2.3 The Unifying Earth Theory Flashcards
1
Q
A lithospheric plate consists of:
Anatomy of a Plate
A
- the crust (either oceanic or continental)
- the uppermost part of the mantle, known as the lithospheric mantle
- All of this moves over the semi-plastic asthenosphere.
2
Q
Continental crust - composition
Anatomy of a Plate
A
- Described as having a FELSIC composition (FEL = feldspar, SIC = silica, as in quartz), similar to the composition of granite
3
Q
Oceanic crust
Anatomy of a Plate
A
- Described as MAFIC (MA = magnesium, FIC = ferric, relating to the high proportion of dense Ferro-Magnesian minerals and lower silica (SiO2) content compared to continental crust).
- The oceanic crust is composed of basalt lava or its coarse-grained intrusive equivalent, gabbro, and contains minerals like plagioclase (a type of feldspar), amphiboles, and pyroxenes.
- These minerals give rocks of basaltic composition a dark colour, and the oceanic crust is typically 7-10 km thick.
4
Q
The lithospheric mantle - composition
Anatomy of a Plate
A
- Composed of a rock called peridotite, which has even lower silica content but a higher MAFIC (Ferro-Magnesian) content than oceanic crust
- Consequently, mantle materials are often described as ULTRA-MAFIC
5
Q
The Earth’s Lithosphere is divided into 8 major plates:
Plates and Plate Tectonics
A
- Eurasia
- Pacific
- India
- Australia
- North America
- South America
- Africa
- Antarctic
6
Q
The Juan De Fuca plate comprises three sub-plates:
Smaller Plates
A
- Gorda
- Juan de Fuca
- Explorer
7
Q
Plates can be composed of more than one type of lithosphere - PASSIVE/ACTIVE MARGINS
A
- The line that marks the boundary between the continental and oceanic lithosphere is called a PASSIVE margin
- Different from the ACTIVE tectonic boundaries that form around the margins of plates (convergent, divergent, transform)
8
Q
The 3 Major Active Tectonic Boundary Types
A
- Divergent boundaries
- Convergent boundaries
- Transform boundaries
9
Q
Divergent Boundaries
The 3 Major Active Tectonic Boundary Types
A
- Two plates move away from each other
- As they separate, magma from the mantle rises to fill the gap, creating new crust. This process is called Seafloor Spreading.
- The most famous example of this is the Mid-Atlantic Ridge, which runs down the center of the Atlantic Ocean. These boundaries are associated with volcanic activity and the formation of new oceanic crust.
10
Q
Convergent Boundaries
The 3 Major Active Tectonic Boundary Types
A
- Two plates move towards each other and collide
- Different geological features (i.e. mountains) can form depending on the type of crust involved (continental or oceanic) and the specific circumstances of the collision.
- When two plates converge, one might be forced beneath the other in a process called subduction
- An example is the Pacific Ring of Fire, known for its intense volcanic and seismic activity.
11
Q
Transform Boundaries
The 3 Major Active Tectonic Boundary Types
A
- Here, two plates slide past each other horizontally in opposite directions
- EX: San Andreas in California
12
Q
Where does new lithosphere material form?
The 3 Major Active Tectonic Boundary Types
A
As new lithosphere is generated at divergent boundaries (seafloor spreading), continents move as the ocean crust spreads from either side of the boundary.
13
Q
If sea-floor spreading occurs, the oceanic crust should be…
AGES AT CERTAIN AREAS
A Test for Seafloor Spreading
A
- youngest at the mid-ocean ridges and progressively older as you move closer to the trenches, where it sinks back into the mantle
14
Q
Magnometer
Different ways to date the ocean crust
A
- What was revealed was a remarkable pattern of symmetrical reversed and normal stripes on either side of the spreading ridges
15
Q
The formation of symmetrical magnetic anomalies around a spreading ridge axis can only be explained by what two things?
A
- Seafloor spreading and the Earth’s changing magnetic field
16
Q
How does seafloor spreading + magnetic field reversal explain symmetrical magnetic anomalies?
A
- As this molten material solidifies and forms new crust, it preserves the normal or reversed orientation of the Earth’s magnetic field at that time
- When the Earth’s magnetic field reverses, the newly formed crust on either side of the spreading ridge records the opposite magnetic polarity.
- As this process proceeds, a pattern of alternating magnetic stripes develops, where rocks with normal polarity are juxtaposed with rocks with reversed polarity (opposite to the present magnetic field) along the spreading ridge axis
17
Q
DATING THE STRIPES:
A
By comparing the patterns of stripes on the seafloor to the magnetostratigraphic record made from volcanoes, we can date the patterns of stripes.
18
Q
Earth’s tectonic plates move at different speeds
Compare Pacific, North American, and Atlantic
A
- The Pacific plate is the fastest, moving about 10 centimetres per year in some areas.
- The North American plate moves more slowly, around 1 CM per year in its southern part
- The Atlantic Ocean is growing wider by about 2 to 5 centimetres per year, which is roughly the same rate as your fingernails grow.
19
Q
Magnetism provides another proof for the opening and closing of ocean basins and the movement of continents in the form of…
A
Magnetic inclination: the angle at which the Earth’s magnetic field intersects with the ground surface
20
Q
HOW CAN WE DETERMINE THE LOCATION OF A CONTINENT/HOW FAR ONE TRAVELS:
A
- In comparing the inclination preserved in the rocks to the current magnetic field at that location today, geologists can determine where (in terms of latitude) a continent was at a particular time in Earth’s history and how far it has travelled from that location since.
21
Q
What are the 4 proposed mechanisms?
Plate Motion Mechanisms
A
- Mantle convection traction
- Ridge push
- Slab pull
- Slab suction
22
Q
Mantle Convection Traction
The 4 Proposed Mechanisms
A
- Discussed the circulation of mantle material from the core-mantle boundary to the base of the lithosphere
- One hypothesis is that these large mantle convection cells drag on the lithospheric plates, creating traction and causing their movement
23
Q
Mantle Convection Traction - what must occur for the dragging hypothesis to be true?
The 4 Proposed Mechanisms
A
For the idea that mantle convection cells drag on lithospheric plates, to be plausible, the mantle would need to move approximately five times faster than the plates
24
Q
Mantle Convection Traction - OPPOSING FINDINGS
The 4 Proposed Mechanisms
A
- Geophysical models do not support such high convection rates (that mantle move 5x faster than the plates)
- Additionally, if mantle traction were the primary driver of plate motion, we would expect plates with larger surface areas to move the fastest, but this is not observed in practice
25
Ridge Push
## Footnote
The 4 Proposed Mechanisms
* Suggests that the **elevated mid-oceanic ridges (divergent) act as gravitational drivers for tectonic plate movement.**
* As volcanic activity creates **new crust** forms at these ridges, it **pushes the older oceanic lithosphere away, like a conveyor belt.**
* The sloping nature of the ridge creates a gravitational force that helps the plates to move away from the ridge axis.
26
Slab Pull
## Footnote
The 4 Proposed Mechanisms
* Suggests that as **a dense oceanic plate subducts into the mantle, it exerts a gravitational pull on the rest of the plate** (some plates can be attached to subducting slabs)
* **This pulling force is likely a significant driving factor**, propelling the rest of the plate toward the subduction zone.
27
Slab Pull - What happens as the plate descends?
## Footnote
The 4 Proposed Mechanisms
* As the plate descends, **the basalt is metamorphosed into a denser rock called *eclogite***, which helps drive this process
28
Slab pull as a mechanism is supported by WHAT?
## Footnote
The 4 Proposed Mechanisms
* Slab pull as a mechanism is supported by the fact that **plates attached to subducting slabs** (e.g., Pacific, Australian, and Nazca Plates) **move much faster than plates that are not** (e.g., North American, South American, Eurasian, and African Plates).
29
Slab Suction - relation to slab pull
## Footnote
The 4 Proposed Mechanism
* Related to slab pull: as the subducting slab descends and **gets older and more dense**, it starts to roll back (similar to folding back), **causing the migration of the ocean trench** (**the area where the subducting slab descends below the overriding plate**).
30
Slab Suction - what develops?
## Footnote
The 4 Proposed Mechanism
* A ”suction current” develops in the mantle wedge (the area between the descending slab and the overriding plate) - kind of a triangle shape
31
Which of the 4 plate mechanisms is strongest? What also contributes to this?
## Footnote
The 4 Proposed Mechanism
* Overall, Slab pull is likely the strongest force in plate motion.
* However, slab suction and ridge push (and possibly a very minor component of mantle traction) also contribute to the overall mechanism.
32
How do plate tectonics foster evolution?
## Footnote
Tectonics on Other Planets
As plate tectonics cycles material between the Earth’s surface and the mantle, it plays a crucial role in regulating the planet’s climate, creating diverse habitats, and facilitating the recycling of essential nutrients
33
All the other terrestrial planets in our solar system, including our Moon, are considered...
## Footnote
Terrestrial Planets and Rocky Moons
* “Stagnant-lid planets,” where the crust has cooled into a single solid surface.
* These surfaces are much older than Earth’s, suggesting that any crustal recycling, like the subduction zones seen on Earth, ceased billions of years ago.
34
What parts of the solar system show particular volcanic activity?
* The inner planets showing signs of lava flows
* Venus and Mars, in particular, have distinct features that can be identified as volcanoes
35
The only confirmed evidence of active volcanic activity beyond Earth comes from...
* Io, one of Jupiter’s moons.
* Observations have captured plumes of material rising from Io’s surface, indicating ongoing volcanic eruptions.
* This volcanic activity is likely caused by Io’s constant squeezing and stretching as it orbits Jupiter
36
Icy Moons
## Footnote
Terrestrial Planets and Rocky Moons
* Moons with **surfaces primarily composed of water ice**, often mixed with other compounds like ammonia or methane
* Beneath their frozen exteriors, many are believed to contain **subsurface oceans of liquid water**, maintained by internal heat from tidal forces and radioactive decay
* **The subsurface oceans may act as a lubricant for the icy shells, allowing surface materials to move.**
* This lubrication, combined with stresses from tidal forces, could lead to the formation of faults, fractures, and other tectonic features. In some areas, these icy “plates” appear to have collided, with one being forced below another in a process akin to subduction
37
Compelling evidence for tectonic activity on icy moons comes from...
* ...observations of their surface features.
* These features resemble those found on Earth’s tectonically active surfaces, suggesting similar forces might be at work.
38
Exoplanets - what two observable factors contibute to plate tectonics?
* **Planet material**: for tectonic activity, a planet needs to be composed of rocks that allow for plates to sink into a mantle. Water is also crucial because, on Earth, it helps reduce friction between the mantle and the crust, facilitating tectonic movement.
* **Planet size**: mantle convection can create stress on the crust, potentially leading to fractures and initiating subduction.
