The LABThe depth of the lithosphere-asthenosphere boundary (LAB) is a hot topic among geologists and rheologists. These scientists study the upper mantle’s viscosity, temperature, and grain size of its rocks and minerals. What they have found varies widely, from a thin, crust-deep boundary at mid-ocean ridges to thick, 200-meter (124-mile) boundary beneath cratons, the oldest and most stable parts of continental lithosphere.LithospheresScientists have identified many ways to define the lithosphere. The “elastic lithosphere” measures its ability to reform itself under stress. The “thermal lithosphere” measures its temperature and the thermal energy—heat—it conducts. The “seismic lithosphere” measures how lithospheric rocks move with seismic shifts and tectonic activity. The “electrical lithosphere” measures the layer’s ability to conduct electricity (much lower than the asthenosphere). Finally, the “petrologic lithosphere” measures the chemical properties of rocks in the lithosphere compared to the asthenosphere.Extraterrestrial LithospheresAll terrestrial planets have lithospheres. The lithospheres of Mercury, Venus, and Mars are much thicker and more rigid than Earth's.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’s structure. It is bounded by the atmosphere above and the asthenosphere (another part of the upper mantle) below.The lithosphere is the most rigid of Earth’s layers. 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—scientists who study the flow of matter—mark the difference in ductility between the two layers of the upper mantle. Ductility measures a solid material’s ability to deform or stretch under stress. The lithosphere is far less ductile than the asthenosphere. The elasticity and ductility of the lithosphere depends on temperature, stress, and the curvature of the Earth itself.The lithosphere is also the coolest of Earth’s layers. In fact, some definitions of the lithosphere stress its ability to conduct heat associated with the convection taking place in the plastic mantle below the lithosphere.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. Continental lithosphere, associated with continental crust, can be much, much thicker than its oceanic cousin, stretching more than 200 kilometers (124 miles) below Earth’s surface.Plate TectonicsThe most well-known feature associated with Earth’s lithosphere is tectonic activity. Tectonic activity describes the interaction of the huge slabs of lithosphere called tectonic plates.The lithosphere is divided into 15 major tectonic plates: 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 mid-ocean ridges, where tectonic plates are shifting apart from one another. At these zones, the lithosphere is only as thick as the crust.How the Lithosphere Interacts with Other SpheresThe cool, brittle lithosphere is just one of five great “spheres” that shape the environment of Earth. The other spheres are the biosphere (Earth’s living things); the cryosphere (Earth’s frozen regions, including both ice and frozen soil); the hydrosphere (Earth’s 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—the 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’s climate zone, in turn, influences adaptations necessary for organisms of the region’s biosphere.
Term Part of Speech Definition Encyclopedic Entry adaptation Noun
a modification of an organism or its parts that makes it more fit for existence. An adaptation is passed from generation to generation.
Encyclopedic Entry: adaptation air pressure Noun
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.
Encyclopedic Entry: atmosphere biodiversity Noun
all the different kinds of living organisms within a given area.
Encyclopedic Entry: biodiversity biosphere Noun
part of the Earth where life exists.
Encyclopedic Entry: biosphere brittle Adjective
fragile or easily broken.
climate zone Noun
area separated from others by its long-term weather patterns.
continental crust Noun
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.
Encyclopedic Entry: crust cryosphere Noun
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.
Encyclopedic Entry: Earth earthquake Noun
the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.
able to bend easily.
conditions that surround and influence an organism or community.
act in which earth is worn away, often by water, wind, or ice.
Encyclopedic Entry: erosion fertile Adjective
able to produce crops or sustain agriculture.
having to do with the physical formations of the Earth.
person who studies the physical formations of the Earth.
mass of ice that moves slowly over land.
Encyclopedic Entry: glacier hill Noun
land that rises above its surroundings and has a rounded summit, usually less than 300 meters (1,000 feet).
Encyclopedic Entry: hill hydrosphere Noun
all the Earth's water in the ground, on the surface, and in the air.
Encyclopedic Entry: hydrosphere landscape Noun
the geographic features of a region.
Encyclopedic Entry: landscape lithosphere Noun
outer, solid portion of the Earth. Also called the geosphere.
Encyclopedic Entry: lithosphere lithosphere-asthenosphere boundary (LAB) Noun
chemical and mechanical distinction between the cool, rigid lithosphere and the warmer, more ductile asthenosphere.
middle layer of the Earth, made of mostly solid rock.
Encyclopedic Entry: mantle mid-ocean ridge Noun
underwater mountain range.
landmass that forms as tectonic plates interact with each other.
oceanic crust Noun
thin layer of the Earth that sits beneath ocean basins.
ocean trench Noun
a long, deep depression in the ocean floor.
Encyclopedic Entry: ocean trench organic Adjective
composed of living or once-living material.
the way mountains are formed.
layer of Earth consisting of soil and all it contains (such as water, air, organisms).
chemical material that can be easily shaped when heated to a high temperature.
all forms in which water falls to Earth from the atmosphere.
Encyclopedic Entry: precipitation precipitation Noun
all forms in which water falls to Earth from the atmosphere.
Encyclopedic Entry: precipitation rain Noun
Encyclopedic Entry: rain remains Noun
materials left from a dead or absent organism.
scientist who studies the flow and shape-changing (deformation) of matter.
rift valley Noun
depression in the ground caused by the Earth's crust spreading apart.
Encyclopedic Entry: rift valley rigid Adjective
natural substance composed of solid mineral matter.
precipitation made of ice crystals.
top layer of the Earth's surface where plants can grow.
to strain or put pressure on.
tectonic activity Noun
movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.
tectonic plate Noun
massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.
degree of hotness or coldness measured by a thermometer with a numerical scale.
Encyclopedic Entry: temperature thermal energy Noun
heat, measured in joules or calories.
depression in the Earth between hills.
liquid that is thick and sticky.
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.
Encyclopedic Entry: volcano weathering Noun
the breaking down or dissolving of the Earth's surface rocks and minerals.
Encyclopedic Entry: weathering wind Noun
movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.