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Taking a Closer Look at the Earth's Three Divisions

Updated on September 8, 2015
Earth's Structural Layers
Earth's Structural Layers

Introduction

The biosphere is the outermost part of the planet’s shell which includes the air, land, surface rocks and water within which life occurs, and wherein biotic processes in turn alter or transform. From the broadest geophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere (rocks), hydrosphere (water), and atmosphere (air). Our planet Earth is the only place where life is known to exist. This biosphere is postulated to have evolved, beginning through a process of biogenesis or biopoesis, at least some 3.5 billion years ago.

Biomass accounts for about 3.7 kg carbon per square meter of the earth’s surface averaged over land and sea, making a total of about 1900 gigatonnes of carbon.

The tectonic plates of the lithosphere on Earth.
The tectonic plates of the lithosphere on Earth.
The two types of lithosphere
The two types of lithosphere

Lithosphere

What characterizes the lithosphere (from the Greek for “rocky” sphere) is being the solid outermost shell of a rocky planet. On the Earth, the lithosphere includes the crust and the uppermost layer of the mantle (the upper mantle or lower lithosphere), which is joined to the crust.

At the cooling surface layer of the Earth’s convection system, the lithosphere thickens over time. It is fragmented into relatively strong pieces, called tectonic plates, which move independently relative to one another. This movement of lithospheric plates is described as plate tectonics.The distinguishing characteristic of the lithosphere is not composition, but its flow properties. Under the influence of the low-intensity, long-term stresses that drive plate tectonics motions, the lithosphere responds essentially as a rigid shell and thus deforms primarily through brittle failure, whereas the asthenosphere (the layer of the mantle below the lithosphere) is heat-softened and accommodates strain through plastic deformation. Both the crust and upper mantle float on the more plastic asthenosphere. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at the Moho discontinuity.

There are two types of lithosphere:

ð oceanic lithosphere

ð continental lithosphere

Oceanic lithosphere is about 70 km thick (but can be as thin as 1.6 km at the mid-ocean ridges), while continental lithosphere is about 150 km thick (and can be considerably thicker at continental collision zones). Oceanic lithosphere consists mainly of mafic and ultramafic rocks and is denser than continental lithosphere, which consists predominantly of felsic rocks. This higher density has the effect that at subduction zones the oceanic plate will invariably sink underneath the continental plate. New oceanic lithosphere is constantly being produced at mid-ocean ridges from mantle material and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 200 million years old. As oceanic lithosphere grows older, it gets cooler and denser, with the result that if two oceanic plates converge, the older one will subduct below the younger one.

Layers of the Earth

The Earth is made up of four layers. The thin outer layer is called the crust, followed by the hot, partly molten rock, the mantle. Beneath it is the outer core, which is a layer of liquid metal. In the center of the Earth is ball of very hot solid metal called the inner core.

Crust

The crust accounts for only about 0.6% of the planet’s volume. It varies in thickness from 248 miles (40 km) beneath the parts of the continents to only 3.1 miles (5 km) under parts of the ocean floor. It is made up of lighter rock than the outer layers. The temperature of the rocks increases by about 86 degrees Fahrenheit (30 degrees Celsius) for every kilometer under the surface. New crust is being made all the time as molten rock bubbles up through the huge cracks between plates in the ocean floor. There are two kinds of earth’s crust: oceanic crust between the seas and the continental crust beneath the land. The oceanic crust has an average thickness of 3.6 miles (5-9 km) and varies comparatively little throughout the world. Its rocks are highly varied, including volcanic lava flows, huge blocks of granite, and sediments laid down in shallow water when parts of the continents were inundated by the sea. Despite the diversity of materials, the average composition is roughly that of rock granite, and the two most common elements in addition to oxygen are silicon and aluminum. The oceanic crust has the much higher average thickness of 18.5 – 25 miles (30-40 km) and varies much more. It is much more of uniform composition and apart from a thin covering of sediment; it is composed largely of the rock basalt. Aside from oxygen, the most common elements in the oceanic crust are the silicon and aluminum with traces of magnesium.

Mantle

The mantle accounts for about 82% of the Earth’s volume and extends from the base of the crust to a depth of about 1,800 miles (2,900 km). The sharp boundary between the crust and the mantle is called Mohorovicic discontinuity after the Yugoslav seismologist who discovered it in 1909.

Aside from oxygen, it is thought to consist largely of peridotite, a rock that contains high proportions of the elements iron, magnesium and silicon. The mantle is inaccessible and although it is mostly solid, it contains a partially molten layer.

Core

The core accounts for about 17% of the Earth’s volume and extends from the base of the mantle of the earth’s center. The discontinuity between the mantle and the core called the core-mantle boundary or Gutenberg discontinuity, after Beno Gutenberg, a German-American seismologist who discovered it. The core is composed of two distinct parts: the outer core and the inner core. The outer core extends down to a depth of about 3,200 miles (5,155 km) and is liquid. The inner core measures about 1,500 miles (2,400 km). The main component of the core is iron and contain some lighter elements possibly carbon, hydrogen, oxygen, silicon, and sulfur.

Trivia about the Earth

The Earth is the place where we all live and it is interesting to note some facts about it. Here they are:

  • The Earth’s total surface area is 196,950,000 sq. miles

(510,100,000 sq. km.).

  • The total land area of the Earth is 57,400,000 sq. miles

(148,800,000 sq. km.).

  • The distance between the Earth and the moon is 238,884 miles

(384,365 km)

  • The average speed of the Earth in orbit is 18.5 miles (29.8)/sec.
  • The Earth’s density is 5,517 times the density of water.
  • The Earth tilts on its axis at an angle of 23.5 degrees.
  • The distance between the Earth and the Moon is 238,884 miles (384,365 km).
  • Light from the Sun takes about 8 minutes to reach the Earth.
  • The Earth’s nearest star, apart from the Sun is Proxima Centauri.
  • Light from Proxima Centauri takes more than four years to reach Earth.

Layers of the Earth

The Composition of the Earth

Questions for Review

1. Describe the layers of the Earth.

2. What is the difference between the oceanic and continental lithosphere?

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    • profile image

      Jelah Mae 

      2 years ago

      REYES, Jelah Mae P. (BSA 1-2)

      Do we mind what is happening to our mother EARTH? It is so wonderful, isn't it? We live here with our family, relatives, best friends, close friends, friends, even our enemies in our mind. But we don't notice that in our everyday living here in our EARTH we didn't know what's going in our EARTH. It is ruin little by little. Our EARTH is ours like a valuable thing that need to takes care of. Just like, we are the Accountants and the EARTH is the money that we are taking care of, that never let to mess our worksheets and takes care of balancing the balance sheet. We should do whatever makes our work done for the best and we should do whatever makes our EARTH wonderful.

    • Geromae Tuzon profile image

      Geromae Anne Prila-Tuzon 

      2 years ago

      Geromae Anne P. Tuzon BS PSY-I

      Describe the layers of the earth.

      Crust: The thin, outermost layer of the earth is called the crust. It makes up only one percent of the earth's mass.

      Mantle: The mantle is the layer below the crust. It makes up almost two thirds of the earth's mass and is about 2900 km thick. The mantel is divided into two regions, the upper and lower sections. Directly below the upper section is the asthenosphere. Heat and pressure cause a small amount of melting to occur in the asthenosphere. While still solid, the asthenosphere is able to flow. The ability of a solid to flow is called plasticity.

      Core: The core, the center of the earth. It makes up nearly one third the mass of the earth. The core is also divided into two regions, the inner core and the outer core. Scientist believed that the inner core was solid and the oter core was liquid.

      Differentiate oceanic from continental crust.

      The continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its bulk composition is more felsic, or granitic, in contrast to the oceanic crust, called sima, whose bulk composition is more mafic or basaltic rock while Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate. The crust overlies the solidified and uppermost layer of the mantle. The crust and the solid mantle layer together comprise oceanic lithosphere. Oceanic crust is the result of erupted mantle material originating from below the plate, cooled and in most instances, modified chemically by seawater. This occurs mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions. It is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium. It is thinner than continental crust, or sial, generally less than 10 kilometers thick; however it is denser, having a mean density of about 2.9 grams per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter.

    • Denise Flores profile image

      Denise Flores 

      2 years ago

      Denise Flores BSA-II ( Earth Science TF 2:00-:30 )

      1. Describe the layers of the Earth.

      Core: Below the mantle is the core, the center of the earth. It makes up nearly one third the mass of the earth. The core is also divided into two regions, the inner core and the outer core. From seismic or earthquake waves, scientists believe the outer core is a liquid and the inner core is a solid. The outer core is made of iron and is very dense.

      Mantle: Under the crust is the rocky mantle, which is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium. It makes up almost two thirds of the earth's mass and is about 2900 km thick. The upper mantle is rigid and is part of the lithosphere (together with the crust). The lower mantle flows slowly, at a rate of a few centimeters per year. The asthenosphere is a part of the upper mantle that exhibits plastic properties. It is located below the lithosphere (the crust and upper mantle), between about 100 and 250 kilometers deep.

      Crust: The thin, outermost layer of the earth is called the crust. It makes up only one percent of the earth's mass. The crust is divided into continental plates which drift slowly (only a few centimeters each year) atop the less rigid mantle. The crust is thinner under the oceans (6-11 km thick); this is where new crust is formed. Continental crust is about 25-90 km thick. The lithosphere is defined as the crust and the upper mantle, a rigid layer about 100-200 km thick. The Mohorovicic discontinuity is the separation between the crust and the upper mantle.

      2. What is the difference between the oceanic and continental lithosphere?

      The crust is subdivided into two types, oceanic and continental. Oceanic crust is found under oceans, and it is about four miles thick in most places. A feature unique to oceanic crust is that there are areas known as mid-ocean ridges where oceanic crust is still being created.

      Magma shoots up through gaps in the ocean’s floor here. As it cools, it hardens into new rock, which forms brand new segments of oceanic crust. Since oceanic crust is heavier than continental crust, it is constantly sinking and moving under continental crust. Continental crust varies between six and 47 miles in thickness depending on where it is found. Continental crust tends to be much older than the oceanic kind, and rocks found on this kind of crust are often the oldest in the world.

    • profile image

      Romalyn Dela Cruz 

      2 years ago

      Geology is the science of the study of the structures of the earth and the processes that affect those structures. Scientists who study the earth are known as geologists.¡¨ One way geologists learn about the inner part of the earth is by studying the way earthquake waves travel through the various layers of the earth.

      The outer layer of the earth is the crust ¡V a layer of solid rock that covers the entire surface of the earth. The thickness of the crust ranges from about 5 miles beneath the oceans to roughly 25 miles beneath the continents. Compared with the other layers, the crust is very rigid, thin, and brittle. We know more about the crust than the other layers because we can see it.

    • profile image

      Romalyn Dela Cruz 

      2 years ago

      Geology is the science of the study of the structures of the earth and the processes that affect those structures. Scientists who study the earth are known as geologists.¡¨ One way geologists learn about the inner part of the earth is by studying the way earthquake waves travel through the various layers of the earth.

      The outer layer of the earth is the crust ¡V a layer of solid rock that covers the entire surface of the earth. The thickness of the crust ranges from about 5 miles beneath the oceans to roughly 25 miles beneath the continents. Compared with the other layers, the crust is very rigid, thin, and brittle. We know more about the crust than the other layers because we can see it.

    • profile image

      Renadel E. Domingo BEED-III 

      2 years ago

      The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Many of the rocks now making up the Earth's crust formed less than 100 million (1×108) years ago; however the oldest known mineral grains are 4.4 billion (4.4×109) years old, indicating that the Earth has had a solid crust for at least that long. Most of the Earth's structure is theoretical, being based on extrapolations of physical evidence which has come from the first few kilometres of the Earth's surface, samples brought to the surface from deeper depths and analysis of signals that pass through it in the electromagnetic spectrum.

      core, mantel, crust.

    • profile image

      Rianne Leigh Hernandez BSBA 1-1 

      2 years ago

      The interior structure of the Earth is layered in spherical shells, like an onion. These layers can be defined by either their chemical or their rheological properties. Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of the internal structure of the Earth is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of the gravitational and magnetic fields of the Earth, and experiments with crystalline solids at pressures and temperatures characteristic of the Earth's deep interior.

    • profile image

      Julie Ann Dela Cruz 

      2 years ago

      Julie Ann Dela Cruz (BSBA1-3)

      1. Describe the layers of the Earth.

      -

      Earth is made up of several different layers, each with unique properties. First is the Earth's Core! You may have heard that ogres are like onions because they have many layers. Well, Earth is the same way! Earth is made up of several different layers, each of which has unique properties.

      Let's start from the inside and work our way out. Earth has a core, but this is really two distinct parts: the inner core and the outer core. Both parts of the core are made up of mostly iron and some nickel. The difference is that in the inner core, those minerals are solid and in the outer core, they're liquid.

      The inner core of the earth- is incredibly hot - so hot that if you tried to dig a hole to China you'd burn up on your way through the earth! What's amazing about the inner core is that even though it's about as hot as the surface of the sun, there's so much pressure from the weight of the world pushing down on it that it can't melt. This is the same reason that water in a pressure cooker doesn't boil, no matter how hot it gets!

      The outer core- is also made up of iron and nickel, but it's quite different because it is a liquid. This is because there is much less pressure on this layer than the one below it (the outer core adds a lot of pressure to the inner core!). Though the flow of this liquid layer is very slow-moving (about a few kilometers a year), it is what produces Earth's magnetic field. Our North and South Poles exist because of this liquid outer core, even though it's almost 2,000 miles below us.

      The Mantle- Sitting on top of the outer core, we find the mantle. This layer is by far the thickest layer of Earth, about 1,800 miles thick! It also makes up about 85% of Earth's volume. Like the core, the mantle contains mostly iron, but in the form of silicate rocks. You might be surprised to learn that this rock actually moves like a fluid, similar to how silly putty moves. If you poke silly putty hard, it acts like a solid, but if you slowly pull it apart, it acts like a liquid. We call this ability of rock to move without breaking plasticity.

      The mantle can also be divided into two portions, the upper mantle and the lower mantle. The lower mantle is completely solid because, like the solid inner core of Earth, the pressure is just too great for it to melt and flow. The upper mantle is also known as the asthenosphere, which flows as convection currents. Convection occurs in all fluids and is the rising of warm particles and sinking of cool particles. So, as the material in the upper mantle warms, it rises straight up, and as it rises, it cools and then sinks back down.

      The Lithosphere. This convection flow of the asthenosphere has a large impact on Earth's lithosphere, the outermost layer of the planet. The lithosphere is only about 60 miles thick and contains both the crust and a small portion of the upper mantle. The lithosphere is very rigid; it does not flow like the asthenosphere but instead floats on top of it like ice on a pond.

      This layer of Earth is broken up into several different pieces, like a jigsaw puzzle called tectonic plates. The convection currents below in the mantle move these plates around on the surface of Earth. Though the 'flow' of the asthenosphere is on par with a snail's pace, these enormous plates running into each other is what causes earthquakes, volcanoes and mountain range formation.

      The crust- is contained in the lithosphere, can also be divided into two parts (have you noticed a pattern yet?). We live on the continental crust, and the ocean floor is made up of the oceanic crust. The continental crust is thicker than the oceanic crust, but it's made up of rock that is less dense than the oceanic crust, so it sits on top of it, above sea level.

      2. What is the difference between the oceanic and continental lithosphere?

      -

      Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle (peridotite) and is denser than continental lithosphere, for which the mantle is associated with crust made of felsic rocks. Oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle and causes the oceanic lithosphere to become increasingly thick and dense with age. The thickness of the mantle part of the oceanic lithosphere can be approximated as a thermal boundary layer that thickens as the square root of time.

      Earth's lithosphere includes the crust and the uppermost mantle, which constitute the hard and rigid outer layer of the Earth. The lithosphere is subdivided into tectonic plates. The uppermost part of the lithosphere that chemically reacts to the atmosphere, hydrosphere and biosphere through the soil forming process is called the pedosphere. The lithosphere is underlain by the asthenosphere which is the weaker, hotter, and deeper part of the upper mantle. The boundary between the lithosphere and the underlying asthenosphere is known as the Lithosphere-Asthenosphere boundary and is defined by a difference in response to stress: the lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure, while the asthenosphere deforms viscously and accommodates strain through plastic deformation. The study of past and current formations of landscapes is called geomorphology.

    • Kate Nicole profile image

      Kate Nicole Pascual 

      2 years ago from Philippines

      Pascual, Kate Nicole (BSBA 1-2)

      1. The Earth layers are: the crust, the mantle, the outer core, and the inner core. The crust comprises the continents and ocean basins. It has a variable thickness, anywhere from 35-70 km thick in the continents and 5-10 km thick in the ocean basins. The crust is composed mainly of alumino-silicates. Just under the crust is the mantle. It is composed mainly of ferro-magnesium silicates. It is about 2900 km thick, and is separated into the upper and lower mantle. This is where most of the internal heat of the Earth is located. The outer core is 2300 km thick and the inner core is 1200 km thick. The outer core is composed mainly of a nickel-iron alloy, while the inner core is almost entirely composed of iron. The outer core contains as much as 10% lighter elements than iron alloy. The inner core is thought to rotate at a different speed than the rest of the Earth and this is thought to contribute to the presence of the Earth’s magnetic field.

      2. Continental lithosphere is thicker and less dense than oceanic lithosphere. Continental crust is primarily composed of granitic rock while oceanic crust is composed primarily of basaltic rock. Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle (peridotite) and is denser than continental lithosphere, for which the mantle is associated with crust made of felsic rocks. Oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge.

    • profile image

      ericssonguevara 

      2 years ago

      john ericsson guevara BSBA 1-3 Economics" has been called "the science of how people make a living." We all have to make a living! Shouldn't we all understand something about economics? Often, though, when people talk about Economics, the words seem to get all twisted around! How do we even know what's being talked about?

      Here's an exercise to give you a taste of how you'll approach things in our Understanding Economics course. Below are twelve images of things one might encounter in our day-to-day economy. There are four ranges on the screen, labeled Land, Capital, Labor and Other.

    • melaaii-bsba profile image

      Carmela Del Rosario 

      2 years ago from Baliuag,Bulacan

      DEL ROSARIO, CARMELA V. BSBA 1-2

      1. Describe the layers of the Earth.

      CRUST --The thin, outermost layer of the earth is called the crust. It makes up only one percent of the earth's mass. This consists of the continents and ocean basins. The crust has varying thickness, ranging between 35-70 km thick in the continents and 5-10 km thick in the ocean basins. Within the crust, intricate patterns are created when rocks are redistributed and deposited in layers through the geologic processes. The crust is composed mainly of alumino-silicates.

      MANTLE -- The mantle is a dense, hot layer of semi-solid rock approximately 2,900 km thick and is composed mainly of ferro-magnesium silicates. This is where most of the internal heat of the Earth is located. Large convective cells in the mantle circulate heat and may drive plate tectonic processes.

      CORE - Below the mantle is the core. It makes up nearly one third the mass of the earth. The Earth's core is actually made up of two distinct parts: a 2,200 km-thick liquid outer core and a 1,250 km-thick solid inner core. The outer core is made of iron and is very dense. As the Earth rotates, the liquid outer core spins, creating the Earth's magnetic field. The inner core is made of solid iron and nickel. Many scientists believe it is kept in the solid state because of the extreme pressure from the other layers.

      2. What is the difference between the oceanic and continental lithosphere?

      Oceanic lithosphere is typically about 50–140 km thick (but beneath the mid-ocean ridges is no thicker than the crust), while continental lithosphere has a range in thickness from about 40 km to perhaps 280 km; the upper ~30 to ~50 km of typical continental lithosphere is crust. The mantle part of the lithosphere consists largely of peridotite. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at the Moho discontinuity.

      Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years but after this becomes increasingly denser than asthenosphere. This is because the chemically differentiated oceanic crust is lighter than asthenosphere, but thermal contraction of the mantle lithosphere makes it more dense than the asthenosphere. The gravitational instability of mature oceanic lithosphere has the effect that at subduction zones, oceanic lithosphere invariably sinks underneath the overriding lithosphere, which can be oceanic or continental. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of the continental lithosphere are billions of years old. The oldest parts of continental lithosphere underlie cratons, and the mantle lithosphere there is thicker and less dense than typical; the relatively low density of such mantle "roots of cratons" helps to stabilize these regions.

    • profile image

      Reibelyn Alabado 

      2 years ago

      Questions for Review

      LAYERS OF THE EARTH

      1. Describe the layers of the Earth.

      Layers of the Earth

      The Earth is made up of four layers. The thin outer layer is called the crust, followed by the hot, partly molten rock, the mantle. Beneath it is the outer core, which is a layer of liquid metal. In the center of the Earth is ball of very hot solid metal called the inner core.

      Crust

      The crust accounts for only about 0.6% of the planet’s volume. It varies in thickness from 248 miles (40 km) beneath the parts of the continents to only 3.1 miles (5 km) under parts of the ocean floor. It is made up of lighter rock than the outer layers. The temperature of the rocks increases by about 86 degrees Fahrenheit (30 degrees Celsius) for every kilometer under the surface.

      Mantle

      The mantle accounts for about 82% of the Earth’s volume and extends from the base of the crust to a depth of about 1,800 miles (2,900 km). The sharp boundary between the crust and the mantle is called Mohorovicic discontinuity after the Yugoslav seismologist who discovered it in 1909.

      Aside from oxygen, it is thought to consist largely of peridotite, a rock that contains high proportions of the elements iron, magnesium and silicon. The mantle is inaccessible and although it is mostly solid, it contains a partially molten layer.

      Core

      The core accounts for about 17% of the Earth’s volume and extends from the base of the mantle of the earth’s center. The discontinuity between the mantle and the core called the core-mantle boundary or Gutenberg discontinuity, after Beno Gutenberg, a German-American seismologist who discovered it. The core is composed of two distinct parts: the outer core and the inner core. The outer core extends down to a depth of about 3,200 miles (5,155 km) and is liquid. The inner core measures about 1,500 miles (2,400 km). The main component of the core is iron and contain some lighter elements possibly carbon, hydrogen, oxygen, silicon, and sulfur.

      2. What is the difference between the oceanic and continental lithosphere?

      Oceanic lithosphere is about 70 km thick (but can be as thin as 1.6 km at the mid-ocean ridges), while continental lithosphere is about 150 km thick (and can be considerably thicker at continental collision zones). Oceanic lithosphere consists mainly of mafic and ultramafic rocks and is denser than continental lithosphere, which consists predominantly of felsic rocks. This higher density has the effect that at subduction zones the oceanic plate will invariably sink underneath the continental plate. New oceanic lithosphere is constantly being produced at mid-ocean ridges from mantle material and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 200 million years old. As oceanic lithosphere grows older, it gets cooler and denser, with the result that if two oceanic plates converge, the older one will subduct below the younger one.

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