Chapter 4: The Earth’s Interior

Chapter 4: The Earth’s Interior What percent of the Earth’s total volume is made of crust? How can we study the interior of the Earth? Why can’t we just drill down to the mantle? 1 % Drilling Seismic waves Earth’s magnetism Measurement of gravity Meteorites Heat flow Crust is too thick Too expensive Takes too long

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

Seismic Reflection

2. Another method is seismic refraction 2. Another method is seismic refraction. With seismic refraction, seismic waves bend (or change paths) as they pass from one material to another. Seismic waves will bend toward the rock layer that is made of lower-velocity (or slower material). Refer to Fig. 4.2 on pg. 110.

Seismic Refraction

What is inside the Earth? Seismic reflection and seismic refraction have enabled scientists to plot the three main zones of the Earth’s interior: Crust - outer layer of rock; thin skin on the surface Mantle - thick shell of rock that separates the crust above from the core below Core - central zone of the earth, probably metallic and probably the source of the Earth’s magnetic field

Interior of the Earth Crust

Apple Analogy

Moho boundary

The Crust Studies of the crust have shown the following: The crust is thinner beneath the oceans than beneath the continents Seismic waves travel faster in oceanic crust than continental crust (so, it’s assumed that each is made of a different type of rock)

Characteristics of Oceanic & Continental Crust Oceanic Crust Continental Crust Avg. thickness 7 km 30-50 km (thickest under mountains) Density 3.0 g/cm3 2.7 g/cm3 Composition Various types of rock Granite rock covered with sedimentary rock layer

The Crust (cont’d) Mohorovičić discontinuity (Moho boundary): This is the boundary that separates the crust from the mantle Note: The mantle lies closer to the Earth’s surface beneath the ocean than it does beneath the continents (The Mohorovičić discontinuity [MOE-HOE-ROE-vee-cheech], usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. Named after the pioneering Croatian seismologist Andrija Mohorovičić)

The Mantle Scientists believe that the mantle is made mostly of solid rock. However, a few isolated chambers of melted rock (magma) do exist. Also, the rock of the mantle is quite different than the rock of the crust. The crust and uppermost mantle together form the lithosphere which is relatively strong and brittle.

The Mantle (cont’d) Beneath the lithosphere is a 200 km thick zone called the asthenosphere. Here, the seismic waves travel more slowly, which suggests that the rocks are closer to their melting point. These rocks may be partially melted forming a “crystal-and-liquid slush”. This is an important fact for two reasons: 1. Magma is probably produced here 2. Rocks have less strength & they probably flow So, the asthenosphere acts as a “lubricating layer” which allows the plates to move.

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

P-wave Shadow Here, P-waves reflect (or bounce) off 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. Here, the P-waves refract (or bend) as they pass though the core

More on the P-wave Shadow

Videos P-wave & S-wave Shadows

The Core (cont’d) S-waves do not travel through the core at all, which indicates that the core is liquid or that it acts like a liquid. The way P-waves behave in the core suggest that the core has two parts: 1. a liquid outer core 2. a solid inner core

What is the composition of the core? The core is made of metal (probably iron), with small amounts of oxygen, silicon, sulphur or nickel). The core is extremely heavy, and has a density of between 10 and 13 g/cm3

How does the elevation of continents change? Isostasy is a balance between blocks of the crust that are floating on the upper mantle. Remember, the crust is not as dense as the mantle, so it floats. The blocks of crust will rise or sink depending on their thickness. Thicker blocks (such as mountains) will extend into the mantle more deeply than other blocks. In other words, the crust rises or sinks gradually until a balance is achieved. This balanced is called isostatic adjustment, and occurs when “high spots” erode or when the crust bounces back after a glacier has melted (please refer to pages 120 & 121 in the soft-covered books for diagrams and more information).

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

Isostasy of Plates

Isostatic Adjustment

What can gravity tell us about the Earth’s crust? The force of gravity is greater between bigger objects. For example, the force of gravity between the moon and the Earth is greater than the force between two bowling balls. Scientists use a gravity meter (a weight on a spring) to sense the amount of gravity. More gravitational attraction is present when a heavy, dense mass of rock is in the crust underneath the gravity meter. Less attraction is present when a cave or light rock is underneath. Such gravity measurements can be used to learn more about the structure of the Earth and to locate valuable metals, minerals, and oil.

Earth’s Magnetic Field What is the magnetic field? A region of magnetism surrounds the Earth. These invisible lines of force surrounding the Earth deflect magnetized objects, such as compass needles. The magnetic lines connect at both the North and South Poles

What are magnetic reversals? How is the magnetic field generated? One widely accepted idea is that the mag. Field is created by currents within the liquid outer core. The outer core is hot and actually flows several kilometres per year. What are magnetic reversals? This happens when the magnetic lines of force run in the opposite direction. So, the South Pole becomes the North Pole and vice versa. In other words, the polarity reverses. Evidence exists for this in rocks that contain metal. One can see the lines in the rock change direction.

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

Geothermal Gradient

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

Geothermal Gradient (cont’d) The temperature gradient of 25°C/km actually decreases substantially a short distance into the Earth, down to about 0.3°C/km within the mantle. The core-mantle boundary has a temperature of about 3800°C, 6300°C at the inner-core/outer-core boundary, and 6400°C at the Earth’s centre. The temperature at the centre of the core is hotter than the surface of the sun!!!

Heat Flow A small amount of measureable heat from the Earth’s interior is gradually being lost through the surface. This gradual loss of heat is called heat flow. This heat could be “original heat” or new heat that is created from radioactive decay. This probably happens within rock that is rich in uranium. Also, the average heat loss is about the same for continental crust and oceanic crust. END OF NOTES BEFORE MID-TERM EXAM!! Next: Ch. 5 and Mineral Term Project (5%)