7: Earth Materials - The Lithosphere - Geosciences
By the end of this chapter you should be able to
- Describe the physical structure of the Earth
- Apply an analogy to the age of the Earth and major events in the development of the biosphere
- Discuss the mechanism that drives plate tectonics
- Review the rock cycle in terms of the processes that form the three major rock types
Thumbnail: Half Dome in Yosemite is a classic granite (a common intrusive igneous rock) dome and popular rock climbing destination (Public Domain; Jon Sullivan).
The lithosphere is the outermost layer that surrounds our planet and is formed by the earth's crust and part of the mantle. The term lithosphere comes from the Greek, lithos which means stone and sphere. It is a solid, hard layer, and it is the most superficial layer that exists. Its external part forms the islands and the continents and because of this we are in direct contact with it. It is formed by solid materials and encloses the continental crust, the oceanic crust and the superficial part of the terrestrial mantle. It is divided into tectonic plates these plates are constantly moving over the asthenosphere.
In his programmatic treatise on structural stability and morphogenesis, René Thom noted that “for centuries the form of living beings has been an object of study by biologists, while the morphology of inert matter seems only accidentally to have excited the interest of physicochemists.” [ 1 ] In relation to design of new materials, this statement is certainly true to the present day. While biomimetics, broadly understood not only as emulation of biological structures but also as a set of guiding principles for design of engineering materials, has long been motivating materials scientists, a treasure-trove of inanimate Nature has been largely overlooked. We would like to draw the attention of the materials research community to a rich gamut of formations that emerge during processes occurring in the inanimate Earth's lithosphere as a potential source of inspiration for design of materials. By analogy with biomimetics, yet in an antonymic sense, we dub this approach lithomimetics. The lithomimetics paradigm implies that processes that occur in the Earth's crust and the patterns they produce can enrich the toolbox used in design of novel materials—again not necessarily offering direct blueprints, but rather providing hints leading to promising material architectures.
Architectured (or architected) materials, also referred to as archimats, have recently come to the fore as a new, special class of materials whose inner make-up represents an additional degree of freedom in engineering design for advanced applications. [ 2 ] According to a definition going back to M.F. Ashby, [ 3 ] an architectured material comprises other materials in such a way that its properties are largely defined by the geometry and mutual arrangement of the constituents. This definition embraces both natural and engineered man-made materials. It encompasses cellular materials, metamaterials, weaves, fiber mats, and many other types of composites. The burgeoning field of architectured materials has been advanced by the seminal work of several research groups. [ 3, 4 ] A key feature borrowed by biomimetics from living organisms and biological materials, such as bones and armor of animals, mollusc shells, insect wings, parts of plants, etc., [ 5-7 ] is their multiscale hierarchical structure. The excellent properties of these natural structures owe to a long evolutionary process, in which mutations led to a great variety of forms, while natural selection manifested those that provide high strength, light weight, fracture toughness, and resistance to cyclic loading. It is thus understandable that researchers turned their interest to such structures first, and that is why many archimats developed so far are bioinspired.
We argue that in designing structural and functional architectured materials we may learn a lot from inanimate Nature. The Earth's lithosphere is a particularly rich depository of structural patterns. [ 8-11 ] Many of them have emerged as a result of transformations induced in rocks by deformation under high pressure. Examples of such patterns are shown in Figure 1I.
We believe that these patterns are of interest, if only because they demonstrate that deformation-induced transitions between the various structural forms of materials are possible without human interference. According to the philosophy of René Thom, one can expect a profound similarity between the geometrical principles governing shape development in animate and inanimate Nature. [ 1 ] However, in contrast to the living world, the development of patterns in inanimate Nature is not subjected to a selection process. This has interesting consequences: on the one hand, due to lack of a biological adaptation mechanism, the properties associated with a pattern formed are not necessarily advantageous for any given function. On the other hand, physicochemical self-organization in inanimate Nature, for example, in the evolving Earth's crust, brings about a broad variety of structural patterns. They represent a rich “atlas” of potential patterns with mostly unknown physical and mechanical properties to choose from. This collection of patterns has good reasons for being more diverse than patterns formed in the living world, as their emergence is not constrained by the limitations of natural selection and biological synthesis ruling out high temperatures and pressures. Besides, the mechanisms of morphogenesis in inanimate Nature governed by physicochemical principles may be easier to emulate than the more intricate biological processes carried out by cells. [ 18 ] What distinguishes the structural patterns generated by morphogenesis in the lithosphere is that some of the underlying mechanisms can be reproduced in a laboratory and used for the design and manufacture of archimats. That is why our focus here is on the lithosphere as a repository of potentially useful structures.
The lithosphere is the solid, outer part of the Earth, including the brittle upper portion of the mantle and the crust.
Earth Science, Geology, Geography, Physical Geography
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The lithosphere is the solid, outer part of the Earth. The lithosphere includes the brittle upper portion of the mantle and the crust, the outermost layers of Earth&rsquos structure. It is bounded by the atmosphere above and the asthenosphere (another part of the upper mantle) below.
Although the rocks of the lithosphere are still considered elastic, they are not viscous. The asthenosphere is viscous, and the lithosphere-asthenosphere boundary (LAB) is the point where geologists and rheologists&mdashscientists who study the flow of matter&mdashmark the difference in ductility between the two layers of the upper mantle. Ductility measures a solid material&rsquos ability to deform or stretch under stress. The lithosphere is far less ductile than the asthenosphere.
There are two types of lithosphere: oceanic lithosphere and continental lithosphere. Oceanic lithosphere is associated with oceanic crust, and is slightly denser than continental lithosphere.
The most well-known feature associated with Earth&rsquos lithosphere is tectonic activity. Tectonic activity describes the interaction of the huge slabs of lithosphere called tectonic plates.
The lithosphere is divided into tectonic plates including the North American, Caribbean, South American, Scotia, Antarctic, Eurasian, Arabian, African, Indian, Philippine, Australian, Pacific, Juan de Fuca, Cocos, and Nazca.
Most tectonic activity takes place at the boundaries of these plates, where they may collide, tear apart, or slide against each other. The movement of tectonic plates is made possible by thermal energy (heat) from the mantle part of the lithosphere. Thermal energy makes the rocks of the lithosphere more elastic.
Tectonic activity is responsible for some of Earth's most dramatic geologic events: earthquakes, volcanoes, orogeny (mountain-building), and deep ocean trenches can all be formed by tectonic activity in the lithosphere.
Tectonic activity can shape the lithosphere itself: Both oceanic and continental lithospheres are thinnest at rift valleys and ocean ridges, where tectonic plates are shifting apart from one another.
How the Lithosphere Interacts with Other Spheres
The cool, brittle lithosphere is just one of five great &ldquospheres&rdquo that shape the environment of Earth. The other spheres are the biosphere (Earth&rsquos living things) the cryosphere (Earth&rsquos frozen regions, including both ice and frozen soil) the hydrosphere (Earth&rsquos liquid water) and the atmosphere (the air surrounding our planet). These spheres interact to influence such diverse elements as ocean salinity, biodiversity, and landscape.
For instance, the pedosphere is part of the lithosphere made of soil and dirt. The pedosphere is created by the interaction of the lithosphere, atmosphere, cryosphere, hydrosphere, and biosphere. Enormous, hard rocks of the lithosphere may be ground down to powder by the powerful movement of a glacier (cyrosphere). Weathering and erosion caused by wind (atmosphere) or rain (hydrosphere) may also wear down rocks in the lithosphere. The organic components of the biosphere, including plant and animal remains, mix with these eroded rocks to create fertile soil&mdashthe pedosphere.
The lithosphere also interacts with the atmosphere, hydrosphere, and cryosphere to influence temperature differences on Earth. Tall mountains, for example, often have dramatically lower temperatures than valleys or hills. The mountain range of the lithosphere is interacting with the lower air pressure of the atmosphere and the snowy precipitation of the hydrosphere to create a cool or even icy climate zone. A region&rsquos climate zone, in turn, influences adaptations necessary for organisms of the region&rsquos biosphere.
Photograph by Jennifer Plourde, MyShot
The depth of the lithosphere-asthenosphere boundary (LAB) is a hot topic among geologists and rheologists. These scientists study the upper mantle&rsquos viscosity, temperature, and grain size of its rocks and minerals. What they have found varies widely, from a thinner, crust-deep boundary at ocean ridges to thick, 200-kilometer (124-mile) boundary beneath cratons, the oldest and most stable parts of continental lithosphere.
a modification of an organism or its parts that makes it more fit for existence. An adaptation is passed from generation to generation.
force pressed on an object by air or atmosphere.
layer in Earth's mantle between the lithosphere (above) and the upper mantle (below).
layers of gases surrounding a planet or other celestial body.
all the different kinds of living organisms within a given area.
part of the Earth where life exists.
area separated from others by its long-term weather patterns.
thick layer of Earth that sits beneath continents.
transfer of heat by the movement of the heated parts of a liquid or gas.
rocky outermost layer of Earth or other planet.
icy part of the Earth's waterincluding icebergs, glaciers, and ice caps.
having parts or molecules that are packed closely together.
varied or having many different types.
ability of a solid material to withstand stress or force by changing form instead of breaking.
our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.
the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.