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Earth’s layered structure

Table of contents

Mantle convection

Action at plate boundaries

Silfra rift in Iceland: swim between the edges of two continents

Continental drift and plate tectonics

Ophiolites – fragments of ocean floor sheared off from subducting crust

Why is the Earth still hot?


How many continents are there?



Remains of lost continents under the Earth

Composition of the Earth

oxygen 46.6 %

silicon 27.7 %

aluminum 8.1 %

iron 5 %

calcium 3.6%

sodium 2.8%

Another way to show the same information:


Two ways of looking at Earth’s layers

Classify by what layers are made of (composition)

crust, mantle, core

Classify by how layers move/bend (mechanical).

lithosphere, aesthenosphere, mesosphere, outer core, inner core

composition and mechanical layers

crust = outermost layer. Thinnest layer

Where mountains, hills, valleys, plains, lakes and oceans are formed.

Continental crust = 35 km deep

Oceanic crust        = 5 to 10 km deep

mantle = highly viscous layer of rock, between crust and the outer core.  Over a period of days or years the mantle behaves like solid rock. But when we observe the mantle over centuries we see that it is deformable, like Play-Doh.

Lithosphere = Rigid, outermost shell of a rocky planet. The upper part of the mantle, and crust. Looking at it over a period of days or years it is solid. Yet it does move over timescales of thousands of years or greater.

From the Greek λίθος [lithos] “rocky” + σφαῖρα [sphaira] “sphere”

Starts right below the crust, and goes down to about 660 km deep.

Astheosphere = middle part of the mantle.  Highly viscous.  Mechanically weak. Does not flow like a liquid, yet it is not a hard solid like a rock or brick. Starts at 660 km deep, and goes down to 700 km deep.

From Greek asthenēs ‘weak’ + σφαῖρα [sphaira] “sphere”.

Mesosphere = lower part of the mantle. From 700 to 2900 km. Tremendous pressure forces the atoms here close together; seismic waves travel faster through this layer.

The core (Inner layers of the Earth)

Outer core of the Earth is from 2900 to 5000 km deep.

Made mostly of liquid iron and nickel. Around 5000 °C

This is where the Earth’s magnetic field is generated: Magnetism and Earth’s magnetic field.

Inner core is very hot: 5,400 °C; 9,800 °F

From 2900 km down to 12,700 km deep

Compare these numbers to distances familiar to us.

Distance scale

Boston, MA to New York City               200 miles (driving) 320 km

Boston, MA to Los Angeles, CA           3,000 miles (driving) 4,800 km

Boston, MA to London, England         3,300 miles (flying) 5300 km

Mohorovičić discontinuity

Also called the Moho, this is the boundary between Earth’s crust and mantle.

Moho discontinuity


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