Earth structure and development
The structure and development of Earth encompass the geological framework and dynamic processes that shape our planet over time. Earth's composition is divided into four primary layers: the solid crust, the mantle, and the dual regions of the core—outer (liquid) and inner (solid). The crust varies in thickness and material, with continental crust predominantly composed of granitic rocks, while oceanic crust consists mainly of basalt. Beneath the crust lies the mantle, a dense layer of silicate rock, and deeper still are the iron-rich outer and inner cores.
These layers are not only defined by their composition but also by their mechanical behavior, leading to the differentiation of the lithosphere (rigid outer layer) and asthenosphere (deformable layer beneath). The movement of tectonic plates, driven by mantle convection, influences geological activity, including the formation of mountains and the shifting of continents, which in turn affects ocean currents, atmospheric patterns, and climate. The historical context of these geological processes is crucial for understanding future climate changes, as the positions of continents and mountain ranges have significant implications for global climate systems.
Earth structure and development
Climate depends strongly on the geometry of the Earth’s surface: Where the continents are, and where the mountain ranges are within them, controls winds, currents, and continental glaciers. The Earth’s surface geometry is a result of its structure and development.
Background
The Earth is a dynamic planet. Heat from its formation and radioactive decay moves to the surface and radiates to outer space. In the process, pieces of the surface are moved around, and mountain ranges are produced. The result is a mosaic of plates, some with continents and mountain ranges on them, that shift around over geologic time periods. The pattern of oceans, continents, and mountains controls ice-sheet formation, ocean currents, and atmospheric circulation patterns that, in turn, control the climate.
Layers Based on Composition
By studying the waves generated by earthquakes, seismologists have been able to determine the elastic properties and density of the Earth as a function of depth. This technique has revealed four major layers: the crust (5 to 70 kilometers thick), the mantle (2,900 kilometers thick), the outer core (2,300 kilometers thick), and the inner core (1,200 kilometers thick). The outer core is liquid; all the other layers are solid. These layers are thought to represent differences in composition.
The continental crust, typically 30 to 70 kilometers thick, is made of granitic rocks, while the somewhat denser oceanic crust, typically 0 to 7 kilometers thick, is made of basaltic rocks. The mantle, a silicate rock called peridotite, is denser than the crust. The core, both inner and outer, is mostly iron, and is about twice as dense as the mantle. The inner core is solid, because it is subjected to higher pressure than the outer core.
Layers Based on Behavior
When Alfred Wegener struggled to convince his skeptics that the continents drifted around, he assumed that the compositional layering described above would also be important mechanically and that the continents moved like giant ships through the mantle and oceanic crust. At the temperatures present in the crust and upper mantle, however, horizontal deformations like this are not possible.
As the evidence for drifting continents accumulated in the 1960’s, a different approach developed. Although the peridotite is denser than granite or basalt, the mechanical behavior of these materials is similar. This behavior depends on temperature, which is known to increase with depth. The cooler, outer part of the Earth (about 150 kilometers thick), which does not deform horizontally, was defined as the lithosphere, and the hotter, deeper part, which does deform horizontally, as the asthenosphere. The theory of continental drift became the theory of plate tectonics, where the continents were seen to be passively riding on much thicker, “rigid” plates of the lithosphere. Plates deform vertically beneath glacial loads, but not horizontally when pushed by convection, because they are only about 150 kilometers thick but thousands of kilometers wide. There are about a dozen major lithospheric plates, and their interactions produce most of the mountains on Earth.
Mantle Convection
In the Earth, heat moves from the hot interior to the surface. Heat from the core enters the lower mantle, expanding some of it, making it buoyant. The mantle is solid, but, because of its immense size, it behaves like silicon putty, deforming to permit the rise of this buoyant, heated rock. As the rock approaches the crust, tens of millions of years later, it spreads out, cools, and sinks. This process is called convection.
The convection cells on Earth rise at mid-ocean ridges, where lithospheric plates grow, and descend at subduction zones. The presence of plates influences the convection and limits the movements it can produce at the surface.
Plate Tectonics and Climate
As convection moves the continents around, their latitude will change, making it more or less likely that continental glaciers will form on them. Currently, glaciers can only form at high latitudes, above 45°. If the rate of convection changes, the atmospheric carbon dioxide (CO2) concentration is likely to change. Volcanism transfers CO2 from within the Earth to the atmosphere; faster convection will result in more volcanism and thus higher levels of atmospheric CO2.
Ocean circulation patterns depend on the shapes of the ocean basins. Before the closing of the Isthmus of Panama, warm, salty water from the Atlantic Ocean was flushed into the Pacific Ocean. After the isthmus’s closure, this water remained in the Atlantic Basin, changing circulation patterns.
Collision of the Indian subcontinent with Asia caused the uplift of the Himalayan plateau, which led to the development of the monsoon winds. These are likely to have resulted in an unusually high rate of weathering, removing CO2 from the atmosphere. The Rocky Mountains have been shown to deflect prevailing westerlies as they move across North America, bringing great quantities of heat to western Europe.
Context
Scientists’ confidence in being able to predict future climate change depends on their understanding of past climate change. Inputs that vary too slowly to affect the historical record, such as the locations of continents and mountain ranges, need to be evaluated from a geological perspective. These formations affect global climate, and their changes in the future will affect the evolution of Earth’s climate on a geological timescale. One way to gauge the accuracy of existing models of geological history is to compare climate signals from times when the elevation and location of a geological formation are known to have differed from their current values. Knowledge of the structure and development of the Earth provides a path to obtain this information.
Key Concepts
asthenosphere: the ductile layer of the Earth, beneath the lithosphere, defined by mechanical behavior; although solid, it deforms horizontally over tens of millions of yearscrust: the uppermost layer of the Earth as defined by seismic properties inferred to represent compositioninner core: the innermost layer of the Earth, extending from 5,150 kilometers to 6,370 kilometers below ground and composed mostly of solid ironlithosphere: the uppermost layer of the Earth, about 150 kilometers thick, as defined by mechanical behavior; it does not deform horizontallymantle: the layer beneath the crust, known to be solid and defined by seismic properties inferred to represent compositionouter core: a liquid region extending from the base of the mantle at a depth of 2,900 kilometers down to the inner core, at a depth of 5,150 kilometers; mostly molten iron
Bibliography
Fowler, C. M. R. The Solid Earth: An Introduction to Global Geophysics. 2d ed. New York: Cambridge University Press, 2005. Excellent discussion of current plate motions, plate rheology, and so on, but somewhat technical in approach. Illustrations, figures, tables, maps, bibliography, index.
Grotzinger, John. Understanding Earth. New York: W. H. Freeman, 2007. Introductory college-level textbook. Good overview of the plate tectonic paradigm. Illustrations, figures, tables, maps, bibliography, index.
Ruddiman, William F. Earth’s Climate Past and Future. 2d ed. New York: W. H. Freeman, 2008. Chapter 6 of this elementary college textbook describes how the locations of continents and mountain ranges may have been responsible for ice ages. Illustrations, figures, tables, maps, bibliography, index.
Seager, R. “The Source of Europe’s Mild Climate: The Notion That the Gulf Stream Is Responsible for Keeping Europe Anonymously Warm Turns Out to Be a Myth.” American Scientist 94 (2002): 340-41. Good description of how the mountains in the American West control the climate of Europe by deflecting prevailing westerlies. Illustrations, maps.
Van der Pluijm, Ben. Earth Structure: An Introduction to Structural Geology and Tectonics. New York: W. W. Norton, 2004. The second half of this book concerns tectonics and describes in detail the formation of many mountain ranges on Earth. Illustrations, figures, tables, maps, bibliography, index.