The Earth’s surface is covered with geographic features. Mountains, canyons, caves, rocky shores, islands, and beaches are just some of the many examples of different landscapes that are present on the planet. One thing in common that all of these features share is that they were created by geomorphic processes. While there are many different types of these processes, they can be categorized into two distinct types, endogenic and exogenic.
Endogenic processes
Put simply, endogenic processes are those that occur beneath the Earth’s surface. While it may seem that the crust of the Earth is static and unmoving, that is actually not the case. The Earth’s crust and the uppermost mantle are known as the lithosphere, which is a rigid layer that floats on the deeper mantle. The lithosphere is broken up into multiple plates of varying sizes, which are known as tectonic plates. There are several large tectonic plates and dozens of minor plates that cover the entire Earth’s surface, both on land and beneath the oceans. The plates move anywhere between zero and 150 millimeters each year. The movement and interactions of the tectonic plates cause changes on the Earth’s surface.
Tectonic plates can either converge, diverge or transform. Converging plates push up against one another and one plate will always begin to sink under the other plate, which is known as subduction. The slow collision and subduction can cause several different geographic features to form, depending on whether the collision of plates occurs on continents, in the ocean or at their intersection. Volcanic activity at the convergence of plates is very common.
When an oceanic plate and continental plate collide, the result is the subduction of the oceanic plate and formation of mountains near the shoreline as the continental plate rises. The collision of two oceanic plates also results in one of the plates being subducted beneath the other. Thin but often deep trenches tend to form in the area of subduction, with volcanic activity being quite common.
Following the trench, the volcanic activity can lead to the formation of island chains as new material from the mantle comes to the surface beneath the water. The final convergence is between two continental plates. In this convergence, and contrary to the other, subduction is much rarer. The two continental plates typically have lighter rock that buckles and is either pushed upward or off to the side. At locations of this type of convergence, large mountain ranges tend to form.
Exactly opposite of convergence, divergence is where the tectonic plates move away from each other. As the plates diverge, the area between the plates is filled by rising magma from the mantle below. The magma rises to the surface, becoming lava, and then cools not only to fill the gap but also to create mountainous areas, volcanoes and other hot spots. Divergence that happens beneath the surface of the oceans often produces underwater mountains and volcanoes which remain unseen.
Tectonic plates are not like jigsaw pieces, and they don’t fit together smoothly. This is where transformation comes into play. In transformation two diverging plates grind against each other as they move apart. The boundary between the tectonic plates can be thought of as more of a zigzag pattern where there are areas in which the two diverging plates touch as they move against each other. While this interaction between plates does not produce volcanic activity or lead to the creation of mountains, the stopping or slowing of the movement, due to the grinding, can lead to significant earthquakes.
Exogenic processes
Unlike the endogenic processes, the exogenic processes occur on the Earth’s surface and are comprised of weathering, both physical and chemical. Physical weathering includes a variety of different processes in which the features of the Earth’s surface are continually altered by nature itself, often without human intervention. When the previous glacial period began to end, large glaciers existed over much of the Earth’s surface. These glaciers did not stay still and moved around several times faster each year than the tectonic plates, and, as a result, they ended up carving valleys into the surface as they moved large amounts of soil. Many large freshwater lakes are a result of these glaciers.
River processes are another way in which the surface materials are deposited, eroded away, and the riverbank as well as the flood plain area is continuously changed by the flow and flooding of the river. Probably the best example of how much a river can change the geomorphology of the land is the Grand Canyon, which was formed almost entirely by the Colorado River.
Wave action of oceans has also played a part in changing the shoreline. As powerful surges of water continually scrape against the coast, rocks are broken down and soil as well as sand can be taken or deposited. The hydrological cycle itself can slowly change the surface of the planet as water is deposited and flows back to the ocean, often taking minerals, salt and other particles with it. Water can also seep into more porous rocks or into cracks and crevices, and, while this may not seem significant, should the water freeze, it will subsequently expand. This can put stress on rock and slowly break it apart. In addition to water, wind also plays a role in weathering of the landscape and can move sand and minerals, as well as wear away rock over time, and is responsible for creating many different features.
Chemical weathering, as the name implies, deals with the interaction of the elements of the Earth’s surface in chemical processes that can alter landforms. Many of the mineral elements present in rock and in the soil are prone to oxidation, better known as rusting. As the minerals oxidize in the presence of oxygen, the integrity of the parent material can be compromised.
Water also reacts with different minerals which are often present in rocks or in the soil. The reaction can produce products that are in solution and can be removed from the parent materials. The air voids of soil are very rich in carbon dioxide, and reacting with water, the product is carbonic acid, which can also react with the minerals present in rock and soil. Due to the presence of carbon dioxide in the atmosphere, rainfall is also slightly acidic and can facilitate reactions with rock. Soil or rock that has significant amounts of calcium carbonate present in it reacts strongly with the carbonic acid and erodes.
