Geology - Denudation
Posted On: 2020-08-15
Geology 101 - Gale Martin - Class Notes
The surface of the earth is constantly being modified by two sets of forces. The internal forces of the earth, tectonics, deform and alter rock producing massive mountains. The external forces, denudation, wear away the mountains through weathering and erosion.
These opposing forces create the topography we see: from majestic mountains to rolling hills, flat plains and deep ocean basins.
Gravity and the hydrologic cycle are major contributors to denudation. As water is recycled on the earth's surface, it assists chemical reactions and moves material to new locations. This is accomplished through gravity driven forces. There are many forms of erosional agents which can be observed: mass wastage, fluvial, groundwater, glacial, eolian, lacustrine, littoral processes and others. This course will present a general overview of only the first few.
Erosion occurs when there is sufficient energy to move sediment. Any moving force, wind, waves, streams, etc., can accomplish this. They are all driven by gravity (Ex.: water flowing from high mountains to lower elevations). Deposition of sediment occurs when there is a decrease in available energy.
Erosion can occur in three ways. Sediment can be picked by through the hydraulic action in the moving medium. Image the forces of currents under your feet in a fast moving stream, the pounding waves on a beach, or the push of wind against your back. Sediment is picked up and carried along with the downhill movement. The grains bounce and hit one anther as they shift downward, producing abrasion on the grain's surfaces or along exposed rock outcrops. This "sand blasting effect" loosens more sediment to be picked up and carried along. Lastly, if the rock is chemically soluble, dissolution will occur as rock is dissolved and remove by the water.
Each erosional agent reacts differently with the rocks they traverse. Glaciers carry masses of rock that have been torn from the outcrops through frost wedging and pulverized by continuous abrasion. Streams roll and push sediment in a continuous "conveyor belt" of sediment to the oceans. Groundwater efficiently carve long networks of caves along fissures in limestone. Each environment will produce unique characteristics that, when preserved in the rock as sedimentary structures, will tell the tales of how that rock formed.
Mass Wastage or Mass Movement
When rock and loose sediment move down slope due to gravity, the process is referred to as mass wastage or mass movement. This process can be sudden and catastrophic in nature or slow and gradual over years time. Many factors control whether sediment or rock remains stable or shifts by mass movement. These include the gradient, water, amount of vegetation and rock characteristics. (It's all a matter of physics. Which is greater: gravity forces? or the cohesive nature of the rock outcrop?)
One of the single most important factors in the stability of a rock outcrop is the gradient of the outcrop. Loose rock material cannot form vertical cliffs. The rock slides down into a conical slope no steeper than the angle of repose (approximately 30). As rock is forced upward into mountains by tectonics, the loose material slides off the ridges into talus slopes.
Several characteristics of the rock itself influence its cohesive nature. Rocks that are well consolidated or crystalline can form steep outcrops that are stable in nature. These include many igneous rocks, nonfoliated metamorphics and silica cemented sedimentary rocks. (Ask any climber, they can tell you.) Rock layers that are oriented opposite the slope direction, i.e. dip into the hill, are more stable than rocks whose beds or foliation planes slope in the same direction as the hill side. Finally the shear weight of the rock or thickness of the bed can be a deciding factor in control of mass wastage.
Water can be important. Though a small amount of water actually increases the cohesiveness of rock material, excessive water allows slope failure. Water reduces friction and acts as a lubricate between grain surfaces in a rock mass. It results in a buoyancy factor, lifting and supporting individual grains. The addition of water to an outcrop also contributes an added weight factor, causing previously stable slopes to fail and move.
Vegetation when present, slows mass wasting by holding and stabilizing the ground with the massive root structures. Fires and land development in steep terrain often lead to unstable slopes that slide with the slightest incentive.
Sometimes rocks can remain in an unbalanced region for long periods of time and only move when another event triggers greater instability. Such triggers include earthquakes, heavy rains, volcanic eruptions, seasonal fires or careless land development.
Though most mass movement events reported in the news are quick catastrophic events, many slower and less traumatic forms of mass wastage are possible. Classification of mass movement is commonly based on the consolidation of the rock material, the amount of water present and the rate at which the movement occurs. The classifications are divided into three general categories: flows, slides and falls.
Flows are characterized by a mass of loose sediment or rock that acts as a unit but contains individual particle motion. Water is a common factor and acts as the medium by which the grains move. Usually the more saturated the flow material is the more rapidly the mass moves. Slow forms include creep. Here the sediment or soil along the surface slowly moves downhill as individual grains are lifted during a freeze and inch downhill with the following thaw. Creep acts at rates which are not visible to an untrained eye. It can be perceived by curved tree trunks, tilted fence posts and bulking of roads in areas prone to creep. Solifluction, also slow in nature, occurs in areas of permafrost: a thin veneer of saturated soil slipping along a frozen surface in high tundra regions. Faster forms of flows include earthflows, debris flows and mudflows. They may result from a heavy rain that produces failure in an accumulation of loose debris on steeper slopes. These flows commonly act as a thick viscous fluid that moves downhill along a previous low lying area. The resulting landforms are hummocky in nature and may appear as "tongues" which extend from mountain valleys into flat lying reaches.
Slides are mass movements that occur as a single consolidated mass along a plane of weakness. They consist of two general styles of movement based on the shape of the failed surface. A slump is a mass of rock that fails along a curved surface. When the rock mass moves it commonly rotates (like a person slipping on a banana peel and falling on their backside). In areas prone to slumps it is common to find curved scrapes along the upper portion of the slump zone. Slides, such as landslides and rock slides, fail along a flat surface (Ex.: a bedding plane). Water may seep into the crack and lubricate the surface to act as a "slide" for the rock to slip across. Trees and plants on the surface of a slide can remain intact during the event and continue to grow after the movement.
A fall is individual rocks that tumble down a steep slope or cliff. The rock bounces and plummets down the slope at great speeds usually free falling through the air. They accumulate at the base of a cliff as talus piles.
The above classification is a generalization. There are many forms of mass movement that can be considered a mixture of all three types. (For example: an avalanche: falling off the mountain ridge, slipping down the slope and cascading over a ridge!) Anywhere there are slopes, either steep or very low, gravity comes into play and mass wastage occurs.