2.2 Deep Forces Flashcards
1
Q
Convection
The Mantle
A
- Done through convection cells
- Heat rises from bottom-up, then cooling at the top falls back to the bottom
2
Q
Convection in the mantle
The Mantle
A
- The high temperatures and pressures experienced at great depths do not allow the rocks to melt but will enable them to move like a super viscous fluid (for a while)
3
Q
Mantle convection - what can the mantle viscosity timeline be observed in?
The Mantle
A
- Glacial rebound
- When the glacier melts, the mantle flows back slowly, uplifting the crust (panel b) until the crust is once again flat laying (panel c) - OVER YEARS
4
Q
Will convection ever stop?
The Mantle
A
- It is thought that convection will stop once the core has cooled to a point where there is not enough heat to overcome the rock’s strength in the mantle.
- This, however, will probably not occur for billions of years.
5
Q
Whole-Mantle Convection Model
Convection Models
A
- Suggests that the entire mantle undergoes convection.
- Implies that heat is transferred from the core-mantle boundary to the Earth’s surface through convection currents extending throughout the entire mantle (like our pan of soup) analogy).
6
Q
Layered Convection Model
Convection Models
A
- suggests that the upper mantle and lower mantle undergo convection independently of each other
WHY?
* differences in viscosity and mineral composition between the upper and lower mantle make it unlikely for the entire mantle to experience uniform convection.
7
Q
To understand whether or not convection will occur, we use…
Convection Models
A
- The Rayleigh Number (Ra): considers the difference between buoyancy and viscosity within a fluid
- Convection will occur when the Rayleigh Number (Ra) exceeds 2000, convection will occur.
8
Q
What happens when the Ra value continues to increase?
Convection Models
A
- Convection becomes more active, turbulent, and chaotic.
- Calculations show that, despite its very high viscosity, the Ra for the mantle is around 1 million or more, far exceeding the 2000 threshold.
- This high Ra suggests that mantle convection is quite complex.
- Instead of having a simple, regularly spaced pattern, the mantle may contain convection cells that are irregularly distributed.
- Additionally, it may feature other convective structures known as mantle plumes
9
Q
Mantle Plumes - how were they used to explain the existence of volcanism far from plate boundaries, such as the Hawaiian Islands?
Convection Models
A
- The hypothesis suggests mantle plumes are generated in the lower mantle close to a zone called the D” layer, just above the liquid iron-rich outer core.
- The heat from the core mobilizes rock in the lower mantle, which rises as a plume.
- The plume consists of a long, thin conduit of material and a bulbous head, giving it a mushroom-like appearance.
10
Q
When the mantle plume reaches the lithosphere, what does the pressure/heat do + produce?
A
- When the plume reaches the base of the lithosphere, it flattens and expands
- This drop in pressure, along with the heat from the plume, allows for partial melting (a process called decompression melting) of the mantle rocks and the formation of basaltic magma (at the lithosphere level)
11
Q
What happens after the mantle plume produces magma at the lithosphere?
A
- The magma rises through the lithosphere, generating volcanoes at the surface
- Deep mantle plumes are thought to be fixed at the core-mantle boundary, resulting in the generation of volcano chains as tectonic plates move over the relatively stationary head of the plume
12
Q
What have large plumes been associated with + their relation to mass extinction
A
- Some large plumes have been associated with extensive basaltic volcanic episodes known as flood basalts, and they may be connected to several mass extinction events that have occurred over the past half-billion years.
- Plumes may also play a role in the fragmentation of large continental masses.
13
Q
What affects convection from earth’s inner -> outer core?
A
Earth’s rotation and the Coriolis effect.
14
Q
The Coriolis Effect
The Outer Core
A
- Occurs when objects or fluids, such as air or water, move within a rotating system like the Earth; causing these moving objects to follow curved paths rather than straight lines.
15
Q
How does the coriolis effect impact the earth’s outer core?
The Outer Core
A
- As the outer core rotates along with the Earth, the Coriolis effect causes the moving molten metal to experience a deflection in its path
- WHY? The deflection occurs because different parts of the outer core move at different speeds depending on their distance from the Earth’s axis of rotation
16
Q
This deflection caused by the Coriolis effect results in…
The Outer Core
A
- Circulating patterns of molten metal within the outer core, forming columns or rolls aligned with the Earth’s rotation axis.
- These patterns of liquid metal movement are crucial for generating Earth’s magnetic field.
17
Q
How does the Dynamo theory explain the magnetosphere/impacts of the Coriolis effect
The Outer Core
A
- According to the dynamo theory, the movement of electrically charged molten metal in the outer core creates electric currents.
- These electric currents then generate the magnetic field that surrounds our planet.
18
Q
Composition of the inner core
The Inner Core
A
- A solid sphere of iron-nickel alloy (approximately 10% nickel and 2% other elements)
- Comprising about 20% of the Earth’s radius.
19
Q
Temperature of the inner core
+ how does pressure affect metals?
The Inner Core
A
- Surface temperature around 5700 K (5430°C), which is similar to the sun’s surface temperature.
- Despite these high temperatures, the immense pressures at this depth—approximately 3.6 million atmospheres or 330 GPa (gigapascals)—prevent the metal from melting.
20
Q
Growth of the inner core
The Inner Core
A
- Believed that the inner core grows at a rate of about 1 mm per year due to the gradual cooling at the boundary between the inner and outer core, which cools at roughly 100°C per billion years.
- Consequently, this process adds approximately 8,000 tonnes of iron to the inner core each second.
21
Q
Rotation of the inner core
The Inner Core
A
- Since the outer core is liquid, the solid inner core is not firmly connected to the rest of the planet.
- While it generally rotates in sync with the Earth, it can sometimes rotate faster or slower than the Earth’s rotation; may cause slight variations in the length of a day and the strength of Earth’s magnetic field
- This oscillation in spin rate is thought to occur over periods of decades.
22
Q
Role of the magnetic field
Earth’s Magnetic Field
A
The Earth’s magnetosphere acts as a shield, deflecting harmful radiation from the Sun that could otherwise strip away the atmosphere.
23
Q
Magnetosphere - visuals
Aurora
Earth’s Magnetic Field
A
- The aurora is caused by charged particles from the Sun interacting with the Earth’s magnetic field, which then directs them toward the poles.
- When these particles collide with atmospheric gases, they excite atoms and produce light.
24
Q
Although a compass needle points toward the magnetic North Pole, it is actually pointing toward the southern pole of Earth’s magnetic field - WHY?
A
Counterintuitively, magnetic field lines emerge from the magnetic northern pole in the southern hemisphere and return to the magnetic southern pole in the northern hemisphere.
25
Magnetic field inclination
* Each point on Earth’s surface, from the poles to the equator (latitude), is characterized by a **specific inclination of the magnetic field lines**
* The field lines are vertical (90°) relative to the Earth’s surface at the poles (left and right) and horizontal at the equator (Figure 11-middle).
* The magnetic field inclination at Vancouver is about 70° into the ground.
26
Magnetic field reversal:
* Earth’s magnetic field undergoes a complete reversal, where the **North and South magnetic poles swap positions**
* During this magnetic field reversal, **the field weakens, becomes disordered, and then reorients with the poles reversed**.
* This process is not instantaneous; it **can take thousands of years to complete a reversal**
27
Certain volcanic or sedimentary rocks may contain...
| + how do they relate to the magnetic field?
## Footnote
Volcanic/sediments and dating
* Certain volcanic or sedimentary rocks may contain tiny **magnetic minerals**, such as **magnetite**.
* **As molten lava solidifies or sediments settle into layers, these minerals align with the prevailing magnetic field**
28
What happens when the lava (with magentic minerals) cools below 570°C? (igneous rock)
* Magnetic minerals become ‘frozen’ in their aligned state, effectively **recording the orientation of Earth’s magnetic field at that time.**
* This ‘remnant magnetism’ preserves the magnetic field’s orientation and its inclination relative to the ground surface when the rock was formed.
* Numerous magnetic field reversals have occurred throughout Earth's history.
29
How else can reversals recorded in igneous rocks be dated?
## Footnote
+ what connections can be made from this?
* Reversals recorded in igneous rocks can also be dated by the **radioactive isotopes they contain**, and as such, **the patterns of reversals can be linked to the geological time scale**
* In this manner, we have estimated that reversals happen on average every few hundred thousand years, although the intervals between them can be highly irregular
30
What causes these magnetic field reversals?
* One hypothesis suggests that subducting slabs of Earth's oceanic lithosphere may eventually reach the core-mantle boundary, disrupting heat flow in the outer core and potentially impacting the dynamo effect that generates Earth's magnetic field
* Another interesting finding was that periods with increased frequency of magnetic reversals tended to be associated with weaker magnetic fields, perhaps implying that the core has not had enough time to build a strong field, making it more susceptible to breakdown and generating another reversal.
31
Venus - has no magnetic field: WHY?
## Footnote
Magnetic Fields on Other Planets
1. Venus has a very slow rotation, meaning **the core would not “churn” in the same manner as we predict for Earth**.
2. The **core is probably cooler than Earth's**, resulting in less heat available to help generate motion in the core.
32
Mars
## Footnote
Magnetic Fields on Other Planets
* Mars **did** have a strong global magnetic field, but the dynamo appears to have **failed about 4.1 billion years ago, probably due to the *cooling of the core***.
* The collapse of a strong magnetic field allowed charged particles from the sun to strip away Mars's atmosphere. **Today, Mars has a very patchy magnetic field**
33
The Outer Planets: Jupiter & Saturn
## Footnote
Magnetic Fields on Other Planets
* Jupiter and Saturn have VERY strong magnetic fields.
* The **gas giants' rapid rotation and metallic hydrogen interiors create a strong dynamo effect and magnetic field.**
34
What is different about Jupiter's Aurora?
* These aurora, however, are **caused by material from Jupiter’s volcanic moon Io interacting with the magnetic field** rather than an interaction of charged particles from the sun.
