Geoclines

Geoclines are trenches on the Earth's surface filled with sediment accumulated from surrounding strata. From the late nineteenth century until the mid-twentieth century, geologists believed that thermal contraction led to the formation of trenches known as geosynclines, which were important in forming mountain ranges. The theory of geosynclines is now known as the unifying theory of plate tectonics, which views the origin of geologic structures as an interplay between the production of volcanic sediment at mid-ocean ridges and the recycling of sediment at subduction zones.

Geoclines in

A geocline is a trough that forms in the Earth's crust and becomes filled with sediment from surrounding strata. It has been an important concept in the development of geomorphology, the branch of geology that studies the formation of geological structures like mountains and volcanoes.

Geologists divide geoclines into two basic categories, depending on the sediment type contained within the geocline's trough. The term “eugeocline” describes those geoclines that contain volcanic sediments and rocks (such as marine shale) characteristic of deep-water environments. Alternatively, miogeoclines are geoclines that lack volcanic sediment and are generally filled with rocks such as limestone and sandstone, which form in shallow marine environments.

Geoclines make up a special type of monocline, which is a bend or dip in an otherwise horizontal stretch of strata. A syncline occurs when there is a dip or fold in the strata, so the sediment at the lowest part of the fold is younger than the sediment surrounding it on either side. As synclines form, they may deepen until the sides of the syncline fold into the center, thereby creating layers of younger (or newer) and older sediment filling the trench. An anticline is an area in which the sediment has become raised with respect to the horizontal plane so that older sediment rises above the younger sediment in surrounding layers.

In modern geology, the term “geocline” describes the basins of sediment that occur at the edge of a continental shelf, where terrestrial strata meet the ocean. In this environment, geologists typically observe a combination of eugeoclines with deep-water sediment and abutting miogeoclines with shallow-water strata.

Geosynclinal Theory and Orogenesis

Initially, scientists believed geoclines were important in orogenesis, or the formation of mountains and other landforms. In 1859, American paleontologist James Hall proposed a theory of orogenesis based on his detailed studies of the northern Appalachian Mountains. Hall noticed that the sediment within the body of the mountain range contained folded shallow-water marine sediment, which was, at minimum, ten to twenty times thicker than unfolded sediment in the surrounding lowlands. Hall theorized that the formation of the mountain range was preceded by a period of subsidence, wherein some portions of the Earth's crust sank, creating a trough or dip in the sediment relative to surrounding layers.

At the time, geologists thought that subsidence was caused by thermal contraction, a process theorized to occur as Earth's interior cooled, causing portions of the Earth's crust to sink. Hall believed that layers of sediment from the surrounding strata collapsed and folded into the gap left by thermal contraction, which continued over long periods of time, giving rise to folded mountains. American geologist James Dwight Dana built on Hall's idea, inventing the term “geosyncline” for these folded subsistence areas that, theoretically, were the basis of orogenesis.

In the early twentieth century, American geologist Marshall Kay built on Dana's and Hall's research and created a detailed terminology scheme for the geosynclinal mountain evolution theory. According to Kay, a geosyncline ideally consisted of a pair of synclines forming on either side of a ridge, called the geanticline, which continued to build until it formed the basis of a mountain range. Kay envisioned these geanticlines forming at the margin between an eugeosyncline containing deep-water and volcanic sediment and a miogeosyncline originating in the shallow-water strata farther inland on the continental shelf. Kay believed the final stages of orogenesis involve vertical elevation of the geanticline, perhaps caused by heat or other geophysical processes.

In the 1960s and 1970s, geologists and paleontologists began working with new theories that had the potential to explain orogenesis, the movement of continents, and the development of strata on the ocean floor. American geologist Robert S. Dietz examined the geosyncline hypothesis in light of newly emerging theories and found that the model of vertical sedimentary movement hypothesized by Kay failed to explain the origin of mountains; experimental models failed to support the theory of thermal contraction, and examinations of existing mountain ranges did not conform with the patterns expected under the geosyncline theory. However, Dietz believed Kay's terminology was still useful for describing the combinations of sediment found in ocean basins. Dietz proposed that the geological features be referred to as geoclines, not geosynclines, because geoclines were no longer considered true synclines, as the term is used in modern geology.

Plate Tectonics

In the early 1960s, Dietz and American geologist Harry Hammond Hess were among a small group of researchers whose studies of deep ocean sediment led to the theory of seafloor spreading, which explains the origin of new sediment at the ocean floor. The theory of seafloor spreading was later combined with the pioneering research of German geophysicist and meteorologist Alfred Wegener, who theorized that the continents are not stable but rather move across the Earth's surface in response to currents within the Earth. Researchers working in the 1960s and 1970s realized that seafloor spreading and Wegener's continental drift hypotheses were part of a unified theory with the potential to explain a number of geological phenomena. Geologists later named this new field of study “plate tectonics.”

Earth's Layers. The mineral structure of the Earth is divided into layers based on the chemical and physical composition of the sediment. The outermost layer, known as the crust, is divided into two portions: oceanic crust and continental crust. The crust is part of the lithosphere, the outermost layer of the Earth, which contains both the crust and the outer layers of the Earth's mantle, the portion of the Earth that consists of sediment in a transitional state between liquid and solid. Beneath the crust, the lithosphere contains both rigid, solid sections and other sections that are more elastic. Beneath the lithosphere is the asthenosphere, which is a layer of superheated yet highly deformable rock. Heat generated from the decay of radioactive materials within the Earth's core causes convection currents to develop within the asthenosphere. The lithosphere, which is divided into sections known as tectonic plates, floats on top of the asthenosphere and is driven by thermal currents.

Crust Development. The Earth's core consists of liquid sediment heated by the decay of radioactive elements. At the center of the ocean, the tectonic plates of the lithosphere spread apart in response to building heat rising from the core. Liquid sediment from the core then pushes through the lithosphere at points where the plates diverge; this leads to the development of a massive volcanic mountain range known as the mid-ocean ridge. As additional liquid sediment pours through the ridge, it adds to the trailing edge of the tectonic plates and produces new layers of ocean crust.

Elsewhere in the ocean, tectonic plates converge, pushing into one another at their leading edges. At these convergence zones, a process known as subduction occurs. The leading edge of the older plate is forced beneath the lithosphere, where it melts into the asthenosphere and eventually returns material to the core. The subduction zones form part of deep ocean trenches, where the substrate sinks to extreme depths. The portion of the lithosphere that sinks into the trenches will eventually be recycled and returned to the crust along the mid-ocean ridge.

The oceanic crust is composed of dense sediment and volcanic rock produced in the mid-ocean ridge. Sediment that is not dense enough to sink into the Earth's core rises to the top of the lithosphere and forms the continental crust. Therefore, all the continents above sea level are formed by accumulating relatively porous sediment that is not recycled because it cannot sink into the Earth. Convection currents cause the tectonic plates to move and, thereby, cause deformations to form in the continental crust.

Modern Perspectives on Orogenesis

Folded Mountains. Some tectonic plates are composed largely of oceanic crust, while others also may carry a portion of the continental crust. When tectonic plates carrying a portion of the continental crust converge, subduction ceases as the overlying continents collide. When this occurs, strata on both sides deform in response to the pressures generated by the collision, and a mountain range will typically form at the collision zone. Geologists refer to mountains formed through continental collisions as folded mountain chains because they contain layers of folded strata that have collapsed onto one another as the continental crust buckles under the impact.

The Himalayan range is an example of a folded mountain chain formed from a collision between the Indian and Asian tectonic plates about 70 to 80 million years ago. As the plates slowly converged, sediment forced into the lithosphere caused volcanoes to emerge in the southern portion of Tibet, as superheated rock responded to pressure and friction by bubbling through the crust's surface. The Asian and Indian plates continue to move toward each other at a rate of about 2 centimeters per year. Because of this continued force, the Himalayan range grows vertically by as much as 5 millimeters annually. The plates are still converging, leading to instability in the sediment surrounding the mountain range and frequent earthquakes in the region.

The Appalachians of the eastern United States are another example of a folded range, resulting from an ancient collision between the plate that would contain North America and the continental mass that would later become South America and Africa. The Appalachians began to form about 420 million years ago, and the range has changed significantly over the ensuing period, largely because erosion and weathering have reduced and reshaped the peaks of the range. In addition, the two plates that collided to form the Appalachian range have since moved apart. Therefore, the region is more geologically stable than areas like the Himalayas, where mountains are still forming.

Volcanic Forces. Though tectonic collisions account for many of the Earth's mountain ranges, various other processes contribute to orogenesis. Occasionally, mountains form from accumulated volcanic rock that originated under the lithosphere. This process occurs at the mid-ocean ridge, where hundreds of miles of volcanic mountains have formed out of the liquid rock, or magma, which is gradually forced to the surface and slowly cools over thousands of years. In areas where the continental crust has been compromised or where the underlying lithosphere is unstable, magma may come to the surface and accumulate sufficiently to form a mountain. Mount Fuji in Japan and Mount Rainier in Washington are two mountains formed through volcanic activity.

Fault Block Mountains. Fault block mountains form along fault lines, where the deformation of the lithosphere has created a weakened section in the continental crust. Movement of the tectonic plates underlying the sediment may then deform the sediment such that a portion of the crust along a fault line collapses, exposing the remaining strata. The Sierra Nevada range in California is an example of fault block formation, where a portion of the crust collapsed, leaving behind jagged blocks of stone along the fault line. Fault block ranges tend to be unstable, as tectonic activity can continue agitating the fault, leading to earthquakes and further strata deformation.

Other Geologic Forces. In some cases, the mechanisms underlying orogenesis do not seem to fall into one of the existing categories. The Rocky Mountain range of the western United States provides one prominent example of a range whose origins are unclear to geologists. Some features of the Rockies indicate origin by collision; however, geologists have been unable to determine when and how a collision could have occurred here, as the mountain range falls inland on the North American plate. Also, there is no evidence that the portion of the plate containing the Rockies ever experienced a tectonic collision. In a 2011 issue of the journal Geosphere, American geologist Craig H. Jones and colleagues from the University of Colorado at Boulder suggested an alternative mechanism for the formation of the Rockies.

Jones and colleagues noted that the portion of the continental crust under the state of Wyoming is exceptionally thick and dips into the fluid portion of the mantle. The researchers believed a portion of the lithosphere containing rigid rock had pushed under this thick portion of the continental crust, causing the more fluid portion of the mantle to flow around it. Acting simultaneously, these forces may have created suction that acted on the crust and, paradoxically, led to a counterforce that caused what would become the Rocky Mountains to thrust up from the surrounding sediment. Jones and colleagues maintained that this counterforce may have drawn liquid material from the mantle through the lithosphere, resulting in unusually rich deposits of gold and other minerals appearing in bands within the Rockies.

While the alternative orogenic mechanism theorized by Jones and colleagues has not been generally accepted, it suggests several related hypotheses that could potentially be tested. If the geological community finds that Jones and his colleagues’ hypotheses stand up to additional investigation, the suction-counterforce theory could become an important new mechanism in helping geologists understand the various processes active in orogenesis. The Rocky Mountains and their uncertain origin provide a prime example of how the forces involved in structural geology are still open to discovery and how new theories can radically reorganize the theoretical model of the Earth's geological function. One new theory posited by researchers at California State University, Northridge, in 2023 suggested that the Rocky Mountains were formed by two distinct collision events 70 and 90 million years ago. This two-stage model joins the other hypothesis of the Rocky Mountains’ origins. Their origin is nuanced and complex. 

Principal Terms

anticline: a monocline in which the strata has been pushed up with respect to the horizontal plane, placing the newest layers of strata at the top of the bend

asthenosphere: a portion of the Earth's mantle that lies beneath the lithosphere and is relatively liquid, allowing the lithosphere to float across its surface

continental crust: a portion of the Earth's surface that is less dense than the oceanic crust and, therefore, rests above sea level

geomorphology: a branch of geology that studies deformations in the Earth's crust

lithosphere: the relatively firm and stiff layer of the Earth's mantle just below the oceanic and continental crusts

monocline: a bend or dip in an otherwise horizontal portion of strata

oceanic crust: a portion of the Earth's mantle that lies at the bottoms of oceans and is produced along volcanic ridges

orogenesis: a deformation of the Earth's surface due to contact between tectonic plates that leads to the formation of mountain ranges

plate tectonics: the branch of geology that combines the ideas of subduction and seafloor spreading to create a theory of continental movement and formation

subduction zone: the portion of the oceanic crust where sediment from the ocean floor is sucked into the Earth's core and melted, recycling material from the mantle

syncline: a monocline in which the strata has been pushed down with respect to the horizontal plane in such a way that the newest layers of strata are lower than older layers of strata

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