10.1 Volcanism at Divergent Plate Boundaries
7 min read•july 30, 2024
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
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- 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 (, Afar region of East Africa)
- Subaerial volcanism at divergent boundaries often manifests as , 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 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 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 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 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
Key Terms to Review (18)
Basaltic volcanism: Basaltic volcanism refers to volcanic activity that primarily produces basalt, a low-viscosity lava that flows easily and results in shield volcanoes and extensive lava flows. This type of volcanism is closely associated with divergent plate boundaries, where tectonic plates move apart, allowing magma to rise and erupt at the surface, often creating new oceanic crust.
Decompression Melting: Decompression melting is the process where a decrease in pressure allows magma to form from solid rock, typically occurring in the Earth's mantle. As tectonic plates move apart or when hot rock rises, the pressure decreases, which can lead to the melting of the rock without the addition of heat. This phenomenon is crucial for understanding magma generation, especially in relation to volcanic activity and plate tectonics.
East African Rift: The East African Rift is a tectonic plate boundary that is currently splitting the African continent into two parts, resulting in a series of geological features such as rift valleys and volcanoes. This divergent boundary, where the African Plate is pulling apart, leads to significant volcanic activity and the formation of new crust as magma rises to the surface.
Fissure eruptions: Fissure eruptions are volcanic events where magma escapes to the Earth's surface through linear fractures or cracks in the crust, rather than a single volcanic vent. These eruptions often produce extensive lava flows that can cover large areas and are typically associated with divergent plate boundaries, where tectonic plates are moving apart, allowing magma to rise easily from the mantle.
Gas Emissions Analysis: Gas emissions analysis refers to the systematic study of gases released from volcanic activity, including their composition, quantity, and behavior. This analysis is crucial in understanding volcanic processes and assessing potential hazards, as it can reveal information about magma movement, eruption forecasts, and environmental impacts. By monitoring gas emissions, scientists can gain insights into the type of volcanic activity taking place and its implications for both local communities and global climate.
Iceland: Iceland is an island nation located at the juncture of the North Atlantic and Arctic Oceans, known for its unique geological features resulting from its position on the Mid-Atlantic Ridge. This location has led to a high degree of volcanic activity, making it a prime example of both effusive eruptions and volcanism occurring at divergent plate boundaries. The interplay between tectonic activity and the island's geology has created a landscape filled with geysers, hot springs, and lava fields.
Lava flow: A lava flow is the movement of molten rock (lava) that erupts from a volcano and flows down its slopes or spreads out across the ground. This geological phenomenon is crucial for understanding the various volcanic hazards, the formation of different landforms, and the impact on surrounding environments.
Mafic magma: Mafic magma is a type of magma that is rich in magnesium and iron, resulting in a lower viscosity and higher temperature compared to other magma types. This composition leads to specific volcanic behaviors and landforms, particularly in areas where tectonic plates diverge or at hotspots. Mafic magma is typically associated with shield volcanoes and basaltic lava flows, which are key features of volcanic activity in certain geological settings.
Magma plume: A magma plume is a column of hot molten rock that rises from deep within the Earth’s mantle to the surface, often causing volcanic activity. These plumes are associated with hotspots and can create large volcanic features as they melt through the Earth's crust, forming islands or continental volcanic ranges over time.
Mid-ocean ridge: A mid-ocean ridge is an underwater mountain range formed by tectonic plate movements, specifically at divergent boundaries where plates are pulling apart. These ridges are significant as they are sites of volcanic activity and the creation of new oceanic crust, influencing the distribution of volcanic landforms and affecting geological processes on Earth.
Pillow Lava: Pillow lava is a type of volcanic rock formation that occurs when lava erupts underwater and cools rapidly, forming pillow-shaped structures. These distinctive formations are characterized by their rounded, bulbous shapes and can often be found in areas where volcanic activity takes place at divergent plate boundaries, such as mid-ocean ridges. The unique structure of pillow lava provides insights into the cooling processes of lava and the geological activity occurring in these underwater environments.
Plate boundary: A plate boundary is a geological term that refers to the edges where two tectonic plates meet. These boundaries are crucial in shaping Earth's surface through processes such as earthquakes, mountain building, and volcanism. The nature of interactions at these boundaries can significantly influence geological activity and the formation of landforms.
Rift valley: A rift valley is a lowland region that forms where tectonic plates are moving apart, leading to the subsidence of the land between them. This geological feature is a clear indicator of divergent plate boundaries, where the Earth's lithosphere is being stretched and pulled apart, often resulting in volcanic activity and the formation of new crust.
Seismic Monitoring: Seismic monitoring refers to the use of instruments and technologies to detect, measure, and analyze seismic waves produced by earthquakes and volcanic activity. This process is crucial for understanding the behavior of volcanoes, assessing hazards, and developing early warning systems to protect communities from potential eruptions.
Submarine volcanoes: Submarine volcanoes are underwater volcanic formations that erupt from the ocean floor, primarily found at divergent plate boundaries where tectonic plates move apart. These volcanoes play a vital role in shaping the oceanic crust and can lead to the creation of new landmasses over time as volcanic materials accumulate. The activity of submarine volcanoes is crucial for understanding the geological processes that occur beneath the sea, as well as their impact on marine ecosystems.
Tectonic divergence: Tectonic divergence refers to the process where tectonic plates move apart from each other, typically occurring at mid-ocean ridges. This movement allows magma to rise from the mantle, creating new oceanic crust and leading to volcanic activity. Divergent boundaries play a crucial role in shaping the Earth's surface and are integral to understanding how volcanic processes operate in these regions.
Volcanic gases: Volcanic gases are gaseous emissions released during volcanic eruptions, primarily consisting of water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide, and other trace gases. These emissions can significantly impact the environment and climate, and they play a crucial role in the dynamics of volcanic eruptions and associated hazards.
Volcanic islands: Volcanic islands are landforms created by volcanic activity, emerging from the ocean when magma erupts from beneath the Earth's crust and solidifies. These islands often form at tectonic plate boundaries, particularly at divergent or convergent boundaries, and can create unique ecosystems and geological features due to their volcanic origins.