Composition of the Earth

The Size of the Earth The Earth is almost spherical with a

radius of nearly 6 400 km. Variations in the value of acceleration

due to gravity “g” at the surface prove that the Earth is not perfectly spherical.

The composition of the Earth varies with depth.

The layers are thought to have developed billions of years ago.

The Earth is Made of Layers

CrustMantleOuter Core

Inner Core

Earth has a layered structure

Atmosphere and Hydrosphere

Low Density Crust

Intermediate Density Mantle

High density core Liquid outer core Solid inner core

Not in your notes.

CrustThis is the outermost layer. We have not been able to penetrate it completely. It varies in thickness from as thin as 5 km under

the oceans to as thick as 60 km under the continents. (Averages less than 20 km thick.)

Continental crust is different from oceanic crust.

Oceanic crust is basalt-like. It is high in silicon and magnesium.

Continental crust is granite-like. It is high in silicon and aluminum.

Mantle Lies beneath the crust. Over 80% of the Earth’s volume. It is 2 900 km thick. Solid, rocky layer It is composed of ultramafic rock

(rock that is rich in iron and magnesium)

Mantle It is thought to have the same

chemical composition throughout but a varying mineral composition (pressure causes a structural change between the atoms resulting in different minerals)

Mantle

Lithosphere Includes the crust and upper

mantle Is approximately 100 km thick It is strong and brittle It makes up the “plates” in plate

tectonics

Asthenosphere Is a layer within the mantle that

extends approximately 150 km below the lithosphere.

It is thought to be the zone of magma production. At this depth rocks are closest to their melting point.

Asthenosphere It contains weaker rock that

deforms easily. It is thought to be a lubricating

layer for the plates. Movements within this layer are

thought to induce motion in the rigid lithosphere above

Outer Core Is 2 250 km thick liquid mostly iron and nickel

Inner Core 2 400 km in diameter Solid Mostly iron and nickel

The Earth’s Magnetic FieldThe Earth acts as if a large bar magnet was buried at its core.

The Earth’s Magnetic FieldSince the Earth’s core is too hot for any material to retain its magnetism (Currie point exceeded) motion must be occurring. This idea is also supported by the fact that the magnetic poles appear to be slowly moving around the geographic poles.

It is thought that the magnetic field is created by electric currents within the slowly circulating liquid part of the core.

Density Varies With Depth

 

Not in your notes.

Temperature Varies With Depth

Not in your notes.

Temperature Varies With Depth Within the upper crust, temperature

increases with depth at 25°C / km Temperature increases then tapers to

1°C / km This means that deeper mines must be

cooled for human survival. This also means that drilling equipment

becomes soft and flexible unless continuously cooled.

Source of Earth’s Heat Heat is thought to come from:

Left-over heat from the Earth’s formation

Friction (rotation; moving plates)Radioactive decay (which is most

important) Since rock is a good insulator, little

heat is lost.

Heat Flow from Oceans and Continents

The Protoplanet Hypothesis

About 5 billion years ago an existing, slowly rotating 10 billion km (diameter) cloud of gas and dust began to collapse.

Collapse caused it to spin faster and heat up.

Most of the mass was concentrated as the sun.

Smaller clusters of matter not captured by the sun began to collect.

These became protoplanets.

Collisions continued over time forming planets and moons.

Uncollected material still exists as asteroids and comets.

The protoplanet Earth evolved as it grew hotter from: Gravitational collapse Radioactive minerals Bombardment by meteor showers

As a result, iron melted and sank toward the centre.

Lighter materials rose. Earth became differentiated into

layers. Escaping steam from the

protoplanet formed our oceans.

Differentiation of the Earth

(a) Early homogenous Earth

(b) Lighter matter “floats” toward surface

(c) Modern structure of the Earth

Not in your notes.

Earth’s CompositionEarth’s Composition

Elements are not distributed evenly throughout the layers.

Not in your notes.

Not in your notes.

Earth’s LayersThickness

(km)Volume1027 cm3

Densityg/cc

Mass1027 kg

MassPercen

t

Atmosphere0.00000

50.0000

9

Hydrosphere 3.80 0.00137 1.03 0.00141 0.024

Crust 17 0.008 2.8 0.024 0.4

Mantle 2883 0.899 4.5 4.016 67.2

Core 3471 0.175 11.0 1.936 32.4

All 6371 1.083 5.52 5.976 100.00

Not in your notes.

Assignments Layers of the Earth assignment

sheet From Chapter One

Review p. 14 Critical Thinking p. 15

From Chapter Two Topic Questions 4, 6,7 Review p. 26 (1-18)