removed either by the water itself, gravity, wind, or other mechanisms. This process of erosion is known as chemical exfoliation. It contributes to natural arch formation in several ways. One of these ways is the creation of potholes, caves, and/or smaller depressions wherever standing, flowing, or seeping water comes in contact with exposed rock. Another is the expansion of joints into air gaps when seeping water gains access to a joint.

Differential erosion. When erosion proceeds at two different rates at the same location, e.g., on adjacent rock surfaces, it is called differential erosion. This can happen wherever the grain and cementing properties of rock vary from place to place in a rock exposure. For example, if the distribution of grain size in the rock matrix is different in one part of the rock exposure than in another, these two places will experience different rates of erosion. Differences in the degree of small-grain fusing, i.e., cementing, will also cause different erosion rates. Such differences commonly occur when a rock exposure comprises more than one geological formation or member. Each member will erode at its own pace. However, many geological members form as the result of a long period of sedimentary deposition. Such a member may consist of several layers laid down at vastly different times. Differences in graining and cementing can certainly occur between such layers. Therefore, differential erosion can occur in a rock exposure that consists of a single member. Differential erosion contributes to the formation of natural arches in several ways, e.g., the undercutting of harder layers of rock that are supported by softer layers.

High gradient of erosion. A rock exposure with a significant slope will erode faster, and be susceptible to more types of erosion, than a similar exposure with a gentler slope. This is simply due to gravity. Gravity can remove fractured rock fragments or loose rock grains from surfaces only if it can overcome friction. For any surface, there is a critical slope at which gravity is able to overcome friction and pull away the detached fragments of rock. This then exposes the next layer of the rock to erosion. The erosion cycle proceeds more efficiently, and hence more rapidly, when it gets this assist from gravity. An exposure with slopes greater than the critical value (which depends complexly on several factors) is said to have a high gradient of erosion. Natural arches are more likely to form on rock exposures with a high gradient of erosion.

Thermal exfoliation. Temperature fluctuation causes rock to expand (as temperature rises) and contract (as temperature falls). This cycle of alternating expansion and contraction frequently leads to the rock fracturing. Fractures preferentially occur along stress patterns in the rock. Fracturing then permits the removal of rock fragments by gravity or water pressure. Even when the ambient temperature is relatively constant, sunlight striking the surface of a rock exposure will create a temperature gradient in the rock. The surface layer of rock will become hotter than deeper layers. The hotter temperature of the surface layer forces it to expand more than the cooler, deeper layers. In effect, the surface tries to bow outward. This can lead to stress fractures parallel to the surface. Should these fractures also be parallel to bedding planes or vertical joints, huge sheets of rock can become detached from the rock exposure. The macroscopic fracturing and removal of rock as the result of temperature fluctuation or temperature gradients is known as thermal exfoliation. This process of erosion contributes to the formation of natural arches in many ways.

Freeze expansion. When seeping water that has permeated a rock joint freezes, it expands. This puts stress on the rock and frequently fractures the rock adjacent to the joint. As the water thaws and is replenished from whatever source is involved, it gains access to these fractures. In this way, repeated cycles of freezing and thawing will break up the rock along a joint into small pieces that can then be removed by gravity or water pressure. The expansion of joints into air gaps via this cyclic process contributes to natural arch formation in many ways. See for example the paragraph on vertical joint expansion.

Weathering. Weathering is the combined effect of precipitation and wind on the surfaces of exposed rock. Frozen precipitation, e.g., snow, can be a steady source of seeping water that can permeate the rock and cause localized chemical exfoliation. Steady or frequent rain may become a similar source. Strong winds can pick up grains and pummel the surface of a rock exposure with them, in effect sandblasting the rock. These processes act in combination to smooth and age the surface of rock. They seldom have sufficient impact to sculpt the rock to any significant degree. Therefore, although weathering sometimes plays a roll in how a natural arch ages, it is not a process that leads to the formation of natural arches.