Geomorphology

GEOMORPHOLOGY IS the study of landforms. Landforms are surface expressions of rocks, as are various features made by rivers, groundwater, waves and currents, winds, glaciers, and corals. Landforms come in all shapes and sizes. A DELTA, PLATEAU, VOLCANO, sinkhole, and beach are all landforms. Geomorphologists are geologists and physical geographers who specialize in the study of landforms.

Geomorphology has two general goals: 1) to explain how landforms vary from place to place; and 2) to develop theories about the origin and development of landforms. In order to achieve these goals, geomorphologists examine the nature of surface rocks and geologic processes, such as soil formation, weathering and EROSION, mass movements, and transportation and deposition of sediments. They use a wide range of techniques for data collection, including field, laboratory, and numerical techniques. Geomorphologic research aids in understanding the role that landform development plays in complex ecosystems. The findings also help prepare for and lessen impacts of hazardous geological events, such as landslides, FLOODS, beach erosion, and slope erosion. The following paragraphs summarize how geologic structure, PLATE TECTONICS, gradational processes, and time influence landform development.

The four factors are interrelated so that their combined effects create an amazing variety of beautiful landforms for us to enjoy. The article concludes with a summary of how geomorphic theory in the United States evolved from a long tradition of geological and geographical research.

Landforms vary, in part, because of geologic structure. Geologic structure refers to the types and arrangement of materials that make up landforms. The materials are extremely diverse, ranging from sediments, such as finely textured clay deposits on a glacial lakebed, to massive large-grained rock that has crystallized from subsurface magma. There are three basic types of rock—igneous, sedimentary, and metamorphic—but each type has a wide variety of examples that differ according to their chemistry, texture, color, hardness, and so on. Common examples of sedimentary rocks are conglomerate, sandstone, siltstone, shale, and limestone.

Igneous rocks form when liquid magma (molten rock) cools and hardens. In the hardening, minerals take on their shape, or crystallize. There are two basic subcategories of igneous rocks—intrusive and extrusive. When the magma cools within Earth’s crust, the rocks are of the intrusive igneous type. Granite is the most common rock of this type. When magma erupts onto Earth’s surface, it cools outside the crust to become an extrusive igneous rock, or lava. Basalt is the most common type of lava.

Metamorphic rock forms when intense pressure and heat cause previously formed rocks (including other metamorphic rock) to deform or metamorphose. (Metamorphose in Latin means “to change form.”) Different rocks have their own metamorphic counterparts. Sedimentary rocks, such as limestone and dolostone, change to marble. Low-grade shale becomes sheets of slate, and sandstone turns to quartzite. Metamorphism may occur in igneous rocks as well. Granite recrystallizes to become banded gneiss (pronounced “nice”). Intense heat and pressure can also turn several kinds of sedimentary rocks or their metamorphic products into gneiss. Schist is a general term for a group of metamorphic rocks that forms under the most intense pressure and heat.

Earth materials exhibit even more variation in their arrangement. Joints divide rocks and internal forces (compression, tension, and shear) break them along faults, thrust them vast distances horizontally, or deform them by heat and pressure. Rocks may or may not be in layers. Sandstone has layers, but granite is a massive rock devoid of them. Layers of sedimentary rocks may be horizontal, tilted, or folded. Rocks also differ in the degree to which they yield to weathering and erosion processes. Hard rocks and soft rocks wear down to differing degrees. Ridges and peaks occur where hard rocks resist the work of weathering, rain, and streams. Valleys that separate the ridges are where rocks are weak. The rocks in valleys weather easily and streams wash their sediments away quickly.

The arrangement of most sediment varies according to a transporting agent (stream, wind, waves, or glacier). For example, meltwater discharging from a retreating glacier deposits stratified (water-sorted) layers of clay, silt, sand and gravel, but it also leaves behind piles of unstratified (unsorted) debris, ranging from clay to boulder-size materials. Like glacial meltwater, other transporting agents stratify or sort their deposits.

Rivers form FLOODPLAINS and deltas; waves leave behind beaches and spits; and winds deposit sand dunes and loess (silt deposits). In contrast, layers of sediment left by corals and groundwater are in situ deposits (the sediments remain at their point of origin). Corals leave behind their skeletons to form limestone reefs, and groundwater forms various types of calcified deposits, such as stalagmites and stalactites.