2.1 - The Earth Inside

Question 1

The average density of the rocks we live on (the continental crust) is approximately…compared to earth…

+ WHAT CAN WE INFER FROM THIS?

Answer 1

• 2.7 g/cm3, compared to 5.5 g/cm3.
• From this, we can infer that much denser materials must exist within the Earth’s interior
• Based on our understanding of the building blocks available to the inner planets, we can also assume that beneath the crust lie denser silicate rocks and a dense metallic core.

Question 2

The Kola Superdeep Borehole

2.1.1.1 Boreholes

Answer 2

• A project to drill through the crust and sample Earth’s mantle - reached a depth of 12.262 km (approximately one-third through the crust), where the rocks reached temperatures around 180°C.
• However, the drilling operations were discontinued due to technical and financial challenges
• The borehole’s depth represents merely 0.19% of the distance to the Earth’s center.

Question 3

There are certain circumstances where rocks and minerals from the mantle can be studied at the surface: DIAMONDS

2.1.1.2 Rocks from the Interior at the Surface

Answer 3

• If you possess a diamond, you most likely have a gem that originated deep within the Earth’s mantle.
• Diamonds are rapidly transported from the mantle to the surface by a rare type of volcanic activity called a kimberlite eruption.

Question 4

There are certain circumstances where rocks and minerals from the mantle can be studied at the surface: DIAMONDS

2.1.1.2 Rocks from the Interior at the Surface

Answer 4

• If you possess a diamond, you most likely have a gem that originated deep within the Earth’s mantle.
• Diamonds are rapidly transported from the mantle to the surface by a rare type of volcanic activity called a kimberlite eruption.
• Kimberlite rocks, along with some other types of volcanic rocks, may also contain complete fragments of mantle material

Question 5

We also get clues about the Earth’s mantle from oceanic crust

Subduction/Obduction

2.1.1.2 Rocks from the Interior at the Surface

Answer 5

• Sometimes, during subduction, entire sections of oceanic crust, including the uppermost portion of the mantle, are “snipped” off and thrust onto the edge of a continent, a process known as obduction.
• The rocks left behind in this sequence are called ophiolites and provide a complete section through the Earth’s crust and the uppermost part of the mantle.

Question 6

To obtain a more comprehensive understanding of the deep ‘picture’ beneath our feet, we need to turn to…

Answer 6

earthquakes

Question 7

Seismic waves are classified into 2 types:

2.1.1.3a) Wave Types

Answer 7

1. Body waves
2. Surface waves

Question 8

BODY waves are classified into 2 types:

2.1.1.3a) Wave Types

Answer 8

primary waves (P-waves) and secondary waves (S-waves).

Question 9

S waves

Seismic waves are classified into 2 types:

2.1.1.3a) Wave Types

Answer 9

• S-waves move by shifting the material side to side, similar to how you might shake a rope
• These waves can only travel through solids
• P-waves travel faster than S-waves.

Question 10

P waves

Seismic waves are classified into 2 types:

2.1.1.3a) Wave Types

Answer 10

• move by compressing and expanding the material they pass through, like how a slinky stretches and contracts
• Can travel through solids, liquids, and gases

Question 11

Surface waves are split into two types:

Seismic waves are classified into 2 types:

2.1.1.3a) Wave Types

Answer 11

1. Love waves
2. Rayleigh waves

Question 12

How do body waves (p/s) provide MORE valuable insights into the Earth’s deep interior

Seismic waves are classified into 2 types:

2.1.1.3a) Wave Types

Answer 12

• As body waves travel through the Earth and encounter materials with different densities, their velocity changes.
• These sudden ‘jumps’ in velocity indicate significant changes in the physical and chemical properties of the Earth at various depths.
• Higher-density materials cause seismic waves to travel faster.
• Additionally, because S-waves cannot pass through liquids, they help identify the presence of liquid layers within the Earth.

Question 13

Tectonic plates

2.1.1.3 b) Earth’s Internal Structure through Seismic Studies

Answer 13

• which consist of the crust and the uppermost part of the mantle
• are so thin compared to the Earth’s overall volume that they are barely visible in this diagram

Question 14

Mantle

2.1.1.3 b) Earth’s Internal Structure through Seismic Studies

Answer 14

• The layer directly below the Earth’s crust—is composed of solid rock that rarely melts, despite being at high temperatures.
• This is due to the extremely high pressures at those depths, which prevent melting.
• Generally, density increases with depth, but there are sudden ‘jumps’ in density likely related to changes in the mineral structure of materials at specific temperatures and pressures within the Earth.

Question 15

As seismic waves travel through the Earth’s crust, what happens to their velocity?

2.1.1.3 b) Earth’s Internal Structure through Seismic Studies

Answer 15

both P-waves and S-waves increase in velocity due to the rising pressure with depth

Question 16

A notable increase in velocity occurs at a depth of 5-10 km in oceanic crust and 20-40 km in most continental crust.
This boundary is known as…

2.1.1.3 b) Earth’s Internal Structure through Seismic Studies

Answer 16

• The Mohorovičić discontinuity (MOHO): marks the transition from the crust to the upper part of the denser mantle, called the lithospheric mantle, which is composed of peridotite rock

Question 17

The crust and the lithospheric mantle together form a solid layer known as the…

2.1.1.3 b) Earth’s Internal Structure through Seismic Studies

Answer 17

lithosphere, which comprises Earth’s tectonic plates

Question 18

Why do seismic waves through dense material arrive at seismic stations faster waves travelling through shallower material

2.1.1.3 b) Earth’s Internal Structure through Seismic Studies

Answer 18

• The MOHO is generally considered the lower limit of the Earth’s crust.
• This transition explains why seismic waves travelling through the denser mantle (with a density of about 3.3 g/cm³) arrive at seismic stations before waves travelling through the shallower, less dense crust, even though the mantle paths are longer.

Question 19

Low-velocity zone

2.1.1.3 b) Earth’s Internal Structure through Seismic Studies

Answer 19

Seismic waves between 100 and 250 km in depth record a decrease in velocity. This region is known as the low-velocity zone

Question 20

What is the low-velocity zone part of?

2.1.1.3 b) Earth’s Internal Structure through Seismic Studies

Answer 20

• associated with a part of the upper mantle called the asthenosphere.
• It is believed that this decrease in velocity might indicate that around 1% of the mantle rocks at this level are partially molten, which contributes to the reduced velocity
• The melting in the asthenosphere may help ‘lubricate’ the undersurface of the tectonic plates, allowing them to move more easily over the mantle.

Question 21

Below the low-velocity zone, in the Upper Mantle, there are additional discontinuities where velocities increase

2.1.1.3 b) Earth’s Internal Structure through Seismic Studies

Answer 21

• These transitions are believed to occur at specific depths and pressures when minerals undergo a phase transition that causes the atoms to become more tightly packed.
• The 660 km discontinuity, which separates the less dense upper mantle from the more dense lower mantle, is particularly distinct in this case.
• This discontinuity likely arises from the phase transformation of the mineral spinel to perovskite, which remains stable at the higher pressures and temperatures found throughout the lower mantle.

Question 22

the most striking change occurs at the core-mantle boundary

2.1.1.3 b) Earth’s Internal Structure through Seismic Studies

Answer 22

• The Gutenberg discontinuity
• At this point, P-wave velocity slows considerably, and S-waves are absent.
• The only possible explanation for this is the presence of a completely liquid layer, slowing P-waves and not permitting S-waves to be transmitted.
• The Gutenberg discontinuity marks the boundary of the liquid metal outer core and the overlying mantle.

Question 23

The Lehman discontinuity

Answer 23

• Marks the boundary between the outer and inner core (Figure 7).
• Although S-waves cannot pass through the liquid outer core, some P-waves are converted into S-waves at the inner-outer core boundary and vice versa when they pass from the inner core.
• The P-waves that emerge on the other side of the core can be identified by seismologists as having originated from S-waves that were converted into P-waves.
• The propagation of S-waves through the inner core indicates that it must be solid.

Question 24

Core-Mantle Boundary

Answer 24

• As S-waves are obstructed at the Core-Mantle Boundary, an S-wave “shadow” is cast on the opposite side of the Earth from the seismic source.
• The angular distance from the seismic source to the shadow zone measures 103° latitude on both sides, resulting in a total angular distance of 154°.
• Utilizing this data, we can deduce the depth of the Core-Mantle Boundary (Figure 8).

Question 25

Initially, it was believed that Earth’s core was entirely liquid. However

Answer 25

* Inge Lehmann identified anomalous P-waves arriving within the P-wave shadow zone that had been reflected off a solid surface within the core, known as the Lehman discontinuity. * These observations led to the identification of Earth’s solid inner core, which has since been confirmed by the P-S-P wave transition mentioned earlier.

Question 26

Order of discontinuities/zones from closest -> furthest into earth's core:

Answer 26

1. Mohorovičić discontinuity 2. Low-velocity zone 3. Gutenberg discontinuity 4. the Lehman discontinuity 5. Core-Mantle Boundary

Question 27

In addition to subdividing the interior of the Earth using changing seismic velocities, we can also define zones within the Earth based on other properties.

Answer 27

* For instance, the term "mantle" refers to a particular layer of the Earth characterized by its **chemical composition**, consisting of specific silicate minerals * We can also classify layers based on their **physical properties**, specifically their response when stress is applied to them.

Question 28

The region we refer to as the "mantle" can be divided into three parts:

Answer 28

1. the solid uppermost lithospheric mantle (located beneath the crust) 2. the plastic asthenosphere 3. the comparatively solid mesosphere

Question 29

Earth’s heat comes from (4) ## Footnote 2.1.2: Earth's Internal Temperature

Answer 29

**1. Residual heat from *accretion*:** When the Earth formed, kinetic energy from the materials impacting and accreting to form the planet was converted into heat. **2. Heat generated by rock compression:** As rocks are buried deeper within the Earth’s interior, they experience increased pressure, which produces heat. **3. Frictional heat from early differentiation:** During the early stages of Earth’s formation, the differentiation process caused molten materials to separate and form distinct layers. This process involved intense heat due to friction between the materials as they moved. **4. Radioactive decay of unstable isotopes**: This is the most significant source of heat in the Earth’s mantle. The decay of isotopes such as uranium-235 (²³⁵U), uranium-238 (²³⁸U), potassium-40 (⁴⁰K), and thorium-232 (²³²Th) releases energy in the form of heat. The heat generated by radioactive decay is about 25% of what it was early in Earth’s history and continues to decrease over time. This reduction is due to the decreasing number of radioactive isotopes and the decay of isotopes with shorter half-lives over geological time.

Question 30

Earth’s temperature increases with depth - 3 points/increases

Answer 30

* Within the crust, the geothermal gradient (the rate at which temperature increases with depth) is approximately **15-30°C** per kilometre * The rate of temperature increase slows through the mantle and then rises towards the mantle’s base, reaching about **3500°C** * In the core, the temperature increases more slowly, reaching an estimated **5200°C**

Question 31

Earth’s temperature profile is crucial for...

Answer 31

understanding where melting occurs and generates molten material

Question 32

Temperature at the Asthenosphere:

Answer 32

* In the Asthenosphere (around 100-250 km), the solid-liquid transition line is very close to the geothermal gradient, causing rocks at this level to be close to melting or partially melted. * In cases of partial melting, only some of the rock’s minerals melt, producing liquid magma. * This partial melting contributes to the low-velocity seismic zone (LVZ) observed at this depth.

Question 33

What about the interiors of other planets in our solar system?

Answer 33

Clues can be gained by studying a planet’s… * Density * The heat it radiates * Its magnetic field

Question 34

Analysis has confirmed that Mars, like Earth, has...

Answer 34

* three distinct layers: the crust, mantle, and core. * The crust may be as deep as 20 km in some areas and appears to be divided into three layers: a top layer of igneous rocks mostly shattered by meteor impacts, a middle layer of more coherent igneous material, and a lower layer with uncertain properties. = * The mantle extends 1560 km below the surface and is relatively cool compared to Earth’s. Mars’ core has a radius of 1830 km and is probably molten, though it appears less dense than Earth’s core.

Question 35

Jovian planet interiors:

Answer 35

* None of these plants have a solid surface. Below the gaseous surface, the atmospheres become increasingly dense and warm, eventually transforming into a liquid at depth. * The intense pressure experienced at depth in Jupiter and Saturn is believed to create a layer of metallic hydrogen. * These planets may have rock and nickel-iron alloy cores of various sizes.