4.1 Mid-ocean ridges and rift valleys
5 min read•august 16, 2024
are underwater mountain ranges where new oceanic crust forms. These geological wonders stretch across the ocean floor, marking the boundaries where drift apart. They're the birthplace of seafloor and play a crucial role in Earth's tectonic dance.
run along the center of these ridges, creating deep trenches where magma rises. The shape and depth of these valleys depend on how fast the plates are spreading. have deeper valleys, while fast-spreading ones are smoother.
Morphology of Mid-Ocean Ridges and Rift Valleys
Topographical Features
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- Mid-ocean ridges form extensive underwater mountain ranges characterized by linear structure and elevated topography compared to surrounding ocean floor
- Rift valleys create elongated depressions along central axis of mid-ocean ridges measuring 1-2 km deep and 20-50 km wide
- Slow-spreading ridges (1-5 cm/year) exhibit prominent rift valleys and rugged topography
- (>9 cm/year) display subdued rift valleys and smoother topography
- in rift valleys produces series of parallel fault blocks stepping down towards ridge's central axis
Geological Characteristics
- Mid-ocean ridges associate with high heat flow, intense volcanic activity, and
- Hydrothermal vents support unique deep-sea ecosystems (giant tube worms, chemosynthetic bacteria)
- Rocks at mid-ocean ridges primarily consist of basaltic composition
- Rapid cooling in deep ocean environment forms distinctive structures:
- (rounded, pillow-shaped formations)
- (flat, layered structures)
- High heat flow contributes to formation of:
- (mineral-rich hydrothermal vents)
- (cooler vents with different mineral composition)
Spreading Rate Variations
- Morphology of mid-ocean ridges varies based on spreading rate
- Slow-spreading ridges (1-5 cm/year):
- Examples: ,
- Characteristics: Deep rift valleys, rugged terrain, fewer volcanoes
- Fast-spreading ridges (>9 cm/year):
- Examples: , Pacific-Antarctic Ridge
- Characteristics: Smooth topography, shallow or absent rift valleys, more volcanic activity
- (5-9 cm/year):
- Examples: Juan de Fuca Ridge,
- Characteristics: Blend features of slow and fast-spreading ridges
Formation of Mid-Ocean Ridges and Rift Valleys
Plate Tectonic Processes
- Mid-ocean ridges and rift valleys form at divergent plate boundaries where tectonic plates move apart
- drives formation of mid-ocean ridges:
- New oceanic crust creates as magma rises from mantle
- Magma fills gap between separating plates
- in Earth's mantle provide driving force for:
- Plate separation
- Upwelling of hot, partially molten material beneath ridge axis
- Diverging plates cause to thin and fracture:
- Creates zone of weakness
- Allows magma to ascend and erupt at surface
Magmatic and Tectonic Activity
- Magmatic activity at ridge axis follows cyclical pattern:
- Periods of intense volcanism alternate with tectonic extension and faulting
- Balance between magma supply and spreading rate determines ridge morphology:
- Higher magma supply relative to spreading rate results in elevated, robust ridge structure
- Lower magma supply relative to spreading rate leads to deeper rift valleys and more faulting
- Central rift valley forms due to:
- pulling crust apart
- Normal faulting creating topographic low along ridge axis
- Magma chambers beneath ridges:
- Act as reservoirs for rising magma
- Contribute to continuous creation of new oceanic crust
- Vary in size and depth depending on spreading rate
Crustal Formation and Evolution
- New oceanic crust forms through combination of processes:
- (vertical magma injection)
- (lava flows on seafloor)
- (formation of at depth)
- As crust moves away from ridge axis:
- It cools and becomes denser
- Subsides and thickens through thermal contraction
- Accumulates sediment over time
- plays crucial role in cooling and altering newly formed crust:
- Seawater penetrates through cracks and fissures
- Becomes heated and chemically modified
- Emerges as hydrothermal fluids, altering surrounding rocks
Significance of Mid-Ocean Ridges in Plate Tectonics
Evidence for Plate Tectonics Theory
- Mid-ocean ridges serve as primary sites of new oceanic crust formation:
- Play crucial role in
- Contribute to evolution of ocean basins
- Discovery of mid-ocean ridges and seafloor spreading provided key evidence supporting plate tectonics theory
- Symmetrical pattern of magnetic anomalies in oceanic crust on either side of mid-ocean ridges:
- Provides evidence for seafloor spreading
- Allows reconstruction of past plate motions
- Mid-ocean ridges act as natural laboratories for studying:
- Magmatic processes
- of Earth's interior
Global Impact and Interactions
- Mid-ocean ridges influence global ocean circulation patterns:
- Create topographic barriers affecting deep ocean currents
- Contribute to thermohaline circulation through hydrothermal activity
- Hydrothermal activity at ridges contributes to chemical composition of seawater:
- Introduces dissolved minerals and gases
- Affects global geochemical cycles (iron, sulfur, manganese)
- Interaction between mid-ocean ridges and mantle plumes leads to formation of:
- Oceanic plateaus (Ontong Java Plateau)
- Hotspot chains (Hawaiian-Emperor Seamount Chain)
- Mid-ocean ridges play significant role in global carbon cycle:
- CO2 degassing from magma and hydrothermal systems
- Carbon sequestration in newly formed oceanic crust
Tectonic Implications
- Mid-ocean ridges drive plate motion through :
- Gravitational sliding of newly formed, elevated crust away from ridge axis
- Contributes to overall plate tectonic system
- Variations in spreading rate and ridge morphology provide insights into:
- Mantle dynamics
- Plate boundary processes
- Evolution of ocean basins
- Mid-ocean ridges interact with other tectonic features:
- offsetting ridge segments
- where oceanic crust is eventually consumed
- where three plate boundaries meet ()
Major Mid-Ocean Ridges and Rift Valleys
Oceanic Spreading Centers
- Mid-Atlantic Ridge:
- Longest mid-ocean ridge extending from Arctic Ocean to Southern Ocean
- Separates North and South American plates from Eurasian and African plates
- Notable features: Azores Plateau, Iceland (emerged portion of ridge)
- East Pacific Rise:
- Fast-spreading ridge in Pacific Ocean
- Separates Pacific Plate from Nazca and Cocos plates
- Notable features: Superfast spreading segments, abundant hydrothermal activity
- Southeast Indian Ridge:
- Connects Mid-Atlantic Ridge to Pacific-Antarctic Ridge
- Separates Antarctic Plate from Australian and African plates
- Notable features: Australian-Antarctic Discordance (area of anomalously deep seafloor)
- Southwest Indian Ridge:
- Extends from Bouvet Triple Junction to Rodrigues Triple Junction
- Separates African Plate from Antarctic Plate
- Notable features: Ultra-slow spreading rate, deep rift valleys
Continental and Transitional Rift Systems
- System:
- Continental rift valley representing early stages of formation
- Extends from Ethiopia to Mozambique
- Notable features: Lake Tanganyika, Olduvai Gorge, active volcanism (Mount Kilimanjaro)
- :
- Intracontinental rift valley in Siberia
- May develop into future ocean basin
- Notable features: Lake Baikal (deepest and oldest freshwater lake)
- :
- Intracontinental rift valley in North America
- Extends from Colorado to Mexico
- Notable features: Rio Grande River, Valles Caldera
- :
- Transitional rift system between continental and oceanic spreading
- Separates African Plate from Arabian Plate
- Notable features: Evaporite deposits, nascent oceanic crust in central axis
Key Terms to Review (48)
Abyssal plain: An abyssal plain is a large, flat area of the ocean floor, typically found at depths between 3,000 and 6,000 meters. These plains are among the flattest and smoothest regions on Earth, formed by the accumulation of sediment over time from various sources like erosion and volcanic activity. Abyssal plains are significant in understanding the geological processes related to mid-ocean ridges and rift valleys as they are often located adjacent to these features, showcasing the dynamic nature of oceanic crust formation.
Afar triple junction: The Afar triple junction is a geologically significant area where three tectonic plates meet: the African Plate, the Arabian Plate, and the Somali Plate. This unique location is characterized by rifting, which is the process of the Earth's crust stretching and thinning, leading to the formation of rift valleys and mid-ocean ridges. The Afar region is a prime example of how tectonic activity shapes the Earth's surface, creating both geological features and seismic activity.
Baikal Rift Zone: The Baikal Rift Zone is a geological rift located in Siberia, Russia, characterized by the stretching and thinning of the Earth's crust that has led to the formation of Lake Baikal, the world's deepest freshwater lake. This rift is a prime example of an active continental rift, showcasing how tectonic forces can shape landscapes and create significant geological features.
Basalt: Basalt is a dark, fine-grained volcanic rock that is formed from the rapid cooling of low-viscosity lava. It is primarily composed of plagioclase and pyroxene minerals, and is the most abundant volcanic rock on Earth. Basalt plays a crucial role in shaping mid-ocean ridges and rift valleys, contributing to the formation of new oceanic crust and influencing geological processes in these dynamic regions.
Black smokers: Black smokers are hydrothermal vents found on the ocean floor, emitting dark, mineral-rich water heated by geothermal energy. These unique geological features are primarily located along mid-ocean ridges and rift valleys, where tectonic plates diverge, allowing seawater to penetrate the Earth's crust and interact with magma. The extreme environments around black smokers support a diverse array of life, showcasing the connections between geological processes and biological ecosystems.
Convection currents: Convection currents are the circular movements of fluid caused by the uneven heating of that fluid, which results in differences in density. These currents are a crucial mechanism in the Earth's mantle that drive the movement of tectonic plates, influencing the formation of mid-ocean ridges, rift valleys, and other geological features.
Dike intrusion: A dike intrusion is a geological feature formed when magma intrudes into pre-existing rock layers, creating a vertical or nearly vertical sheet of igneous rock. This process often occurs in tectonically active regions such as mid-ocean ridges and rift valleys, where magma is generated and forces its way upward through cracks and fissures, solidifying as it cools. Dike intrusions can reveal important information about the history of volcanic activity and the movement of tectonic plates.
Divergent boundary: A divergent boundary is a tectonic plate boundary where two plates move away from each other, allowing magma from the mantle to rise and create new crust. This process plays a crucial role in the formation of ocean basins and rift valleys, contributing to the geological features and topography of Earth.
Earthquakes: Earthquakes are sudden releases of energy in the Earth's crust, resulting from tectonic movements that create seismic waves. These movements can occur at different types of plate boundaries, affecting geological formations and human structures alike, and they are often linked to various geological processes such as subduction, rifting, and faulting.
East African Rift: The East African Rift is a tectonic plate boundary that stretches from the Afar Triangle in Ethiopia down to Mozambique, characterized by a series of rift valleys formed by the divergence of the African tectonic plate. This region is an active area of continental rifting, leading to the formation of large lakes, volcanoes, and a unique geological landscape that connects to processes like mid-ocean ridges and the creation of new oceanic crust.
East Pacific Rise: The East Pacific Rise is a mid-ocean ridge located along the floor of the eastern Pacific Ocean, characterized by its high volcanic activity and a significant rate of seafloor spreading. This ridge serves as a divergent boundary between the Pacific Plate and the North American Plate, playing a crucial role in the creation of new oceanic crust and shaping oceanic features such as rift valleys and hydrothermal vents.
Extensional forces: Extensional forces are stress forces that act to pull apart or stretch the Earth's crust. These forces are significant in shaping geological features, particularly in creating rift valleys and mid-ocean ridges where tectonic plates diverge, allowing magma to rise and create new crust.
Extrusive volcanism: Extrusive volcanism refers to the process by which magma from beneath the Earth's crust erupts onto the surface, resulting in the formation of volcanic rocks and landforms. This type of volcanism occurs when magma reaches the surface quickly, often through fissures or vents, allowing it to cool and solidify rapidly. As a key aspect of geological activity, extrusive volcanism is closely associated with mid-ocean ridges and rift valleys, where tectonic plates diverge and create pathways for magma to escape.
Fast-spreading ridges: Fast-spreading ridges are mid-ocean ridges characterized by rapid volcanic activity and the swift creation of new oceanic crust, resulting from the movement of tectonic plates. These features play a crucial role in seafloor spreading and the overall dynamics of plate tectonics, influencing the geological and ecological aspects of ocean environments.
Gabbro: Gabbro is a coarse-grained igneous rock formed from the slow crystallization of magma beneath the Earth's surface. It is primarily composed of plagioclase feldspar and pyroxene, which gives it a dark color. Gabbro is significant as it is often found in oceanic crust and plays a critical role in understanding mid-ocean ridges and rift valleys, where tectonic processes create new oceanic crust through volcanic activity.
Harry Hess: Harry Hess was a prominent American geologist and a key figure in the development of the theory of plate tectonics, particularly known for his contributions to understanding seafloor spreading. His work helped establish the mechanisms of plate movement and the formation of ocean basins, connecting various geological features and processes within the Earth's lithosphere.
Hydrothermal circulation: Hydrothermal circulation refers to the process of water circulation through oceanic crust, particularly around mid-ocean ridges and rift valleys. This movement of water is driven by geothermal heat from the Earth's interior, leading to the heating of seawater, which then rises through cracks in the crust and interacts with hot volcanic rocks. As the heated water rises, it can dissolve minerals and transport nutrients, contributing to unique ecosystems and geological features.
Hydrothermal vents: Hydrothermal vents are fissures in the Earth's surface where heated water, rich in minerals, is expelled from the ocean floor. These vents are often found along mid-ocean ridges and rift valleys, playing a crucial role in the ecosystem by supporting unique communities of organisms that thrive in extreme conditions and contribute to the geological processes at plate boundaries.
Intermediate-spreading ridges: Intermediate-spreading ridges are mid-ocean ridges where tectonic plates move apart at a moderate rate, typically between 2 to 6 centimeters per year. These ridges play a critical role in the process of seafloor spreading, contributing to the creation of new oceanic crust as magma rises from the mantle and solidifies at the ocean floor. They are characterized by various geological features, including volcanic activity and hydrothermal vent systems, which support diverse ecosystems.
John Tuzo Wilson: John Tuzo Wilson was a Canadian geophysicist and geologist known for his pioneering contributions to the understanding of plate tectonics and the concept of transform faults. His work helped explain the movement of tectonic plates and their role in forming accretionary wedges, rift valleys, and orogenic belts. He is also recognized for formulating the theory of the Wilson cycle, which describes the lifecycle of supercontinents and the cyclical nature of continental formation and breakup.
Lithosphere: The lithosphere is the rigid outer layer of the Earth, encompassing the crust and the uppermost part of the mantle. This layer is crucial in understanding how tectonic plates interact, as it affects everything from isostatic adjustments to the formation of geological features like continents and ocean basins.
Magma generation: Magma generation refers to the process through which molten rock is formed within the Earth's mantle and crust, leading to volcanic activity and the formation of igneous rocks. This process is intricately linked to tectonic activity, particularly in regions where plates converge or diverge, playing a critical role in shaping the planet's geological features and contributing to the rock cycle.
Mantle composition: Mantle composition refers to the specific materials and minerals that make up the Earth's mantle, which lies between the crust and the outer core. This layer plays a crucial role in plate tectonics, as its characteristics influence the movement of tectonic plates and the formation of geological features such as mid-ocean ridges and rift valleys. Understanding mantle composition helps explain the processes of magma generation, thermal convection, and the overall dynamics of the Earth’s interior.
Mid-Atlantic Ridge: The Mid-Atlantic Ridge is a vast underwater mountain range that runs down the center of the Atlantic Ocean, marking the boundary between the Eurasian and North American tectonic plates in the north and the African and South American plates in the south. It is a prime example of a mid-ocean ridge, where new oceanic crust is formed through volcanic activity, contributing to seafloor spreading and playing a vital role in plate tectonics and geological processes.
Mid-ocean ridges: Mid-ocean ridges are underwater mountain ranges formed by tectonic plate movements, specifically at divergent boundaries where two oceanic plates pull apart. These features are critical in understanding the process of seafloor spreading and are often associated with volcanic activity, as magma rises to create new oceanic crust, impacting both marine ecosystems and global geology.
Normal faulting: Normal faulting occurs when the Earth's crust is extended, causing one block of rock to move downward relative to another block. This type of fault is commonly associated with tectonic processes at mid-ocean ridges and rift valleys, where the stretching of the crust leads to the formation of faults that accommodate this extension. As tectonic plates pull apart, normal faults help shape the landscape and contribute to geological features such as oceanic ridges and continental rifts.
Ocean basin: An ocean basin is a vast underwater depression that covers a significant portion of the Earth's surface, primarily formed by tectonic activity and the movement of the Earth's lithospheric plates. These basins are crucial for understanding the geology of the ocean floor, including features such as mid-ocean ridges, deep-sea trenches, and rift valleys, all of which play essential roles in the processes of plate tectonics and the dynamic nature of the Earth.
Pangaea: Pangaea was a supercontinent that existed during the late Paleozoic and early Mesozoic eras, roughly 335 to 175 million years ago, when it began to break apart. This vast landmass is significant as it provides insights into the historical movements of tectonic plates, influencing geological formations and the distribution of ancient flora and fauna across the planet.
Pillow Lavas: Pillow lavas are a type of volcanic rock formation that forms when lava erupts underwater, resulting in a distinctive, pillow-like shape. This occurs mainly at mid-ocean ridges and rift valleys, where tectonic plates diverge and create conditions for underwater volcanic activity, leading to the formation of these unique structures.
Plutonic Activity: Plutonic activity refers to the processes associated with the formation of igneous rocks that solidify below the Earth's surface, resulting from the cooling and crystallization of magma. This activity is a key driver in the formation of geological features such as mid-ocean ridges and rift valleys, where tectonic movements allow magma to rise and intrude into surrounding rock layers, creating new landforms and influencing the structure of the Earth's crust.
Red Sea Rift: The Red Sea Rift is a tectonic plate boundary that marks the divergent movement of the African and Arabian plates, creating a rift valley that is both geologically active and significant in the study of plate tectonics. This rift is characterized by seafloor spreading, volcanic activity, and the formation of new oceanic crust as the plates move apart, contributing to the widening of the Red Sea and the geological evolution of the surrounding regions.
Ridge push force: Ridge push force is a tectonic process that occurs at mid-ocean ridges, where the elevated position of the ridge causes the lithosphere to slide away from the ridge crest. This force results from the cooling and sinking of newly formed oceanic crust, which leads to a gravitational force pushing the tectonic plates apart. Ridge push contributes significantly to the movement of tectonic plates and is closely related to phenomena like seafloor spreading and rift formation.
Rift valleys: Rift valleys are elongated lowlands formed by the tectonic forces that pull apart the Earth's crust, typically found at divergent plate boundaries. These valleys are significant geological features that indicate areas where continental plates are moving away from each other, leading to the formation of new crust and often associated with volcanic activity. Rift valleys not only provide insights into the process of plate tectonics but also reveal the dynamic nature of Earth's surface over time.
Rio Grande Rift: The Rio Grande Rift is a geological feature that represents a continental rift zone where the North American continent is being pulled apart, creating a series of basins and mountain ranges. It stretches from Colorado to Mexico, showcasing how tectonic forces can lead to the formation of rift valleys similar to those found at mid-ocean ridges.
Seafloor Spreading: Seafloor spreading is the process by which new oceanic crust is formed at mid-ocean ridges as tectonic plates move apart. This geological phenomenon plays a crucial role in the formation of ocean basins and influences various tectonic activities, including the generation of rift valleys and the distribution of magnetic anomalies on the seafloor.
Sheet flows: Sheet flows refer to the movement of water over land in a thin, continuous layer, often occurring in areas with gentle slopes. These flows can carry sediment and nutrients, impacting soil and water quality. In the context of geological features, sheet flows can influence the formation and evolution of mid-ocean ridges and rift valleys by altering the landscape and contributing to erosion and deposition processes.
Slow-spreading ridges: Slow-spreading ridges are mid-ocean ridges where tectonic plates separate at a slower rate, typically less than 2.5 centimeters per year. These ridges are characterized by rugged topography, and volcanic activity is generally less intense compared to fast-spreading ridges. The slower movement leads to the formation of more prominent features such as rift valleys and fault systems, which contribute to the overall geological diversity of the ocean floor.
Southeast Indian Ridge: The Southeast Indian Ridge is a mid-ocean ridge located in the southern Indian Ocean, forming part of the divergent boundary between the African and Indo-Australian tectonic plates. This underwater mountain range is characterized by volcanic activity and new oceanic crust formation as tectonic plates move apart, contributing to the ongoing processes of seafloor spreading and shaping the ocean floor.
Southwest Indian Ridge: The Southwest Indian Ridge is a divergent tectonic plate boundary located in the southern Indian Ocean, where the African Plate and the Indo-Australian Plate are moving apart. This mid-ocean ridge is part of the global system of mid-ocean ridges that serve as sites for seafloor spreading, which occurs when magma rises from the mantle, creating new oceanic crust as the tectonic plates separate.
Subduction Zones: Subduction zones are regions where one tectonic plate moves under another plate and sinks into the mantle, leading to various geological activities. These areas are critical for understanding volcanic activity and earthquake generation, as they often coincide with major volcanic arcs and earthquake-prone regions.
Supercontinent cycle: The supercontinent cycle refers to the process by which Earth's continents merge to form a supercontinent and then break apart over geological time scales. This cycle plays a significant role in the movement of tectonic plates, leading to the creation of mid-ocean ridges and rift valleys during continental fragmentation, while also influencing the formation of supercontinents through processes like subduction and plate tectonics.
Tectonic plates: Tectonic plates are massive, irregularly shaped slabs of solid rock that make up the Earth's lithosphere, which includes both the crust and the uppermost mantle. These plates constantly move and interact at their boundaries, leading to various geological features and processes such as earthquakes, volcanic activity, and the formation of mountains. Their movement is influenced by convection currents in the underlying mantle and is fundamental to understanding features like mid-ocean ridges, rift valleys, and the creation of new oceanic crust.
Thermal structure: Thermal structure refers to the variation of temperature within the Earth's layers, particularly in relation to how heat is distributed and transferred through geological formations. This concept is crucial for understanding processes such as magma formation, plate tectonics, and the dynamics of mid-ocean ridges and rift valleys, as the thermal profile influences the behavior of materials within the Earth, leading to tectonic activity and the formation of new crust.
Transform boundary: A transform boundary is a type of tectonic plate boundary where two plates slide past each other horizontally. This movement creates friction and can lead to significant seismic activity, often resulting in earthquakes, as the plates get stuck and release energy suddenly when they finally move.
Transform faults: Transform faults are types of plate boundaries where two tectonic plates slide past each other horizontally. This movement can cause significant geological activity, including earthquakes, as the friction between the sliding plates leads to stress accumulation until it's released. These faults are essential in understanding how the Earth's crust is shaped and restructured, especially in areas near mid-ocean ridges and rift valleys where tectonic movements are prevalent.
Triple Junctions: Triple junctions are points on the Earth's surface where three tectonic plates meet, resulting in complex interactions between these plates. These junctions can significantly influence geological processes, including earthquakes and volcanic activity, and are often associated with unique geological features like mid-ocean ridges or rift valleys. The dynamics at triple junctions vary based on the types of plate boundaries involved—whether they are divergent, convergent, or transform boundaries.
White smokers: White smokers are hydrothermal vents found on the ocean floor, characterized by their white mineral deposits, which are primarily formed from compounds like barium, calcium, and silicon. These unique features distinguish them from black smokers, as they emit cooler fluids that create distinct ecosystems. The presence of white smokers is closely associated with mid-ocean ridges and rift valleys, where tectonic activity leads to the circulation of seawater through the oceanic crust.
Wilson Cycle: The Wilson Cycle describes the cyclical process of continental rifting, ocean basin formation, and subsequent closure leading to the creation and breakup of supercontinents over geological time. This concept helps us understand the dynamic nature of plate tectonics, emphasizing how mid-ocean ridges, seafloor spreading, and the formation of rift valleys contribute to the recycling of the Earth's lithosphere.