🌎Plate Tectonics Unit 4 – Divergent Boundaries: Seafloor Spreading

Key Concepts

• Divergent boundaries occur where two tectonic plates move away from each other
• Seafloor spreading is the process by which new oceanic crust is formed at mid-ocean ridges
• Magma rises from the mantle to fill the gap created by the diverging plates, cools, and solidifies to form new oceanic crust
• The age of the oceanic crust increases with distance from the mid-ocean ridge
• Hydrothermal vents are often found near divergent boundaries, supporting unique ecosystems
• Magnetic anomalies in the oceanic crust provide evidence for seafloor spreading
  • The Earth's magnetic field has reversed polarity multiple times throughout history
  • These reversals are recorded in the oceanic crust as it forms at the mid-ocean ridge

Historical Background

• Alfred Wegener proposed the theory of continental drift in 1912, suggesting that continents were once joined together and had since moved apart
• However, Wegener's theory lacked a mechanism to explain how the continents could move
• In the 1960s, Harry Hess and Robert Dietz independently proposed the concept of seafloor spreading
• Hess's hypothesis was based on the discovery of mid-ocean ridges and the realization that the oceanic crust was younger near these ridges
• The development of plate tectonic theory in the late 1960s incorporated seafloor spreading as a key component
• Technological advancements, such as deep-sea drilling and marine geophysical surveys, provided further evidence to support the theory

Plate Tectonic Theory Basics

• The Earth's lithosphere is divided into several large plates that move relative to one another
• Plates can be oceanic (composed of dense oceanic crust) or continental (composed of less dense continental crust)
• There are three main types of plate boundaries: divergent, convergent, and transform
• Plate motion is driven by convection currents in the Earth's mantle
• At divergent boundaries, plates move apart, allowing magma to rise and create new oceanic crust
• Convergent boundaries occur where plates collide, resulting in subduction, mountain building, or continental collision
• Transform boundaries are characterized by plates sliding past each other horizontally, often along transform faults

Anatomy of Divergent Boundaries

• Divergent boundaries are characterized by a rift valley, where the two plates are moving apart
• The rift valley is flanked by steep, fault-bounded cliffs called escarpments
• As the plates continue to diverge, the rift valley widens and deepens, eventually forming a new ocean basin
• The center of the rift valley is marked by a mid-ocean ridge, a long, linear volcanic feature
• The mid-ocean ridge is composed of basaltic rock and is the site of active seafloor spreading
• Fracture zones, large-scale linear features that offset mid-ocean ridges, are common at divergent boundaries
  • These zones accommodate the differential spreading rates along the ridge

Seafloor Spreading Process

• As plates diverge at a mid-ocean ridge, the underlying mantle rises to fill the gap
• The rising mantle undergoes decompression melting, producing magma
• The magma rises through the overlying crust and erupts along the mid-ocean ridge, creating new oceanic crust
• The newly formed crust cools and solidifies, adding to the edges of the diverging plates
• As the plates continue to move apart, the process repeats, causing the seafloor to spread laterally
• The rate of seafloor spreading varies among different mid-ocean ridges (1-20 cm/year)
• Faster spreading rates result in a more gradual slope of the mid-ocean ridge, while slower rates produce a steeper profile

Evidence for Seafloor Spreading

• Age of the oceanic crust: The oceanic crust is youngest at the mid-ocean ridge and becomes progressively older with distance from the ridge
• Magnetic anomalies: The Earth's magnetic field has reversed polarity throughout history, and these reversals are recorded in the oceanic crust as it forms
  • The pattern of magnetic anomalies is mirrored on either side of the mid-ocean ridge, providing evidence for symmetric spreading
• Thickness of sediment: Sediment accumulation on the seafloor increases with distance from the mid-ocean ridge, as older crust has had more time to accumulate sediment
• Heat flow: Heat flow is highest at the mid-ocean ridge due to the presence of magma and decreases with distance from the ridge
• Seismic activity: Divergent boundaries are characterized by shallow earthquakes caused by the stretching and thinning of the lithosphere

Types of Divergent Boundaries

• Mid-ocean ridges: The most common type of divergent boundary, found in all major ocean basins (Atlantic, Pacific, and Indian)
  • Examples include the Mid-Atlantic Ridge and the East Pacific Rise
• Back-arc basins: Divergent boundaries that form behind subduction zones, often associated with volcanic island arcs (Mariana Trough)
• Continental rifts: Divergent boundaries that form within a continent, potentially leading to the formation of a new ocean basin (East African Rift)
• Microplates: Small tectonic plates that form at complex divergent boundaries where three or more plates interact (Juan de Fuca Plate)

Geological and Environmental Impacts

• Creation of new oceanic crust and lithosphere
• Formation of rift valleys, mid-ocean ridges, and submarine volcanic features
• Hydrothermal vent systems that support unique chemosynthetic ecosystems
• Metallic mineral deposits (copper, zinc, gold) associated with hydrothermal vents
• Geothermal energy potential due to high heat flow near divergent boundaries
• Influence on ocean circulation patterns and heat distribution
• Volcanic activity and associated hazards (submarine eruptions, lava flows)
• Seismic activity and potential for underwater landslides and tsunamis

Real-World Examples

• Mid-Atlantic Ridge: A prominent mid-ocean ridge in the Atlantic Ocean, separating the North American and Eurasian plates from the South American and African plates
  • Iceland, located on the Mid-Atlantic Ridge, is an example of a subaerial expression of a divergent boundary
• East Pacific Rise: A fast-spreading mid-ocean ridge in the Pacific Ocean, separating the Pacific Plate from the North American, Cocos, and Nazca plates
• East African Rift: A continental rift system that extends from the Afar Triangle in the north to Mozambique in the south, showcasing the early stages of divergent boundary formation
• Red Sea: An example of a young ocean basin formed by seafloor spreading, separating the African Plate from the Arabian Plate

Current Research and Discoveries

• Advancements in deep-sea exploration technologies (remotely operated vehicles, autonomous underwater vehicles) have allowed for more detailed studies of divergent boundaries and associated features
• Improved understanding of hydrothermal vent ecosystems and their role in the evolution of life on Earth
• Research into the microbial communities and biogeochemical processes associated with hydrothermal vents
• Investigations into the role of divergent boundaries in the global carbon cycle and the exchange of elements between the Earth's interior and the oceans
• Ongoing studies of the relationship between seafloor spreading rates, magma supply, and the morphology of mid-ocean ridges
• Exploration of the potential for seafloor massive sulfide deposits as a future resource for critical metals
• Monitoring of seismic and volcanic activity at divergent boundaries to improve hazard assessment and risk management strategies