Plate Tectonics
Structure of the Earth
The Earth is described in terms of three primary layers: crust, mantle, and core.
Crust is the outermost layer of the Earth and is subdivided into continental crust and oceanic crust. It extends from the surface to a depth of about .
Mantle lies beneath the crust and extends down to the outer core. It is divided conceptually into the upper mantle and the lower mantle; a key subdivision is the asthenosphere.
Core is at the center of the Earth and has two distinct layers: the outer core (which is liquid) and the inner core (which is solid).
The radius of the Earth is about
Lithosphere vs Asthenosphere:
Lithosphere = crust and the uppermost solid mantle; it behaves as a rigid shell.
Asthenosphere = a portion of the mantle that flows like molten plastic while remaining solid, enabling plate movement.
Mantle characteristics:
Mantle is a semi-solid, rocky, and very hot layer composed of ferromagnesian silicate rocks.
Core characteristics:
Outer core is liquid and lies beneath the mantle.
Inner core is solid and lies at the Earth's center.
Crust, Mantle, Core Depths and Boundaries
Crust: (outermost layer of the Earth).
Mantle thickness: ~ from the crust to the outer core boundary.
Outer core: to depth (liquid).
Inner core: to depth (solid).
The boundary between mantle and core occurs at about below the surface (varies by source).
The Lithosphere comprises the crust and the uppermost solid portion of the mantle; the Asthenosphere lies beneath it and allows movement of tectonic plates.
Plate Tectonics: Overview and History
Plate tectonics theory states that the Earth’s solid outer crust, the lithosphere, is partitioned into plates that move over the semi-fluid asthenosphere.
Plates interact at their boundaries, and oceanic and continental plates come together, move apart, or slide past one another.
Historical milestones:
Sir Francis Bacon (1620): suggested that South America and Africa fit together.
Antonio Snyder (1655): sketch showing South America and Africa together.
Benjamin Franklin (1782): proposed the crust floats on a fluid interior, implying possible movements.
Alfred Wegener (early 1900s, 1915): proposed Continental Drift, arguing that continents were once joined in a supercontinent called Pangaea.
1962: Harry Hess published the idea of seafloor spreading.
1965: John Wilson proposed that the Earth’s crust is divided into several plates that fit together like a puzzle along cracks in the lithosphere.
Core idea: The combination of continental drift and seafloor spreading supports the modern Plate Tectonics Theory.
Continental Drift: Evidence for a Moving World
Wegener proposed in the early 1900s that continents were once connected in Pangaea.
Evidence summarized as four main lines: 1) Fit of the coastlines: coastlines of continents appear to fit together like a jigsaw, though gaps/overlaps exist due to later movements. 2) Fossil evidence: identical or related fossils found on now-distant landmasses, suggesting a former connection. Examples from the slides include:
Glossopteris: fossils found across Europe, South America, Africa, Madagascar, Antarctica, and Australia; a woody plant with heavy seeds.
Mesosaurus: a freshwater reptile with limbs for swimming and land locomotion; fossils found in South Africa and South America.
Cynognathus: a land-dwelling reptile about 3 meters long; fossils found in Africa and Argentina.
Lystrosaurus: herbivorous land-dwelling reptile; fossils found in Africa, India, Antarctica.
3) Rocks and mountain ranges: identical rock formations and ages found along coasts that now separate continents; mountain ranges align across continents.
4) Paleoclimatic evidence: evidence of past climates that don’t match current positions, including glacial striations and coal deposits; e.g., glacial marks found on continents now near the equator, and coal deposits indicating tropical flora in polar regions.
Paleoclimatic details:
Glacial striations indicate past glaciation on continents now far apart.
Coal deposits from tropical flora (e.g., Glossopteris) found in Antarctica, suggesting Antarctica was once closer to the equator.
Timeline and Reconstructions: From Pangaea to Today
225 million years ago (Ma): A single supercontinent called Pangaea existed.
150 Ma: Breakup into two large landmasses—Laurasia (northern) and Gondwana (southern).
100 Ma: Continents drifted toward their present positions; Australia and Antarctica separated; equatorial and hemispheric arrangements began to resemble today.
These timelines are used to illustrate the progressive breakup and continental drift that preceded today’s arrangement.
Paleontological and Geological Evidence Details
Fossil evidence (detailed):
Glossopteris: found in Europe, South America, Africa, Madagascar, Antarctica, Australia.
Mesosaurus: freshwater reptile; fossils limited to South Africa and South America.
Cynognathus: found in Africa and Argentina.
Lystrosaurus: found in Africa, India, Antarctica.
Rock and mountain evidence:
Identical rock formations along western Africa and eastern South America, among others.
Mountain belts that align when continents are rearranged (e.g., edges of Africa and South America pairing).
Paleoclimatic evidence:
Glacial striations indicate historic glaciers on continents now near the equator.
Coal deposits in Antarctica reflect a warmer, swampy climate at the time these rocks formed.
Seafloor Spreading and Ocean Basins
Harry Hess (1962) proposed Seafloor Spreading, based on sonar observations of the ocean floor and the publication The History of Ocean Basins.
Key discoveries:
Old oceanic crust lies farther from the mid-ocean ridge; newly formed crust forms at the ridge and moves outward.
The mid-ocean ridge is the locus of new crust formation; oceanic crust ages with distance from the ridge.
Evidence and concepts:
The spreading of the seafloor drives plates apart at divergent boundaries.
The process is accompanied by upwelling magma at ridges and downward sinking of older crust at trenches (subduction zones).
Ridge push analogy:
Ridge push (gravitational sliding) is the force where the elevated mid-ocean ridge mass pushes the lithosphere away from the ridge, akin to a wedge of honey with a sloping surface.
Slab pull concept:
Denser, older oceanic lithosphere sinks into the mantle at subduction zones, pulling the rest of the plate along.
Mantle convection:
The initial mechanism proposed for plate movement; proposed by Arthur Holmes in 1929.
Heat from radioactive decay in the Earth's core heats the mantle, driving convection currents that move plates.
Quick-check concepts from the slides:
Convection depends on changes in Temperature and Density.
The source of heat driving convection is Radioactive decay.
Convection occurs in the upper mantle or the Asthenosphere.
Plate Tectonic Theory and Mechanisms of Motion
Plate tectonics combines continental drift and seafloor spreading into a single framework:
Earth’s lithosphere consists of tectonic plates that ride atop the semi-fluid asthenosphere.
Plates interact at their boundaries: diverge, converge, and transform.
Mechanisms driving plate motion:
Mantle convection (driven by radioactive heat): causes upwelling at ridges and downwelling at subduction zones.
Ridge Push (Gravitational Sliding): elevated ridges push plates apart.
Slab Pull: sinking, dense slabs pull the rest of the plate along.
Key figures in plate tectonics:
Alfred Wegener: Continental Drift (early 20th century).
Harry Hess: Seafloor Spreading (1962).
John Wilson: Plate tectonics (1965) – proposed that the crust is divided into plates that move.
World Plate map concepts:
There are 7 major tectonic plates: Pacific, North American, Eurasian, African, Antarctic, Australian, South American.
Numerous minor plates exist, such as Nazca, Cocos, Juan de Fuca, Caribbean, Philippine, Indian, and others.
Hotspots and mantle plumes:
Hotspots create volcanic tracks as the plate moves over fixed mantle plumes (e.g., the Hawaii-Emperor seamount chain).
Hawaii hotspot chain shows progressive aging of volcanic islands as the Pacific Plate moves over the hotspot.
Plate Boundaries: Types, Features, and Examples
Three main types of plate boundaries:
1) Divergent boundaries – where plates move away from each other; associated with seafloor spreading and mid-ocean ridges; features include a ridge and sometimes a rift valley.
2) Convergent boundaries – where plates move toward each other; major processes include subduction and mountain-building; features include trenches, volcanic arcs, and mountain belts.
3) Transform boundaries – where plates slide horizontally past one another; features include transform faults and significant earthquakes; typically little or no volcanic activity.Major plates and boundary examples:
Major plates: Pacific, North American, Eurasian, African, Antarctic, Australian, South American.
Others: Nazca, Cocos, Juan de Fuca, Caribbean, Philippine, Indian, etc.
Divergent boundaries (Divergence and seafloor spreading):
Often located along mid-ocean ridges (e.g., the Mid-Atlantic Ridge).
Rift valleys may form on continents prior to seafloor spreading creating an ocean basin.
Convergent boundaries: three subtypes with typical features and examples:
Oceanic–Oceanic convergence: trenches form; island arcs can appear (e.g., western Pacific Islands, Mariana Trench).
Oceanic–Continental convergence: trenches and volcanic arcs form on the continental margin (e.g., Andes).
Continental–Continental convergence: collision forms large mountain belts (e.g., the Himalayas, Swiss Alps).
Transform boundaries:
Characterized by transform faults connecting segments of divergent boundaries (mid-ocean ridges) and/or connecting different plate boundaries.
San Andreas Fault (California) is a classic example; earthquakes are common; largely no volcanism.
Regional illustrations and examples:
The Philippines region illustrates oceanic–continental convergence with numerous active volcanoes (e.g., Mayon, Taal, Bulusan, etc.). PHIVOLCS monitors these volcanoes.
The Pacific Northwest and Japan reflect subduction-related volcanism along continental margins.
The Mid-Atlantic Ridge demonstrates seafloor spreading at a divergent boundary.
The Himalayan region exemplifies continental–continental convergence.
Major Plates and Regional Tectonics
The seven major tectonic plates:
Pacific plate
North American plate
Eurasian plate
African plate
Antarctic plate
Australian plate
South American plate
Notable tectonic features and plates on the map:
Plate boundaries include the Pacific–Nazca boundary, the boundaries around the Caribbean, and the Indian plate interacting with Eurasian and African plates.
Regional roles:
East African Rift: an example of continental rifting showing uplift and eventual formation of a new ocean basin; melting and upwelling of mantle material create rift valleys and can lead to ocean formation over time.
The Hawaiian–Emperor seamount chain: a classic hotspot track demonstrating plate movement over a stationary mantle plume; island ages decrease away from the hotspot track.
Regional Case Studies and Visual References
East African Rift: shows continental rifting, uplift of heated lithosphere, and formation of a rift valley; potential progression toward a new ocean basin (read: splitting of the continent and possible future sea). This is described as a process where the crust is pulled apart and large blocks subside forming a rift valley.
The Philippines and surrounding region: a complex zone of subduction giving rise to an archipelago; numerous active volcanoes and deep-sea trenches; a vivid real-world example of Oceanic–Continental convergence and subduction dynamics as described in plate tectonics.
Hotspots and Pacific plate movement: hotspot data (Kauai, Oahu, Molokai, Maui, Hawaii) illustrate the age progression of volcanic islands along the Pacific Plate due to clockwise (or otherwise directed) plate motion over a relatively stationary mantle plume; ages range from about on Kauai to about on Hawaii, indicating ongoing plate motion over the hotspot.
Boundary cross-sections and plate maps:
Types of boundaries are illustrated with diagrams showing plates, lithosphere, and asthenosphere, as well as ridges, trenches, and transform faults.
The global plate map lists major and minor plates and their approximate relative positions and interactions.
Quick-Check and Concept Checks
Key questions and answers (from the slides):
What are the two key factors driving convection? Temperature and Density.
What is the source of heat driving convection? Radioactive decay.
In which layer does mantle convection occur? The Upper Mantle or the Asthenosphere.
What is the mechanism of Ridge Push? It occurs because the mid-ocean ridge is elevated, and the mass of the ridge pushes the plates away, similar to a wedge of honey with a sloping surface.
What is the mechanism of Slab Pull? The sinking, dense oceanic plate pulls the rest of the plate downward into the mantle due to its weight.
Relationship to Real-World Geology and Implications
Plate tectonics explains a wide range of geological phenomena including earthquakes, volcanic activity, mountain building, and the distribution of fossils and rocks across continents.
The theory integrates evidence from paleontology, geology, geophysics, and oceanography to explain how continents drift and how ocean basins and mountain belts form.
Practical implications include understanding earthquake and volcanic hazards, resource distribution, and insights into Earth’s geologic history.
Summary of Key Concepts (Recap)
Earth’s internal structure consists of crust, mantle, and core, with the lithosphere as a rigid outer shell and the asthenosphere as a partially molten layer beneath it that allows plate movement.
Continental drift proposed by Wegener is supported by fossil correlations, matched coastlines, rock/mountain continuity, and paleoclimatic evidence; the mechanism was later explained by seafloor spreading and mantle convection.
Seafloor spreading at mid-ocean ridges creates new oceanic crust and drives plate motion; older crust moves away from ridges and is recycled at trenches via subduction (slab pull).
The Plate Tectonics Theory integrates the movement of lithospheric plates over the asthenosphere, driven by convection, ridge push, and slab pull, and involves divergent, convergent, and transform boundaries.
Major plates include Pacific, North American, Eurasian, African, Antarctic, Australian, and South American; numerous minor plates and hotspots further shape Earth’s dynamic surface.
Regional examples such as the East African Rift and the Philippine subduction zone illustrate active plate boundary processes and real-world geologic activity.
Chronology of major ideas: Bacon/Franklin (early ideas of fit and floating crust), Wegener (Continental Drift), Hess (Seafloor Spreading), Wilson (Plate Tectonics).