Composition of the Earth

The Size of the Earth The Earth is almost spherical with a radius of nearly 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 Crust Mantle Outer 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.

Crust This 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 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 km thick liquid mostly iron and nickel

Inner Core km in diameter Solid Mostly iron and nickel

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

The Earth’s Magnetic Field Since 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 Composition Elements are not distributed evenly throughout the layers. Not in your notes.

Earth’s Layers Thickness (km) Volume cm 3 Density g/cc Mass kg Mass Percent Atmosphere Hydrosphere Crust Mantle Core All 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)