Chapter 4: The Earth’s Chapter 4: The Earth’s InteriorInterior

Chapter 4: The Earth’s Chapter 4: The Earth’s InteriorInterior

What percent of the What percent of the Earth’s total volume Earth’s total volume is made of crust?is made of crust?

How can we study the How can we study the interior of the Earth?interior of the Earth?

Why can’t we just Why can’t we just drill down to the drill down to the mantle? mantle?

1 %

1. Drilling2. Seismic waves3. Earth’s magnetism4. Measurement of

gravity5. Meteorites6. Heat flow

1. Crust is too thick2. Too expensive3. Takes too long

What can we learn from the What can we learn from the study of seismic waves?study of seismic waves?

1. One important way for learning about 1. One important way for learning about the Earth’s interior is the study of the Earth’s interior is the study of seismic reflectionseismic reflection. With seismic . With seismic reflection, seismic waves bounce (or reflection, seismic waves bounce (or reflect) from a rock boundary deep within reflect) from a rock boundary deep within the Earth, and return to a seismograph the Earth, and return to a seismograph station on the surface. This is just like station on the surface. This is just like light bouncing off a mirror. Scientists light bouncing off a mirror. Scientists can use this process to calculate the can use this process to calculate the depth of the rock layer.depth of the rock layer.

Seismic ReflectionSeismic Reflection

2. Another method is 2. Another method is seismic seismic refractionrefraction. With seismic refraction, . With seismic refraction, seismic waves bend (or change seismic waves bend (or change paths) as they pass from one paths) as they pass from one material to another. Seismic waves material to another. Seismic waves will bend will bend towardtoward the rock layer that the rock layer that is made of lower-velocity (or slower is made of lower-velocity (or slower material). Refer to Fig. 4.2 on pg. material). Refer to Fig. 4.2 on pg. 110.110.

Seismic Seismic RefractionRefraction

What is inside the Earth?What is inside the Earth?Seismic reflection and seismic refraction

have enabled scientists to plot the three main zones of the Earth’s interior:

1.Crust - outer layer of rock; thin skin on the surface

2.Mantle - thick shell of rock that separates the crust above from the core below

3.Core - central zone of the earth, probably metallic and probably the source of the Earth’s magnetic field

Interior of the EarthInterior of the Earth

Crust

Apple AnalogyApple Analogy

Moho boundary

The CrustThe Crust

Studies of the crust have shown the Studies of the crust have shown the following:following:

1.1. The crust is thinner beneath the The crust is thinner beneath the oceans than beneath the continentsoceans than beneath the continents

2.2. Seismic waves travel faster in Seismic waves travel faster in oceanic crust than continental crust oceanic crust than continental crust (so, it’s assumed that each is made (so, it’s assumed that each is made of a different type of rock)of a different type of rock)

Characteristics of Oceanic & Characteristics of Oceanic & Continental CrustContinental Crust

CharacteristicCharacteristic Oceanic CrustOceanic Crust Continental Continental CrustCrust

Avg. Avg. thicknessthickness

7 km7 km 30-50 km 30-50 km (thickest under (thickest under mountains)mountains)

DensityDensity 3.0 g/cm3.0 g/cm33 2.7 g/cm2.7 g/cm33

CompositionComposition Various types Various types of rockof rock

Granite rock Granite rock covered with covered with sedimentary sedimentary rock layerrock layer

The Crust (cont’d)The Crust (cont’d)Mohorovičić discontinuity (Moho boundary):Mohorovičić discontinuity (Moho boundary):

This is the boundary that separates the crust This is the boundary that separates the crust from the mantle from the mantle

Note:Note: The mantle lies closer to the Earth’s The mantle lies closer to the Earth’s surface beneath the ocean than it does surface beneath the ocean than it does beneath the continentsbeneath the continents

((The The Mohorovičić discontinuityMohorovičić discontinuity [MOE-HOE-ROE-vee-cheech], usually referred [MOE-HOE-ROE-vee-cheech], usually referred to as the to as the MohoMoho, is the boundary between the Earth's crust and the mantle. , is the boundary between the Earth's crust and the mantle. Named after the pioneering Croatian seismologist Andrija Mohorovičić)Named after the pioneering Croatian seismologist Andrija Mohorovičić)

The MantleThe Mantle

Scientists believe that the mantle is Scientists believe that the mantle is made mostly of solid rock. However, made mostly of solid rock. However, a few isolated chambers of melted a few isolated chambers of melted rock (magma) do exist. Also, the rock (magma) do exist. Also, the rock of the mantle is quite different rock of the mantle is quite different than the rock of the crust.than the rock of the crust.

The crust and uppermost mantle The crust and uppermost mantle together form the together form the lithospherelithosphere which which is relatively strong and brittle.is relatively strong and brittle.

The Mantle (cont’d)The Mantle (cont’d) Beneath the lithosphere is a 200 km thick Beneath the lithosphere is a 200 km thick

zone called the zone called the asthenosphereasthenosphere. Here, the . Here, the seismic waves travel more slowly, which seismic waves travel more slowly, which suggests that the rocks are closer to their suggests that the rocks are closer to their melting point. These rocks may be partially melting point. These rocks may be partially melted forming a “crystal-and-liquid slush”.melted forming a “crystal-and-liquid slush”.

This is an important fact for two reasons:This is an important fact for two reasons:1. Magma is probably produced here1. Magma is probably produced here2. Rocks have less strength & they probably 2. Rocks have less strength & they probably flowflow

So, the asthenosphere acts as a “lubricating So, the asthenosphere acts as a “lubricating layer” which allows the plates to move.layer” which allows the plates to move.

The CoreThe Core

Seismic wave data tells us a great Seismic wave data tells us a great deal about the core. P-waves bounce deal about the core. P-waves bounce off the core or refract through the off the core or refract through the core. But there is a “P-wave core. But there is a “P-wave shadow” that has allowed scientists shadow” that has allowed scientists to calculate the size and shape of the to calculate the size and shape of the core.core.

P-wave ShadowP-wave Shadow

Here, P-waves reflect (or bounce) off the core

Here, the P-waves refract (or bend) as they pass though the core

Here, the size and shape of the P-wave shadow can be used to determine the size and shape of the entire core.

More on the P-wave ShadowMore on the P-wave Shadow

VideosVideos

P-wave & S-wave Shadows

The Core (cont’d)The Core (cont’d)

S-waves do not travel through the S-waves do not travel through the core at all, which indicates that the core at all, which indicates that the core is liquid core is liquid oror that it acts like a that it acts like a liquid.liquid.

The way P-waves behave in the core The way P-waves behave in the core suggest that the core has two parts:suggest that the core has two parts:

1. a liquid outer core1. a liquid outer core

2. a solid inner core2. a solid inner core

What is the composition of the What is the composition of the core?core?

The core is made of metal (probably The core is made of metal (probably iron), with small amounts of oxygen, iron), with small amounts of oxygen, silicon, sulphur or nickel).silicon, sulphur or nickel).

The core is extremely heavy, and has The core is extremely heavy, and has a density of between 10 and 13 a density of between 10 and 13 g/cmg/cm33

How does the elevation of How does the elevation of continents change?continents change?

IsostasyIsostasy is a balance between blocks of the crust is a balance between blocks of the crust that are floating on the upper mantle. Remember, that are floating on the upper mantle. Remember, the crust is not as dense as the mantle, so it floats.the crust is not as dense as the mantle, so it floats.

The blocks of crust will rise or sink depending on The blocks of crust will rise or sink depending on their thickness. Thicker blocks (such as their thickness. Thicker blocks (such as mountains) will extend into the mantle more mountains) will extend into the mantle more deeply than other blocks. In other words, the crust deeply than other blocks. In other words, the crust rises or sinks gradually until a balance is achieved.rises or sinks gradually until a balance is achieved.

This balanced is called isostatic adjustment, and This balanced is called isostatic adjustment, and occurs when “high spots” erode or when the crust occurs when “high spots” erode or when the crust bounces back after a glacier has melted (please bounces back after a glacier has melted (please refer to pages 120 & 121 in the soft-covered books refer to pages 120 & 121 in the soft-covered books for diagrams and more information).for diagrams and more information).

IsostasyIsostasy

Crust that is less dense will float higher than crust this is more dense.

Isostasy of PlatesIsostasy of Plates

Isostatic AdjustmentIsostatic Adjustment

What can gravity tell us about What can gravity tell us about the Earth’s crust?the Earth’s crust?

The force of gravity is greater between bigger The force of gravity is greater between bigger objects. For example, the force of gravity objects. For example, the force of gravity between the moon and the Earth is greater than between the moon and the Earth is greater than the force between two bowling balls.the force between two bowling balls.

Scientists use a gravity meter (a weight on a Scientists use a gravity meter (a weight on a spring) to sense the amount of gravity.spring) to sense the amount of gravity.

More gravitational attraction is present when a More gravitational attraction is present when a heavy, dense mass of rock is in the crust heavy, dense mass of rock is in the crust underneath the gravity meter. Less attraction is underneath the gravity meter. Less attraction is present when a cave or light rock is underneath.present when a cave or light rock is underneath.

Such gravity measurements can be used to Such gravity measurements can be used to learn more about the structure of the Earth and learn more about the structure of the Earth and to locate valuable metals, minerals, and oil.to locate valuable metals, minerals, and oil.

Earth’s Magnetic FieldEarth’s Magnetic FieldWhat is the magnetic field?What is the magnetic field?A region of magnetism surrounds the Earth. A region of magnetism surrounds the Earth.

These invisible lines of force surrounding These invisible lines of force surrounding the Earth deflect magnetized objects, the Earth deflect magnetized objects, such as compass needles. The magnetic such as compass needles. The magnetic lines connect at both the North and South lines connect at both the North and South PolesPoles

What are magnetic reversals?What are magnetic reversals?

One widely accepted idea is that the mag. Field is One widely accepted idea is that the mag. Field is created by currents within the liquid outer core. created by currents within the liquid outer core. The outer core is hot and actually flows several The outer core is hot and actually flows several kilometres per year.kilometres per year.

How is the magnetic field How is the magnetic field generated?generated?

This happens when the magnetic lines of force run This happens when the magnetic lines of force run in the opposite direction. So, the South Pole in the opposite direction. So, the South Pole becomes the North Pole and vice versa. In other becomes the North Pole and vice versa. In other words, the polarity reverses. Evidence exists for words, the polarity reverses. Evidence exists for this in rocks that contain metal. One can see the this in rocks that contain metal. One can see the lines in the rock change direction.lines in the rock change direction.

What are magnetic What are magnetic anomalies?anomalies?

Variations (or anomalies) in the Variations (or anomalies) in the magnetic field can indicate different magnetic field can indicate different types of rocks. Scientists use types of rocks. Scientists use instruments called magnetometers to instruments called magnetometers to measure the strength of the measure the strength of the magnetic field. For example, rocks magnetic field. For example, rocks with more iron or metal will give off a with more iron or metal will give off a stronger magnetic field.stronger magnetic field.

Geothermal GradientGeothermal Gradient

Geothermal Gradient:Geothermal Gradient:

This is the rate of temperature This is the rate of temperature increases with depth. The average increases with depth. The average temperature increase is 25temperature increase is 25°C for every °C for every kilometre of depth for the first few km’s. kilometre of depth for the first few km’s. Some areas have a much higher Some areas have a much higher gradient, and some have potential for gradient, and some have potential for geothermal energy (such as Iceland). geothermal energy (such as Iceland). This temperature gradient makes mines This temperature gradient makes mines hot (near the boiling point of 100°C in hot (near the boiling point of 100°C in South Africa) and makes drilling deep South Africa) and makes drilling deep oil wells difficult.oil wells difficult.

Geothermal Gradient Geothermal Gradient (cont’d)(cont’d)

The temperature gradient of 25The temperature gradient of 25°C/km °C/km actually decreases substantially a actually decreases substantially a short distance into the Earth, down to short distance into the Earth, down to about 0.3about 0.3°C/km within the mantle.°C/km within the mantle.

The core-mantle boundary has a The core-mantle boundary has a temperature of about 3800°C, 6300°C temperature of about 3800°C, 6300°C at the inner-core/outer-core boundary, at the inner-core/outer-core boundary, and 6400°C at the Earth’s centre. The and 6400°C at the Earth’s centre. The temperature at the centre of the core temperature at the centre of the core is hotter than the surface of the sun!!!is hotter than the surface of the sun!!!

Heat FlowHeat Flow

A small amount of measureable heat from the A small amount of measureable heat from the Earth’s interior is gradually being lost Earth’s interior is gradually being lost through the surface. This gradual loss of through the surface. This gradual loss of heat is called heat flow. This heat could be heat is called heat flow. This heat could be “original heat” or new heat that is created “original heat” or new heat that is created from radioactive decay. This probably from radioactive decay. This probably happens within rock that is rich in uranium. happens within rock that is rich in uranium. Also, the average heat loss is about the Also, the average heat loss is about the same for continental crust and oceanic same for continental crust and oceanic crust.crust.

END OF NOTES BEFORE MID-TERM EXAM!!END OF NOTES BEFORE MID-TERM EXAM!!

Next: Ch. 5 and Mineral Term Project (5%)Next: Ch. 5 and Mineral Term Project (5%)