Divergent plate boundaries are hotspots of volcanic activity, where new crust forms as plates move apart. These areas are home to unique underwater landscapes and ecosystems, shaped by the constant flow of lava and mineral-rich fluids.

Mid-ocean ridges, the most common divergent boundaries, produce over 70% of Earth's volcanism. Here, magma rises from the mantle, creating new seafloor through a process called seafloor spreading. This volcanic activity shapes our planet's surface and influences ocean chemistry.

Volcanism at Divergent Boundaries

Types of Volcanism

• Divergent plate boundaries, where tectonic plates are moving apart, are characterized by effusive volcanism producing fluid basaltic lava flows
• Submarine volcanism dominates at mid-ocean ridges, the most extensive divergent boundaries on Earth, forming new oceanic crust
  • Mid-ocean ridges account for over 70% of Earth's volcanism
  • Submarine volcanism at mid-ocean ridges is responsible for creating the majority of Earth's oceanic crust (seafloor spreading)
• Subaerial volcanism can occur at divergent boundaries where mid-ocean ridges emerge above sea level (Iceland, Afar region of East Africa)
  • Subaerial volcanism at divergent boundaries often manifests as fissure eruptions, lava fountains, and the formation of shield volcanoes
  • The Mid-Atlantic Ridge emerges above sea level in Iceland, resulting in extensive subaerial volcanic activity
• Volcanic activity at divergent boundaries is typically less explosive compared to convergent boundaries due to the low silica content and low viscosity of the magma
  • Basaltic magma at divergent boundaries has a low silica content (45-52 wt% SiO2), resulting in low viscosity and more fluid lava flows
  • The low explosivity of volcanism at divergent boundaries is due to the easy escape of volcanic gases from the fluid magma

Hydrothermal Activity

• Hydrothermal vents and black smokers are unique manifestations of submarine volcanism at mid-ocean ridges, driven by the interaction of seawater with hot volcanic rocks
  • Seawater percolates through fractures in the oceanic crust, becomes heated by magma or hot rocks, and rises back to the seafloor as hydrothermal fluids
  • Black smokers are chimneys formed by the precipitation of dissolved minerals (sulfides) as the hot hydrothermal fluids mix with cold seawater
  • Hydrothermal vents support unique chemosynthetic ecosystems, with organisms deriving energy from chemical reactions rather than sunlight
  • Examples of organisms found in hydrothermal vent communities include giant tube worms, clams, and sulfur-oxidizing bacteria

Magma Generation at Mid-Ocean Ridges

Decompression Melting

• Magma generation at mid-ocean ridges occurs through decompression melting of the upper mantle as tectonic plates diverge and hot mantle rock rises to fill the gap
  • As tectonic plates move apart, the underlying mantle rises to fill the space, causing a decrease in pressure (decompression)
  • Decompression of the mantle results in partial melting, as the decrease in pressure lowers the melting temperature of mantle rocks
• Partial melting of the mantle peridotite produces basaltic magma with a relatively low silica content (45-52 wt% SiO2) and high temperature (1200-1300°C)
  • Mantle peridotite is composed primarily of olivine, pyroxene, and garnet/spinel
  • Partial melting of peridotite preferentially melts the more easily fusible components (pyroxene and garnet), resulting in a basaltic melt composition
• The extent of partial melting depends on factors such as the rate of upwelling, the temperature of the mantle, and the presence of volatiles (water, carbon dioxide)
  • Faster upwelling rates lead to greater decompression and higher degrees of partial melting
  • Higher mantle temperatures promote more extensive melting
  • The presence of volatiles (water, carbon dioxide) lowers the melting temperature of mantle rocks, enhancing partial melting

Magma Focusing and MORB Composition

• Magma generation is focused beneath the ridge axis, where the mantle upwelling and decompression are greatest, resulting in the formation of a magma chamber
  • Magma accumulates in a shallow magma chamber beneath the ridge axis, where it undergoes further differentiation and crystallization
  • The magma chamber feeds dikes and sills that transport magma to the surface, resulting in the formation of new oceanic crust
• The composition of mid-ocean ridge basalts (MORB) is relatively uniform globally, reflecting the homogeneous nature of the upper mantle source
  • MORB is characterized by low concentrations of incompatible elements (potassium, rubidium, cesium) and high concentrations of compatible elements (magnesium, iron, calcium)
  • The uniform composition of MORB suggests that the upper mantle source is well-mixed and homogeneous on a global scale
• Trace element and isotopic variations in MORB can provide insights into mantle heterogeneity and the contribution of enriched mantle components (recycled oceanic crust, mantle plumes)
  • Enriched MORB (E-MORB) shows higher concentrations of incompatible elements and more radiogenic isotopic signatures compared to normal MORB (N-MORB)
  • E-MORB may reflect the influence of recycled oceanic crust or mantle plumes on the magma generation process

Volcanic Landforms at Divergent Boundaries

Submarine Volcanic Features

• Mid-ocean ridges are characterized by a linear, elevated topography with a central axial valley or ridge, flanked by steep normal faults and abyssal hills
  • The axial valley is a depression that runs along the crest of the mid-ocean ridge, formed by the spreading and faulting of the oceanic crust
  • Abyssal hills are elongated, fault-bounded ridges that parallel the mid-ocean ridge, formed by a combination of volcanism and tectonic activity
• Submarine lava flows at mid-ocean ridges typically form pillow basalts, which are ellipsoidal or tubular structures formed by rapid cooling of lava in contact with cold seawater
  • Pillow basalts are the most common volcanic rock type found at mid-ocean ridges
  • The distinctive pillow shape is a result of the rapid quenching and solidification of the lava upon contact with cold seawater
• Sheet flows and lobate flows are also common submarine volcanic features, formed by the coalescence of multiple pillow lavas or the rapid spreading of lava on the seafloor
  • Sheet flows are extensive, flat-lying lava flows that cover large areas of the seafloor
  • Lobate flows are characterized by a series of overlapping, tongue-shaped lobes of lava
• Seamounts and volcanic ridges can develop near mid-ocean ridges due to off-axis volcanism or the interaction of the spreading center with mantle plumes or hotspots
  • Seamounts are isolated, conical submarine volcanoes that rise from the seafloor
  • Volcanic ridges are elongated, linear chains of submarine volcanoes that may form due to the interaction of a mid-ocean ridge with a mantle plume or hotspot

Subaerial Volcanic Landforms

• Hydrothermal vent fields, characterized by mineral-rich chimneys and unique chemosynthetic ecosystems, are closely associated with active submarine volcanism at mid-ocean ridges
  • Hydrothermal vent fields are areas where hydrothermal fluids discharge through the seafloor, forming mineral-rich chimneys and mounds
  • The mineral-rich fluids provide the energy and nutrients necessary to support diverse chemosynthetic communities
• Subaerial volcanic landforms at divergent boundaries, such as in Iceland, include shield volcanoes, fissure vents, lava fields, and geothermal features (hot springs, geysers)
  • Shield volcanoes are broad, gently sloping volcanoes built by the accumulation of fluid basaltic lava flows (Mauna Loa, Hawaii)
  • Fissure vents are linear cracks or fractures in the Earth's surface from which lava erupts, often forming extensive lava fields (Laki, Iceland)
  • Geothermal features, such as hot springs and geysers, are manifestations of the high heat flow associated with divergent boundaries (Strokkur geyser, Iceland)

Environmental Impacts of Submarine Volcanism

Chemical and Thermal Exchange

• Submarine volcanism at mid-ocean ridges plays a crucial role in the chemical and thermal exchange between the Earth's interior and the oceans
  • Hydrothermal circulation transfers heat from the Earth's interior to the oceans, influencing global heat budgets and ocean circulation patterns
  • Submarine volcanism and hydrothermal activity contribute to the chemical composition of seawater, providing a source of dissolved elements and nutrients
• Hydrothermal circulation driven by submarine volcanism transfers heat and dissolved chemicals (metals, sulfur, carbon dioxide) from the crust to the ocean, influencing ocean chemistry and nutrient cycles
  • Hydrothermal fluids are enriched in dissolved metals (iron, manganese, copper, zinc) and other elements (sulfur, silicon) that are essential for marine life
  • The release of carbon dioxide and other gases from hydrothermal vents can influence ocean chemistry and contribute to the global carbon cycle
• Hydrothermal vent fluids can reach temperatures up to 400°C and are enriched in dissolved metals and sulfides, which precipitate to form massive sulfide deposits on the seafloor
  • Massive sulfide deposits formed by hydrothermal activity are potential sources of economically valuable metals (copper, zinc, gold, silver)
  • The precipitation of metal sulfides around hydrothermal vents creates distinctive chimney structures known as black smokers

Unique Ecosystems and Potential Hazards

• Chemosynthetic microbial communities thrive in hydrothermal vent environments, utilizing the chemical energy from vent fluids to support unique ecosystems in the absence of sunlight
  • Chemosynthetic bacteria form the base of the food chain in hydrothermal vent communities, converting the chemical energy from vent fluids into organic matter
  • Hydrothermal vent ecosystems are characterized by high biomass and unique adaptations to extreme conditions (high temperature, high pressure, toxicity)
• Submarine volcanic eruptions and associated earthquakes can trigger underwater landslides and generate local tsunamis, potentially impacting coastal communities and marine habitats
  • Submarine slope failures and landslides can be triggered by volcanic activity or earthquakes, displacing large volumes of water and generating tsunamis
  • Local tsunamis generated by submarine volcanic activity can pose a significant hazard to nearby coastal communities and marine infrastructure
• The release of volcanic gases (carbon dioxide, sulfur dioxide) during submarine eruptions can contribute to ocean acidification and affect marine life in the vicinity of the vents
  • The dissolution of volcanic gases in seawater can lower the pH, leading to localized ocean acidification
  • Ocean acidification can have detrimental effects on marine organisms that rely on calcium carbonate for their shells or skeletons (corals, mollusks)
• Studying submarine volcanism and hydrothermal systems provides insights into the origin of life on Earth and the potential for life on other planetary bodies with similar conditions
  • Hydrothermal vents are considered potential sites for the origin of life on Earth, as they provide the necessary energy, nutrients, and protected environments
  • The discovery of hydrothermal activity and potential subsurface oceans on other planetary bodies (Europa, Enceladus) raises the possibility of extraterrestrial life in similar environments