The Plate Tectonic System

Question 1

When was the Plate Tectonic Theory accepted?

Answer 1

Later 1960s

Question 2

What processes of the Earth system does Plate Tectonic theory help describe?

Answer 2

Earthquakes, volcanism, mountain building and sea floor spreading

Question 3

What was Wegener’s continental drift hypothesis?

Answer 3

A proposed theory suggesting that the continents had once been joined together in a supercontinent (Pangea) and had since drifted apart

Question 4

How does the fit of coastlines around the Atlantic support the theory of continental drift?

Answer 4

Geological fit, they align to form one land mass when pulled together

Question 5

How does geological fit support the theory of continental drift?

Answer 5

Rock formations and mountain ranges, now separated by oceans once aligned

Question 6

How does fossil distribution support the theory of continental drift?

Answer 6

Shows that identical species once lived on continents now separated by vast oceans

Question 7

How does the distribution of late Paleozoic glaciation support the theory of continental drift?

Answer 7

Evidence of glaciation has been found in regions which are now tropical or temperate, which in the model of Pangea align to form a single southern region, near the south pole

Question 8

How does the distribution of climate belts support the theory of continental drift?

Answer 8

Past climatic conditions align with the positions of the continents when they were part of Pangea, for example evidence of tropical coal deposits, desert sandstones and glacial formation

Question 9

Why was continental drift initially rejected?

Answer 9

Wegener had no explanation for how the plates moved

Question 10

What scientific development led to the formation of plate tectonic theory? How?

Answer 10

Advances in oceanography and seafloor mapping provided new data. The discovery of mid-ocean ridges and magnetic striping on the seafloor suggested that the seafloor was spreading, providing the mechanism that Wegener lacked.

Question 11

Describe the basic bathymetric characteristics of mid-ocean ridges, deep-ocean trenches and seamount chains

Answer 11

Mid-ocean ridges are underwater mountain ranges that form at divergent plate boundaries. Bathymetrically, they are characterised by a central rift valley, elevated terrain that gradually slopes downward. They extend for thousands of kms across ocean basins.

Deep-ocean trenches are the deepest parts of the ocean, formed at convergent plate boundaries where one tectonic plate is subducting beneath another. These trenches can reach depths of 8-11 km below sea level. Bathymetrically, they are narrow and elongated, with steep sides plunging toward the seafloor. Trenches are often found adjacent to volcanic arcs and are associated with intense geological activity like earthquakes and volcanic eruptions.

Seamount chains are underwater mountain ranges or isolated volcanic peaks that rise from the ocean floor but do not reach the surface. Formed by volcanic activity, seamounts can rise 1-4 km above the seafloor and are often found in linear chains, which is created by a tectonic plate moving over a stationary hotspot. The bathymetry of seamounts includes steep, rugged slopes.

Question 12

Describe the hypothesis of sea-floor spreading. What is the evidence for it?

Answer 12

New oceanic crust is continuously formed at mid-ocean ridges and spreads outward, pushing older crust away from the ridge. As magma rises from beneath the Earth’s mantle, it cools and solidifies, creating new seafloor.
Mid-Ocean Ridge Topography, Magnetic Striping, Age of Oceanic Crust and Sediment Thickness

Question 13

What is paleomagnetism and how does it form?

Answer 13

Paleomagnetism is the study of Earth’s ancient magnetic field as recorded in rocks, when iron-rich minerals in molten rock align with Earth’s magnetic field as the rock cools and solidifies. Once the rock cools, the magnetic orientation is locked in, preserving a record of the magnetic field’s direction at the time. Over geological time, it reveals changes in Earth’s magnetic field, such as magnetic reversals, and provide evidence for processes like seafloor spreading and continental drift.

Question 14

What is a magnetic reversal?

Answer 14

A magnetic reversal is where Earth’s magnetic field reverses polarity, meaning that the magnetic north and south poles switch places. This process occurs irregularly over geological time

Question 15

What is a marine magnetic anomaly?

Answer 15

Variation in the Earth’s magnetic field detected in oceanic regions, typically associated with the patterns of magnetic reversals, recorded at mid-ocean ridges. As new seafloor forms, the orientation of magnetic minerals reflects the Earth’s magnetic field at that time, creating a symmetrical pattern of positive and negative anomalies on either side of the ridge. This is evidence for seafloor spreading.

Question 16

What is the basis for the distinction between the lithosphere and asthenosphere? What are the key differences?

Answer 16

Based on their physical properties and behavior. The lithosphere comprises the crust and uppermost mantle, asthenosphere lies beneath the lithosphere.

Lithosphere: cool, strong outershell, rigid and brittle
Asthenosphere: hot, weaker interior and flows like a ductile solid

Question 17

How does the lithosphere respond to stress?

Answer 17

Faulting

Question 18

What is the difference between the lithosphere and asthenosphere and the crust and mantle?

Answer 18

The lithosphere includes the rigid outer layer of Earth, comprising the crust and the uppermost part of the mantle. The asthenosphere, beneath the lithosphere, is a partially molten layer that can flow. In contrast, the crust is the Earth’s outermost layer, while the mantle lies beneath it, extending deeper and consisting of solid rock that can behave plastically over time.

Question 19

What are the tectonic plates made of? How many plates are there and are they all the same shape and size?

Answer 19

Tectonic plates are made up of the lithosphere, which is composed primarily of silicate minerals.

There are 7 major tectonic plates (Pacific, North American, Eurasian, African, South American, Antarctic, and Indo-Australian) along with several smaller plates.

The plates vary significantly in shape and size; the Pacific Plate is the largest, covering over 103mkm^2, while the Juan de Fuca Plate, is much smaller.

Question 20

On which tectonic plate is Britain located?

Answer 20

Eurasian

Question 21

What are the 3 types of plate boundaries? How do the plates move with respect to each other at each boundary type?

Answer 21

Convergent, divergent and transformed

Question 22

What is the relationship between earthquake activity/ volcanism and plate margins?

Answer 22

Earthquake activity and volcanism are concentrated along plate margins, with divergent and convergent boundaries experiencing more significant geological events compared to transform boundaries, which primarily produce earthquakes.

Question 23

What are the key differences in the behaviour of continental and oceanic lithosphere at plate boundaries?

Answer 23

Continental lithosphere is more buoyant and less dense, leading to different tectonic interactions, while oceanic lithosphere is denser and more actively involved in processes like subduction and seafloor spreading.

Question 24

How do continental-oceanic versus continental-continental convergent margins differ?

Answer 24

The differences between continental-oceanic and continental-continental convergent margins involve the subduction of the oceanic plate, leading to volcanic arcs and trenches in c-o, whilst c-c results in mountain building and intense crustal deformation without subduction.

Question 25

At what rates do the plates move? How do the rates of spreading compare at the 3 different types of boundaries? If new material is constantly being added to the Earth, why is it not expanding?

Answer 25

A few cm per year. The fastest spreading rates occur at divergent boundaries (2 to 20 cm/year), with convergent boundaries showing slow movement toward one another, and transform boundaries exhibiting moderate lateral sliding rates (2 to 6 cm/year). Over time, this new crust is pushed laterally, causing the oceanic plates to move. Eventually, the older crust is subducted back into the mantle at deep-ocean trenches, completing a cycle of crustal recycling.

Question 26

Where are the current highest and lowest rates of spreading and convergence?

Answer 26

Eastern Pacific Ocean, the Gulf of California to south of the Galapagos Islands, averaging 10- 20cm/year in some areas. Arctic Ocean, between Greenland and Siberia, around 1cm/year
Himalayan between the Indian Plate and the Eurasian Plate, approximately 5cm/year. Mediterranean Ridge, between the African Plate and the Eurasian Plate, around 1- 2cm/year

Question 27

What role does mantle convection play in plate tectonics? What two mechanisms drive plate movement?

Answer 27

Mantle convection drives plate tectonics by creating convection cells that move the lithosphere. The two primary mechanisms driving plate movement are slab pull, where sub-ducting plates pull the rest of the plate with them, and ridge push, where elevated mid-ocean ridges push plates away from the ridge due to gravity.

Question 28

What are the following and where do they occur with respect to plate boundaries: mid ocean ridges, volcanic (Island) arcs, deep ocean trenches, Wadati-Benioff zones?

Answer 28

Mid-Ocean Ridges: underwater mountain ranges formed at divergent plate boundaries
Volcanic Arcs: curved chains of volcanoes formed above sub-ducting plates at convergent boundaries
Deep Ocean Trenches: deep, narrow depressions in the ocean floor at convergent boundaries where one plate is sub-ducted beneath another
Wadati-Benioff Zones: Sloping zones of seismic activity associated with subduction, found at convergent boundaries, illustrating the depth of earthquakes as a plate descends.

Question 29

What is the composition of the oceanic crust? Where is the majority of new oceanic crust formed?

Answer 29

Oceanic crust primarily consists of: basalt, gabbro and sediments (felsic) and are formed at mid-ocean ridges

Question 30

What is the evidence for sea-floor spreading?

Answer 30

Magnetic stripes on the ocean floor, the age distribution of oceanic crust, sediment thickness patterns, geological activity, seafloor topography, and fossil distribution

Question 31

How does the age of the oceanic crust vary with distance from the spreading centres?

Answer 31

The age of the oceanic crust increases with distance from the spreading centres, such as mid-ocean ridges. New oceanic crust is formed and is youngest, typically only a few million years old. As you move away, the crust becomes older, reaching up to about 200 million years near continental margins.

Question 32

How does the rock cycle relate igneous, metamorphic and sedimentary rocks?

Answer 32

The rock cycle is the interactions that form on type of rock from the other two.

Question 33

How does plate tectonics control the rock cycle?

Answer 33

Convergent, divergent and transform boundaries all provide molten magma which can cause melting of rocks or high temp/pressure to metamorphise rock and faulting which can lead to erosion for later deposition and formation of sedimentary rock

Question 34

What is a supercontinent?

Answer 34

One continent on which a singular plate (or stagnant lid) is held

Question 35

What is the supercontinent cycle? What events (evidence) are associated with continental break-up and assembly of supercontinents?

Answer 35

Supercontinent cycle is the formation and disassembly of a supercontinent. Fossil distribution provides evidence for continental break-up and tectonic activity and matching geological features provide evidence for supercontinent assembly

Question 36

What is an orogeny? In which plate tectonic settings do the most intense orogenesis occur? Present day orogens are identified by finding regions of high, rugged peaks. How can geologists identify orogens that have been eroded away?

Answer 36

Orogeny refers to mountain formation, resulting from tectonic plate interactions e.g., collision, subduction, or continental convergence. The most intense orogenesis occurs at convergent plate boundaries.
Metamorphic Rocks: The presence of high-grade metamorphic rocks indicates former mountain-building processes.
Fossil and Sedimentary Records: Analyzing sedimentary basins can reveal past geological environments and tectonic activity.

Question 37

What is the nature of the relationship between supercontinent formation, global sea level and global climate?

Answer 37

Formation of supercontinents leads to land area increasing, leading to lower global sea levels due to the reduced volume of ocean basins. This can cause changes in ocean circulation patterns, affecting climate and weather systems.
Conversely, during supercontinent break-up, increased volcanic activity can release greenhouse gases, raising global temperatures.