Chapter 3- Earth's Interior (Week 2)

Question 1

Rock Coloration

Answer 1

ed Rocks: Color attributed to iron-bearing minerals reacting with oxygen.
Natural State: Unaltered rocks are dark green
Vegetation Absence: Lack of vegetation due to insufficient nutrients in the rocks

Question 2

Meteorites and Earth’s Interior:

Answer 2

Source: Meteorites derived from differentiated bodies, such as asteroids that separated into mantle and core.

Mineral Composition: Asteroids formed at a similar distance from the sun as Earth, having a mineral composition akin to Earth’s.

Types of Meteorites:
-Stony Meteorites: Result from shattered mantle rock.
-Iron Meteorites: Result from shattered core.
-Mixed Fragments: Some fragments exhibit a mix of both mantle and core materials

Question 3

Limitation of Direct Observation

Answer 3

Inaccessibility: Geologists cannot physically go underground to visually examine Earth’s interior.
Methods: Information is gathered through indirect methods like studying rocks, meteorites, and seismic data.

Question 4

Earth’s Layers

Answer 4

Composition: Earth is composed of three main layers - the crust, the mantle, and the core.

Distribution:
-Core: Accounts for nearly half of Earth’s radius but constitutes only 16.1% of Earth’s volume.
-Mantle: Represents the majority of Earth’s volume, approximately 82.5%.
-Crust: Constitutes a small fraction of Earth’s volume, about 1.4%.

Importance: Understanding the composition and distribution of these layers is fundamental for studying Earth’s structure and geological processes.

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Question 5

What is the Earth’s outermost layer?

Answer 5

The Earth’s outermost layer is called the crust.

Question 6

What are the two kinds of crust on Earth?

Answer 6

Continental crust and ocean crust.

Continental crust is thicker than ocean crust.

Question 7

What is the predominant composition of continental crust?

Answer 7

Continental crust is predominantly felsic in composition, meaning it contains minerals richer in silica.

Continental crust is less dense due to its felsic composition, which includes minerals richer in silica.

Question 8

What is the predominant composition of ocean crust?

Answer 8

Ocean crust is predominantly mafic in composition, containing minerals with less silica but more iron and magnesium.

Ocean crust is denser due to its mafic composition, which includes minerals with less silica but more iron and magnesium.

Question 9

How does the crust interact with the mantle?

Answer 9

The crust floats on the mantle.

Continental crust floats higher due to its lower density (compared to ocean), which is a result of its felsic composition.

Question 10

What happens when tectonic plates with ocean crust and continental crust collide?

Answer 10

The plate with ocean crust is forced down into the mantle beneath the plate with continental crust.

Question 11

What is the composition of the Earth’s mantle?

Answer 11

The mantle is ultramafic, meaning it has more iron and magnesium than mafic rocks and less silica. It is almost entirely solid rock but flows very slowly.

Rocks can have different mineral compositions but still share the same chemical composition due to the reconfiguration of mineral structures caused by increasing pressure in the mantle.

Question 12

How does the mantle’s composition change with depth?

Answer 12

Although the mantle has a similar chemical composition throughout, increasing pressure deeper in the mantle causes mineral structures to be reconfigured.

Deeper in the mantle, extreme pressures transform mineral compositions. For example, rocks higher in the mantle are typically peridotite, while lower in the mantle, minerals reconfigure to form rocks like eclogite, which contains garnets.

Question 13

What is the dominant rock composition higher in the mantle?

Answer 13

Rocks higher in the mantle are typically composed of peridotite, a rock dominated by the minerals olivine and pyroxene.

Question 14

Describe the rock composition lower in the mantle.

Answer 14

Lower in the mantle, extreme pressures transform minerals and create rocks like eclogite, which contains garnets.

Question 15

Is the mantle stationary or in constant motion?

Answer 15

The mantle is in constant motion, flowing very slowly despite being almost entirely solid rock.

Question 16

What is the lithosphere composed of?

Answer 16

The lithosphere consists of both crust and the uppermost layer of the mantle, forming a rigid outer shell of the Earth.

The lithosphere cannot be neatly classified as either crust or mantle as it is a combination of both.

Question 17

How is the lithosphere formed?

Answer 17

The lithosphere is formed from the crust and the uppermost layer of the mantle, which is attached to the underside of the crust.

Question 18

What are tectonic plates made of?

Answer 18

Tectonic plates are fragments of the lithosphere.

Question 19

What lies beneath the lithosphere?

Answer 19

The asthenosphere is located beneath the lithosphere.

Question 20

How does the asthenosphere differ from the lithosphere in terms of strength?

Answer 20

The asthenosphere is weaker compared to the lithosphere due to tiny amounts of melted rock dispersed through an otherwise solid structure.

Question 21

Why is the weakness of the asthenosphere important for plate tectonics?

Answer 21

The weakness of the asthenosphere is crucial for plate tectonics as it allows for deformation, enabling fragments of lithosphere to move around upon and through it.

Question 22

What would happen without a weak asthenosphere in terms of plate movement?

Answer 22

Without a weak asthenosphere, plates would be locked in place, unable to move as they currently do in the dynamic process of plate tectonics.

Question 23

What is the D” layer?

Answer 23

The D” (dee double prime) layer is a mysterious layer located approximately 200 km above the core-mantle boundary.

Question 24

How do we know about the existence of the D” layer?

Answer 24

The D” layer is known to exist due to changes in the speed of seismic waves as they move through it.

Question 25

Why is the D” layer considered mysterious?

Answer 25

The D” layer is mysterious because it’s not clear why it’s different from the rest of the mantle. Various theories exist, but the exact reason remains uncertain.

Question 26

What are some proposed explanations for the differences in the D” layer?

Answer 26

Possible explanations include mineral transitions due to pressure and temperature conditions, the presence of small pools of melt, or differences in seismic properties caused by subducted slabs of lithosphere on the core-mantle boundary.

Question 27

What is the primary composition of the Earth’s core?

Answer 27

The core is primarily composed of iron, with lesser amounts of nickel. Lighter elements such as sulfur, oxygen, or silicon may also be present.

Question 28

How hot is the Earth’s core?

Answer 28

The core is extremely hot, ranging from approximately 3500° to more than 6000°C.

Question 29

Why is only the outer core liquid?

Answer 29

Despite the high temperatures, only the outer core is liquid. The inner core is solid because the pressure at that depth is so high that it prevents the core from melting.

Question 30

What prevents the inner core from melting?

Answer 30

he inner core is solid due to the extremely high pressure at that depth, which prevents it from melting, even though it is as hot as the surface of the sun.

Question 31

What is seismology?

Answer 31

Seismology is the study of vibrations within Earth caused by events such as earthquakes, extraterrestrial impacts, explosions, storm waves hitting the shore, and tides.

Question 32

How is seismology applied to Earth’s interior?

Answer 32

Seismic waves, generated by various events, travel through different materials at different speeds. By studying how these waves interact with materials, we can understand Earth’s layers and internal structures.

Question 33

What are P-waves and S-waves in seismology?

Answer 33

P-waves can travel rapidly through both liquids and solids, while S-waves can only travel through solids and are slower than P-waves.

We can identify regions within Earth that are melted.

Seismic waves travel in all directions from their source, but their paths can be conveniently represented as seismic rays.

Question 34

What happens when seismic waves encounter a different rock layer?

Answer 34

Some waves may bounce off the layer, or reflect, while others will travel through the layer. If the wave travels at a different speed in the new layer, its path will be bent, or refracted.

Seismic velocities are higher in more rigid layers, so they generally get faster deeper within the Earth due to higher pressures that make layers more rigid.

Seismic waves tend to take curved paths due to refraction

Question 35

What was one of the first discoveries about Earth’s interior made through seismology?

Answer 35

In the early 1900s, Croatian seismologist Andrija Mohorovičić discovered that seismic waves arrived at more distant seismic stations before closer ones, indicating different speeds. This led to the identification of the Mohorovičić discontinuity, or Moho.

Mohorovičić observed that seismic waves traveling farther were faster, bending down and moving faster through different rocks (mantle) before being bent upward into the crust.

The Moho is the boundary between the Earth’s crust and mantle, identified by differences in seismic wave speeds. Its depth is between 60 – 80 km beneath major mountain ranges, 30 – 50 km beneath most continental crust, and 5 – 10 km beneath ocean crust.

Question 36

What evidence supports the existence of a liquid outer core?

Answer 36

A distinctive seismic wave signature in the global distribution of earthquakes shows S-wave shadow zones, indicating that S-waves cannot travel through the liquid outer core.

The S-wave shadow zone is evidence that S-waves cannot pass through the liquid outer core.

The S-wave and P-wave shadow zones indicate that the outer core is liquid.

Question 37

How has the change in seismic wave velocity with depth in Earth been determined?

Answer 37

The change in seismic wave velocity with depth has been determined by analyzing seismic signals from large earthquakes worldwide over the past several decades.

Earth’s layers are detectable as changes in velocity with depth. The asthenosphere appears as a low velocity zone within the upper mantle.

The core-mantle boundary (CMB) is apparent as a sudden drop in P-wave velocities, marking the transition from solid mantle to liquid outer core.

The boundary between the outer core and inner core is marked by a sudden increase in P-wave velocity after 5000 km, indicating the transition from a liquid to a solid state.

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Question 38

What technique is used to create images from the seismic properties of the mantle?

Answer 38

Seismic tomography is the technique used to create images from the seismic properties of the mantle, using data from many seismometers and earthquakes.

Seismic tomography maps out slabs of lithosphere by identifying regions with higher seismic velocities, indicating cooler and more rigid slabs that allow seismic waves to travel faster.

Higher-than-average seismic velocities, indicated in dark blue in tomograms, suggest the presence of cool and rigid slabs of lithosphere.

Seismic tomography helps in understanding plate tectonic structures by mapping out slabs of lithosphere entering or disappearing within the mantle, providing insights into their depth and extent.

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Question 39

How does Earth’s temperature change with depth?

Answer 39

Earth’s temperature increases with depth, but not at a uniform rate.

The temperature drops dramatically through the mantle, increases more quickly at the base of the mantle, and then increases slowly through the core.

The temperature is approximately 1000°C at the base of the crust, around 3500°C at the base of the mantle, and approximately 6,000°C at Earth’s center.

Question 40

Why are mantle rocks almost entirely solid despite high temperatures?

Answer 40

High pressures within the Earth’s interior keep mantle rocks from melting, despite the high temperatures.

The geothermal gradient is to the left of the red line, except in the asthenosphere, where small amounts of melt are present.

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Question 41

What is the evidence of convection in the Earth’s mantle?

Answer 41

The lower temperature gradient in the main part of the mantle compared to the lithosphere is interpreted as evidence of convection in the mantle.

During mantle convection, heat is transferred through the mantle by physically moving hot rocks.
Mantle convection is initiated by heat transfer from the core to the base of the lower mantle.
-Similar to a pot of soup on a hot stove, the material near the heat source (bottom of the pot) becomes hot, expands, becomes less dense, and rises due to buoyancy. Cooler material flows in from the sides, creating a convection cycle.

Question 42

How does mantle convection contribute to plate tectonics?

Answer 42

Mantle convection is an essential feature of plate tectonics because it transfers heat to the surface of the mantle more rapidly than conduction. This higher rate of heat transfer is necessary to keep the asthenosphere weak, facilitating plate movement.

Convection involves the physical movement of hot material, while conduction is heat transfer by collisions between molecules. Convection is much faster than conduction.

If mantle convection stops, plate tectonics will cease. This has already occurred on smaller planets like Mercury and Mars, as well as on Earth’s moon.

Question 43

What is the concept of whole-mantle convection in Earth?

Answer 43

Whole-mantle convection suggests that hot rock from the base of the mantle moves all the way to the top of the mantle before cooling and sinking back down.

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Question 44

What is double-layered convection, and why do some geologists propose this model?

Answer 44

Double-layered convection is a model where the upper and lower mantle convect separately. Some geologists propose this model based on observations of slabs of lithosphere sinking into the mantle and chemical differences in magma.

Question 45

Third perspective on mantle convection

Answer 45

Some scientists argue that convection may occur differently in various locations—some with whole-mantle convection and some without

Question 46

What are some sources of heat within Earth’s interior?

Answer 46

Sources of heat within Earth’s interior include the heat contained in objects that accreted to form Earth, heat produced by collisions during Earth’s formation, compression and heating due to increased pressure as Earth grew larger, and heat generated by friction during the redistribution of melted material forming the core and mantle.

As Earth grew larger, the increased pressure on its interior caused it to compress and heat up.

Radioactivity is a major source of heat in Earth’s interior. The decay of radioactive isotopes, such as uranium-235 (235U), uranium-238 (238U), potassium-40 (40K), and thorium-232 (232Th) in Earth’s mantle, releases energy.
The heat contributed by radioactivity is now roughly a quarter of what it was when Earth formed.

Question 47

What causes Earth’s liquid iron core to convect?

Answer 47

Earth’s liquid iron core convects because it is heated from beneath by the inner core.

Question 48

How is Earth’s magnetic field generated within the outer core?

Answer 48

Earth’s magnetic field is generated within the outer core by the convective movement of liquid iron, which, being a metal, conducts electricity and generates a magnetic field.

Question 49

How is Earth’s magnetic field defined, and what do the north and south poles represent?

Answer 49

Earth’s magnetic field is defined by north and south poles, representing lines of magnetic force flowing into Earth in the northern hemisphere and out of Earth in the southern hemisphere.

The tilt or inclination of magnetic field lines is represented by the tilt of compass needles. At the north and south poles, the magnetic force is vertical, while it is horizontal at the equator.

Although convection in the outer core is continuous, Earth’s magnetic field is not stable. Periodically, the magnetic field decays and then becomes re-established. Changes are caused by the convective movement of liquid iron in the outer core.

Question 50

What is required for Earth’s magnetic field to flip during a reversal?

Answer 50

Both the outer core and the solid inner core are required for Earth’s magnetic field to flip during a reversal. Reversals in the outer core do not always coincide with reversals in the inner core.

Over the past 250 million years, there have been hundreds of magnetic field reversals. The shortest identified reversals lasted only a few thousand years, and the longest one was more than 30 million years during the Cretaceous Period.

Question 51

How does the ability of the mantle to convect impact tectonic plates?

Answer 51

The mantle’s ability to convect allows tectonic plates to float on it, similar to a raft floating in water.

Question 52

What is the role of isostasy in the relationship between tectonic plates and the mantle?

Answer 52

Isostasy is the state in which the force of gravity pulling a tectonic plate toward Earth’s center is balanced by the resistance of the mantle to letting the plate sink.

isostasy is demonstrated with rafts floating in a swimming pool full of peanut butter. The elevation of the floating rafts depends on the balance between gravity pulling them down and the buoyancy of the peanut butter resisting their downward motion.

Peanut butter is used in the example because its viscosity (resistance to flowing) more closely represents the relationship between tectonic plates and the mantle. While peanut butter has a similar density to water, its higher viscosity causes a slower settling when weight is added to the raft.

The relationship is similar to the rafts in peanut butter; when weight is added to the crust through processes like mountain building, the crust sinks into the mantle. When the weight is reduced through erosion, the crust rebounds, and mantle rock flows back.

Question 53

Why does the crust float on the mantle, and how does isostasy come into play during mountain building and erosion?

Answer 53

The crust floats on the mantle because it is less dense. Isostasy comes into play during mountain building as the added weight causes the crust to sink, and during erosion, the reduced weight allows the crust to rebound.

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Question 54

How does glaciation affect the crust and mantle in terms of isostasy?

Answer 54

Thick accumulations of glacial ice add weight to the crust, causing subsidence and pushing the mantle aside. When the ice melts, the crust and mantle rebound.

Question 55

What happens to the crust and mantle during glacial rebound?

Answer 55

During glacial rebound, the crust and mantle slowly rebound after the melting of glacial ice.

Full rebound after glacial melting typically takes more than 10,000 years.

Large parts of Canada, northern Europe (Fenno-Scandian Ice Sheet), and eastern Antarctica are still rebounding due to the loss of glacial ice.

The highest rate of uplift is in a large area west of Hudson Bay

Glacial rebound in one location leads to subsidence in surrounding areas. Regions that were lifted during mantle rock displacement now subside as the mantle rock flows back

Question 56

How can Earth’s mantle be both solid and exhibit flow-like behavior?

Answer 56

The mantle behaves as a non-Newtonian fluid, responding differently to stresses depending on how quickly the stress is applied (depending on speed of application)

Silly Putty is a good example—it bounces when compressed rapidly, breaks if pulled sharply, and deforms like a liquid when stress is applied slowly.

Similar to Silly Putty, the mantle will flow when placed under the slow but steady stress of a growing (or melting) ice sheet.