Earth's Structure and Plate Tectonics
Earth's Core
Accounts for one-sixth of Earth's volume but one-third of its mass due to its high density.
## Outer Core
Liquid, based on the absence of S waves traveling through it.
Thickness: 2270 km.
Density: 9.9 g/cm^3.
Composed mostly of iron with some nickel.
## Inner Core
Solid.
Radius: 1216 km.
Density: 13 g/cm^3.
Growing as Earth cools, at the expense of the outer core.
Rotates faster and moves independently of the crust and mantle.
Earth's Layers and Plate Tectonics Fundamentals
The tectonic system is powered by Earth's internal heat.
## Asthenosphere
More plastic than both the overlying lithosphere and the underlying lower mantle.
## Lithospheric Plates
Rigid lithospheric plates split and move apart as single mechanical units.
Molten rock from the asthenosphere swells up to fill the void created by splitting plates, thus creating new lithosphere.
## Mantle Convection
Very slow convection occurs in the mantle.
Locally, convection in the deep mantle creates rising mantle plumes.
## Continental Crust
Some plates contain blocks of thick, lower-density continental crust.
This continental crust cannot sink into the denser mantle.
## Lithosphere Structure
Above the plastic asthenosphere, the relatively cool and rigid lithosphere is broken into a mosaic of moving plates.
These plates separate, collide, and slide past one another.
## Plate Margins
The most active areas on Earth.
Sites of the most intense volcanism, seismic activity, crustal deformation, and mountain building.
Continental Drift Hypothesis
Proposed in 1915 by Alfred Wegener (1880$-$1930), a German meteorologist.
Considered "an idea before its time."
Hypothesized a supercontinent called Pangea (derived from Greek
Παγγαία, meaning 'all earth') that began breaking apart about 200 million years ago.## Evidence for Continental Drift
Evidence 1: Fit of continents.
Evidence 2: Fossil evidence suggesting common past landmasses.
Evidence 3: Similar types of rocks, rock structures, and ages of rocks found across different continents.
Evidence 4: Paleoclimate evidence (e.g., ancient glacial deposits in present-day tropical areas).
## Objections to Continental Drift Hypothesis
Wegener was a meteorologist, not a geologist.
No convincing driving mechanism was initially proposed.
The
fixist theory(continents are stationary) was considered satisfactory by most geologists.Strong opposition from the majority of the geologic community.
Few scientists considered Wegener's ideas plausible at the time.
The drift hypothesis was correct in principle but had incorrect details.
## Possible explanations for observed phenomena (excluding Continental Drift)
Rafting
Landbridges
Island Stepping Stones
Paleomagnetism and Seafloor Spreading
## Earth's Magnetic Field
Consists of lines of force, similar to those produced by a giant bar magnet at Earth's center.
Strength of the magnetic field varies, often weakening just before reversals.
Earth has two dominant magnetic poles and several very weak poles (mathematically about 8 in number).
## Magnetic Field Generation (Dynamo Theory)
Generated by circulating currents of charged particles in the electrically-conducting molten material of the outer core.
It is impossible to monitor these fluid motions directly due to the depth (4,000 miles to Earth's center).
The Sun, other planets, and the Milky Way galaxy are also magnetized and probably undergo reversals.
Dynamo theory explanation: Interactions between the twisting flow of molten material in the outer core generate electrical currents. These currents create new magnetic energy, sustaining the magnetic field like a perpetual machine.
## Rock Magnetism
Earth's magnetic field periodically reverses: the north magnetic pole effectively becomes the south magnetic pole, and vice versa.
Magnetic minerals (iron-bearing) in rocks align with Earth's magnetic poles as they form.
This preserved magnetic orientation provides a record of where the rocks formed (e.g., near poles or equator).
## Apparent Polar-Wandering Paths
The more westerly path determined from North American data is thought to have been caused by the westward drift of North America by about 24 degrees from Eurasia.
The positions of these wandering paths align when the landmasses are reassembled in their pre-drift locations.
## Ocean Floor as a Magnetic Recorder
Extensive mapping of the ocean floor in the 1950s and 1960s was crucial.
Magnetic intensities are recorded when a magnetometer is towed across segments of the oceanic floor.
Symmetrical
stripesof low- and high-intensity magnetism parallel the axis of mid-ocean ridges (e.g., Juan de Fuca Ridge).High-intensity stripes occur where normally magnetized oceanic rocks enhance the existing magnetic field.
Conversely, low-intensity stripes are regions where the crust is polarized in the reverse direction, weakening the existing magnetic field.
## Seafloor Spreading Hypothesis (Hess, 1962)
New crust forms near ridges in the middle of oceans.
Old crust is consumed at the edges of the ocean basins.
Magnetic stripes in the ocean crust near ridges are directly tied to Hess's concept of seafloor spreading.
## Evidence for Seafloor Spreading
Magnetic Reversals: When new basaltic rocks form at mid-ocean ridges, they magnetize according to Earth's existing magnetic field. Oceanic crust therefore provides a permanent record of each reversal of our planet's magnetic field over the past 200 million years.
Sediment Accumulation: Data from deep-sea drilling shows that the ocean floor is indeed youngest at the ridge axis and sediments become progressively older away from the ridges.
Age of Ocean Floor: The youngest rocks are found at ridges, while the oldest rocks on the seafloor are approximately 180 million years old.
Global Heat Flow: Heat flow is highest at ridges and decreases systematically away from the ridges, measured in mW/m^2.
Plate Tectonics: Synthesis
The combination of Continental Drift + Seafloor Spreading + Paleomagnetism = Plate Tectonics.
## Pattern Recognition
Earth's lithosphere is broken up into plates.
Plates move very slowly (a few inches/cm per year).
Plates continually change in shape and size.
## Plate Tectonic 'Drivers'
Convection in the mantle: The primary driving mechanism.
Analogy: Convection in a cooking pot – as a stove warms water at the bottom, heated water expands, becomes less dense (more buoyant), and rises. Simultaneously, cooler, denser water near the top sinks.
Models of mantle convection illustrate this process.
## Forces Acting on Lithospheric Plates
The primary forces are related to mantle convection and gravity acting on the plates themselves (e.g., ridge push, slab pull).
Plate Configurations: The Breakup of Pangea
The First Major Event (150 Million Years Ago): Separation of North America and Africa, marking the opening of the North Atlantic Ocean.
By 90 Million Years Ago: The South Atlantic had opened. Continued breakup in the Southern Hemisphere led to the separation of Africa, India, and Antarctica.
About 50 Million Years Ago: Southeast Asia had docked with Eurasia, while India continued its northward journey.
By 20 Million Years Ago: India had begun its ongoing collision with Eurasia, creating the Himalayas and the Tibetan Highlands.
Future Plate Configurations (Idealized): Reconstructions project the world's appearance 50 million years from now and 250 million years from now, based on the assumption that current processes continue.
## Plate Motions
Measurements are in cm/yr (inches per year).
Red arrows show plate motion based on GPS data; longer arrows indicate faster spreading rates.
Small black arrows and labels show seafloor spreading velocities based mainly on paleomagnetic data.
Plate Boundaries
Interactions between plates occur along boundaries.
## Types of Boundaries
Divergent Boundaries (Constructive Margins): Plates move apart.
Convergent Boundaries (Destructive Margins): Plates move towards each other.
Transform Boundaries (Conservative Margins): Plates slide past one another.
Divergent Boundaries
## Mid-Ocean Ridge System
Extends as a major structural feature around the entire globe.
Marks divergent plate boundaries.
The interconnected ridge system exceeds 70,000 km (43,000 mi) in length.
New ocean floor is created at ridges, composed of mafic igneous rocks (e.g., basalt and gabbro).
## Seafloor Spreading Rates
Slow: <5 cm/yr ($<2$ inches/yr).
Intermediate: 5-9 cm/yr ($2-3.5$ inches/yr).
Fast: >9 cm/yr ($>3.5$ inches/yr).
## Rift Valley
Example: Thingvellir National Park, Iceland, is located on the western margin of a rift valley roughly 30 kilometers (20 miles) wide.
This rift valley is connected to a similar feature that extends along the crest of the Mid-Atlantic Ridge.
The cliff approximates the eastern edge of the North American plate.
## Continental Rifting
Process that splits landmasses into segments.
Involves the initial formation of a rift valley.
Can lead to the formation of a shallow sea.
Ultimately results in the formation of a mid-ocean ridge and new ocean basins.
Passive Margins
Location: Found along most coastal areas of the Atlantic Ocean.
Characteristics: Not associated with plate boundaries; therefore, they experience little volcanism and few earthquakes.
## Components
Continental Shelf: A flooded extension of the continent with an average slope of one-tenth of 1 degree.
Continental Slope: The boundary between the continental and oceanic crust, with an average slope of about 5 degrees.
Continental Rise: The edge of the oceanic crust, an accumulation of sediment.
Convergent Boundaries
Marked either by deep trenches (where oceanic lithosphere descends) or by high folded mountain belts.
Earthquakes and magma generation are common.
## Ocean-Continent Convergence
Oceanic lithosphere returns into the asthenosphere at
destructive plate margins(also known assubduction zones).An ocean trench is found where the plate descends.
Melting of mantle rock generates magma.
Magma rises and forms a
volcanic mountain chainorcontinental volcanic arc(e.g., Andes, Cascades).Zones of both contact and regional metamorphism occur.
Example: Mount Hood, Oregon, is one of more than a dozen large composite volcanoes in the Cascade Range, a continental volcanic arc formed by oceanic-continental convergence.
## Ocean-Ocean Convergence
Often leads to volcanoes on the ocean floor.
Some volcanoes emerge as
island arcs(e.g., Japan, Aleutian Islands).Zones of both contact and regional metamorphism occur.
## Continent-Continent Convergence
Two continents collide, forming a
suture zone.Collision produces high mountains (e.g., Himalayas).
Note that no volcanoes form because the asthenosphere can no longer be hydrated to generate magma in the subduction process.
Zones of both contact and regional metamorphism occur.
Example: Continental collision and the formation of the Himalayas.
## Convergence & Collision: Accretion of Terranes
Collision and accretion of small crustal fragments (
terranes) to a continental margin.West Coast Terranes: Terranes added to western North America during the past 200 million years.
Paleomagnetic studies and fossil evidence indicate some of these terranes originated thousands of kilometers to the south of their present locations.
Associated with East Coast Terranes & Mountain Building (Orogeny).
Gravity & Isostasy
Gravity: Plays a fundamental role in Earth's dynamics and is responsible for adjustments of the crust's elevation.
Isostasy: The universal tendency of segments of Earth's crust to establish a condition of gravitational balance.
Differences in both thickness and density can cause isostatic adjustments in Earth's crust.
## Principles of Isostasy
Low-density blocks float on a denser liquid.
If blocks have equal densities, thicker blocks rise higher and sink deeper than thinner blocks.
High mountains in low-density crust are balanced by a deep root that extends into the mantle.
Floating blocks of unequal density: a block with denser portions sinks, and its surface is lower than adjacent blocks, even if its thickness is the same.
A deep basin may form if the rocks beneath it are denser than surrounding rocks.
## Effects of Isostatic Adjustment and Erosion
The combined effects of erosion and isostatic adjustment result in a thinning of the crust in mountainous regions, as material is removed from the top and the underlying
rootuplifts to compensate.
Transform Boundaries
The major transform plate boundaries and associated oceanic fracture zones are related to spreading at mid-ocean ridges.
Other transform boundaries are related to convergent margins in regions of complex plate movement.
The trend of a transform fault is parallel to the direction of relative motion between plates.
## Types of Transform Fault Connections
Ridge-ridge transform fault.
Ridge-trench transform fault.
Trench-trench transform fault.
## Characteristics
Plates slide past one another.
Most transform faults join mid-ocean ridge segments.
## Facilitating Plate Motion
Example 1 (Juan de Fuca Ridge): Seafloor generated along the Juan de Fuca Ridge moves southeastward, past the Pacific plate. Eventually, it subducts beneath the North American plate. This transform fault connects a spreading center (divergent boundary) to a subduction zone (convergent boundary).
Example 2 (San Andreas Fault): A transform fault that connects a spreading center located in the Gulf of California and the Mendocino Fault. Movement along the San Andreas Fault is a key example of plates sliding past each other.
Hotspots (Hawaii Example)
Not located at a plate boundary.
Occur where a plate moves over a
hotspot, which is a mantle magmatic center (mantle plume).Islands are produced sequentially as the plate moves over the stationary hotspot.
## Hawaiian Hotspot Track
The Hawaiian islands get progressively older towards the northwest.
For example:
Hawaii: 0.7 million years ago to present.
Maui: less than 1.0 million years ago.
Molokai: 1.3$-$1.8 million years ago.
Oahu: 2.2$-$3.3 million years ago.
Kauai: 3.8$-$5.6 million years ago.
The older islands form the Hawaiian chain, which extends into the Emperor Seamount chain (e.g., Suiko at 65 million years ago).
## Other Hotspot Tracks
Globally, other hotspots include Iceland, Yellowstone, Galapagos, Reunion, and many more, each leaving a track of volcanic activity as plates move over them.