Chapter 10 – Plate Tectonics Study Notes
Physical Geography: The Earth and Humanity - Chapter 10 – Plate Tectonics
10.0 Learning Objectives
Understand the importance of plate tectonics
Discuss the evolution of the Theory of Plate Tectonics
Explain the difference between geographic and geomagnetic north
Compare and contrast the three different types of plate boundaries
10.1 A Broken Earth
Concept of Earth's Crust:
The Earth's crust is composed of separate pieces called plates.
Plates are perpetually moving slowly (comparable to fingernail growth).
Movement is not easily visible; however, over millions of years, the effects are significant.
Plate Actions:
Can collide (converge), pull apart (diverge), or slide past each other (transform).
Consequences include the formation of mountains and earthquakes.
Theory of Plate Tectonics:
Encompasses the division, movement, and results of the crustal plates.
The term 'tectonic' originates from the Greek word 'tektonikos', meaning 'to construct' or 'build'.
10.2 Importance of Earthquakes
Case Study: Chile:
Table 10.1 illustrates significant earthquakes that have occurred in Chile, showing examples of notable magnitudes:
May 21, 1960: Magnitude 7.9
May 22, 1960: Magnitude 9.5 (record breaking)
February 27, 2010: Magnitude 8.8
April 1, 2014: Magnitude 8.2
September 16, 2015: Magnitude 8.3
The occurrence of these earthquakes is linked to the collision of the Nazca Plate and the South American Plate.
The implications of earthquakes will be elaborated in Chapter 11.
Summary of Plate Tectonics:
The crust is segmented into plates.
The seafloor undergoes continual movement and regeneration.
Movement is driven by convection from radioactive decay of isotopes within the Earth.
Phenomena Resulting from Plate Tectonics:
Upwelling of magma
Seafloor spreading
Continental drift
Earthquakes
Volcanoes
Orogenesis (the formation of mountains)
10.3 Origins of a Theory
Historical Context:
Early skepticism existed about continents being able to move.
Example: Fossils such as the Mesosaurus found in both South America and Southern Africa posed questions about continental mobility.
Alfred Wegener's Contribution:
Published The Origin of Continents and Oceans in 1915, introducing the concept of continental drift.
Argued that continents were once connected as a supercontinent called Pangaea, existing approximately 300-250 million years ago.
Although he claimed that continental movement was due to centrifugal forces, this was later disproven.
Evidence Presented by Wegener:
Continental fit observed in map studies (e.g., South America and Africa).
Geological strata similarities across continents (e.g., Antarctica, Australia, India, Africa, South America).
Orogenesis evidence through the formation of mountains like the Appalachians due to plate collisions.
Fossil distributions, including Mesosaurus and the plant Glossopteris, across distant continents further supported his theory.
Skepticism Around Wegener's Theory:
Lack of a definitive mechanism for continental drift led to rejection by many geologists.
10.4 The Role of War in Advancing Geological Understanding
Significant Discoveries:
During World War II, Harry Hammond Hess mapped the Atlantic Ocean floor, leading to the idea of seafloor spreading.
Seafloor spreading involves magma rising and creating new ocean floor, pushing older crust outward.
Publication of his theory in 1962 titled “History of the Ocean Basins.”
Magnetic polarity patterns in ocean-floor rock provided data supporting seafloor spreading though empirical evidence was initially limited.
10.5 Magnetic Polarity and Its Geological Importance
Magnetic vs. Geographic North:
Distinction explained: Magnetic north does not align perfectly with geographic north.
Variations and Declinations:
The term 'variation' for sailors and 'declination' for land navigators.
Polar Wandering:
Magnetic north constantly moves, known as polar wandering.
Overview of geomagnetic reversals, which have occurred at least nine times throughout Earth’s history.
Basalt Magnetism:
When basalt cools, the iron preserves the position of the magnetic north pole, revealing patterns of magnetic polarity aligned along the Mid-Atlantic Ridge.
Indicates sea-floor spreading, providing additional evidence for continental drift.
10.6 Plate Dynamics
Plate Boundaries:
Recognize three types of boundaries:
1. Spreading Boundaries (Divergent)
Example: Mid-Atlantic Ridge, where North American Plate and Eurasian Plate pull apart.
Characterized by extensional forces leading to normal faulting.
Notably visible at Iceland and the Great Rift Valley of East Africa.
2. Convergent Boundaries
Occur where two plates collide, resulting in compression forces.
Continental-continental convergence leads to mountain building (e.g., Himalayas).
The Indian subcontinent’s collision with Asia exemplifies this dynamic.
Ocean-continental convergence entails subduction, leading to magma generation and volcanic activity.
3. Transform Boundaries
Defined by plates sliding past each other (e.g., San Andreas Fault).
No volcanic activity occurs, but earthquakes are notable risks associated with these boundaries.
10.7 Hot Spots and Volcanism
Hot Spots Defined:
Unusually hot regions in the mantle creating volcanic islands like the Hawai´ian Islands, not located at tectonic boundaries.
Process describes how islands are formed and displaced over time as plates shift.
Seamounts Tracking:
Mapping the ocean floor reveals seamounts, or submerged mountains, that once existed over hot spots but now are in a different location due to plate movement.
10.8 Chapter Summary
Theory of Plate Tectonics Overview:
Driven by convection caused by the radioactive decay of isotopes within Earth, leading to magma motion.
Seafloor is continuously generated, resulting in the outward movement of older seafloor and subduction of older crust into the mantle.
Explanatory power of the theory extends to continental drift, marine geology, seismic activity, and volcanism.
The theory emerged from many decades of research, embodying the dynamic nature of Earth's lithosphere.
Based on the notes, magnetic north refers to the Earth's magnetic pole, which is constantly moving, a phenomenon known as polar wandering. True north, also called geographic north, is a fixed point on the Earth's axis. They are different because magnetic north does not perfectly align with true north, and its position is subject to continuous shifts, whereas true north remains constant. The difference between them is referred to as 'variation' for sailors and 'declination' for land navigators.