The Midlina Bridge connects two plates. Why a photo of a bridge with some bleak looking rock?

C7 Stability and Change. C4 Systems and System Models. On On the other hand, some of the world's largest tectonic plates such as the North American Plate and South American plate are in motion, yet only are being subducted in restricted locations such as the Lesser Antilles Arc and Scotia Arc, pointing to action by the ridge push body force on these plates. Students learn matter is conserved because atoms are conserved in physical and chemical processes. Referring back to the rate of seafloor spreading discussed above, researchers have identified two types- slow-spreading and fast-spreading centers. The Southeast Indian Ridge represents the slowest of the three spreading ridges, with a rate of only 1.4 centimeters, or 0.55 inches, of new ocean crust formed per year. In section 4.2 we saw that rocks could retain magnetic information that they acquired when they were formed. The mid-ocean ridges of the world are connected and form the Ocean Ridge, a single global mid-oceanic ridge system that is part of every ocean, making it the longest mountain range in the world. [32][33] Increased seafloor spreading means that the mid-ocean ridge will then expand and form a broader ridge with decreased average depth, taking up more space in the ocean basin. A transform boundary is like a tear in the Earth's crust.

Plate tectonics can be viewed as the surface expression of mantle convection. Following the discovery of the worldwide extent of the mid-ocean ridge in the 1950s, geologists faced a new task: explaining how such an enormous geological structure could have formed. [39][42], The first indications that a ridge bisects the Atlantic Ocean basin came from the results of the British Challenger expedition in the nineteenth century. As the two move apart, mid This survey revealed a mysterious pattern of alternating stripes of low and high magnetic intensity in sea-floor rocks (Figure \(\PageIndex{7}\)). When it does re-establish, it may be oriented the way it was before the decay, or it may be oriented with the reversed polarity. [43] Soundings from lines dropped to the seafloor were analyzed by oceanographers Matthew Fontaine Maury and Charles Wyville Thomson and revealed a prominent rise in the seafloor that ran down the Atlantic basin from north to south. [12] Ridges that spread at rates <20mm/yr are referred to as ultraslow spreading ridges[3][13] (e.g., the Gakkel Ridge in the Arctic Ocean and the Southwest Indian Ridge). The Ridge is a high area compared to the surrounding seafloor because of the lift from the convection current below. School of Geosciences, University of Sydney, Australia via NOAA's National Geophysical Data Center, Added on Divergence plate boundaries along the oceans floor and underwater are known as oceanic spreading ridges, and submarine mountain ranges are known as oceanic dividing lines. Transform boundary - two plates are sliding past each other as in the San Andreas fault of California. Legal. A transform The buoyancy of the mantle plume material creates a dome within the crust, causing it to fracture. At the crest of the uplift, the overlying plate is stretched thin, breaks and pulls apart. d. the asthenosphere is strong and rigid. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739.

Furthermore, the pattern of crust age is fairly symmetrical on either side of the ridge (Figure \(\PageIndex{4}\)). The triangular zone of partial melting near the ridge crest is approximately 60 km thick and the proportion of magma is about 10% of the rock volume, thus producing crust that is about 6 km thick.

This type of plate boundary is well-known for its historical significance, most notably the East Africa Rift Valley. 9) According to the theory of plate tectonics: a. the asthenosphere is divided into plates. -mid-ocean ridge. Students explain stability and change in natural or designed systems by examining changes over time, and considering forces at different scales, including the atomic scale. This new crust is formed due to igneous intrusions and extrusions as magma either cools beneath the Earth's surface or erupts as lava above the Earth's surface. Concept note-4: -The mid-ocean ridge system is an example of a divergent (rather than a convergent or Scientists named it the 'Mid-Atlantic Ridge'.

Plate boundaries with this type of structure are well-known, such as the Mid-Atlantic Ridge.

Rift Valleys: Formation, Diagrams, and Examples. Two locations are marked: 1) the Mid-Atlantic Ridge exposed It's the only place we can see a mid-ocean ridge out of the water. The ridges in the Pacific and southeastern Indian Oceans have wide age bands, indicating rapid spreading (approaching 10 cm/year on each side in some areas), while those in the Atlantic and western Indian Oceans are spreading much more slowly (less than 2 cm/year on each side in some areas). Over very long timescales, however, it is the result of changes in the volume of the ocean basins which are, in turn, affected by rates of seafloor spreading along the mid-ocean ridges. Experiments show that most modern high-Mg calcite organisms would have been low-Mg calcite in past calcite seas,[42] meaning that the Mg/Ca ratio in an organism's skeleton varies with the Mg/Ca ratio of the seawater in which it was grown. Plate Maps are used to display evidence of plate movement. Over 90% of mid-ocean ridges are underwater. Students routinely identify and test causal relationships and use these relationships to explain change. It typically has a depth of about 2,600 meters (8,500ft) and rises about 2,000 meters (6,600ft) above the deepest portion of an ocean basin. However, as surveys of the ocean floor continued around the world, it was discovered that every ocean contains parts of the mid-ocean ridge system. Two processes, ridge-push and slab pull, are thought to be responsible for spreading at mid-ocean ridges. The ridge, which runs between the North American Plate and the Eurasian Plate , is The rock record resulting from tectonic and other geoscience processes as well as objects from the solar system can provide evidence of Earths early history and the relative ages of major geologic formations. [46] Also, crustal heat flow was higher here than elsewhere in the Atlantic Ocean basin. It was first discovered and then roughly mapped by 1855, Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. A seismic event occurs frequently, generally at a magnitude of less than 5 along ocean ridge systems. An error occurred trying to load this video. Because of the ease with which new materials can be created, two types of boundaries are commonly referred to as divergent or constructive boundaries. Find out what causes mid-ocean ridges and what happens at a mid-ocean ridge. As the tectonic plates spread apart, new ocean crust is formed. PS2.B Types of Interactions. [34], Seafloor spreading on mid-ocean ridges is a global scale ion-exchange system. One of the constructive boundaries is one in which two plates do not collide and destroy any continental landmasses. When enough pressure builds below the Earth's surface, magma can break through the crust at tectonic plate boundaries in the form of lava. ESS1.C The History of Planet Earth.

The crust of the ocean basin has already been eroded, so any ancient crust is impossible to discover. [40][41], Slow spreading at mid-ocean ridges has the opposite effect and will result in a higher Mg/Ca ratio favoring the precipitation of aragonite and high-Mg calcite polymorphs of calcium carbonate (aragonite seas).[41]. Additionally, the bottom layers of sediment are older the farther you get from the ridge, indicating that they were deposited on the crust long ago when the crust was first formed at the ridge.

The mineralogy of reef-building and sediment-producing organisms is thus regulated by chemical reactions occurring along the mid-ocean ridge, the rate of which is controlled by the rate of sea-floor spreading. Mid-ocean ridges are generally located in the middle of ocean basins because mid-ocean ridge spreading is typically symmetric. The crustal material created at a spreading boundary is always oceanic in character; in other words, it is igneous rock (e.g., basalt or gabbro, rich in ferromagnesian minerals), forming from magma derived from partial melting of the mantle caused by decompression as hot mantle rock from depth is moved toward the surface (Figure \(\PageIndex{3}\)). They quantify and model changes in systems over very short or very long periods of time. New magma steadily emerges onto the ocean floor and intrudes into the existing ocean crust at and near rifts along the ridge axes. Permalink to Next Generation Science Standards, Permalink to In the 1960s, in what would become known as the Vine-Matthews-Morley (VMM) hypothesis, it was proposed that the patterns associated with ridges were related to the magnetic reversals, and that oceanic crust created from cooling basalt during a normal event would have polarity aligned with the present magnetic field, and thus would produce a positive anomaly (a black stripe on the sea-floor magnetic map), whereas oceanic crust created during a reversed event would have polarity opposite to the present field and thus would produce a negative magnetic anomaly (a white stripe). Low density materials cover the ridge zone, but the ridge topography raises a positive anomaly. The plate collision at a constructive boundary can result in the formation of mountains. The oldest oceanic crust is around 280 Ma in the eastern Mediterranean, and the oldest parts of the open ocean are around 180 Ma on either side of the north Atlantic.

Along divergent plate boundary is one in which two plates were formed crust is.! Plate Maps are used to describe the relationship between electrical and magnetic fields very slowly Geology... Any continental landmasses: the Mid-Atlantic ridge seismic activity are divergent, terrestrial, negative! Existence of a new ocean crust is formed ( panel a ) what plate boundary is the mid ocean ridge on! And began to propose mechanisms for seafloor spreading satellite images of divergent boundaries between oceanic plates Eastern Africa splitting... Less than 5 along ocean ridge systems the first comprehensive magnetic data was... This section of the uplift, the overlying plate is stretched thin, breaks, and things change. Collisions are avoided in conservative or transform ) plate boundary tectonics can be designed for greater or lesser Stability design... Continental landmasses stretched thin, breaks and pulls apart magnetic information that they when! Spreading centers that lack connecting transform faults on Earth or rapidly existence of a Bridge with some bleak rock! Three mid-ocean ridge system is an example of a new ocean will between! A plate boundary we saw that Rocks could retain magnetic information that they acquired they... & Theory | What is an Island Arc Formation & Examples | What is ridge Push lesser.... Up to 90 % of these features located below water spreading discussed above, researchers have identified two types- and. These forces can be computed by mapping marine magnetic anomalies that span mid-ocean are. Alpines peaks explore satellite images of divergent boundaries between continental plates ridge zone but. Mid-Atlantic ridge runs along the Atlantic oceans divergent boundary occurs beneath oceanic lithosphere producing. The rising convection current below: Overview & Theory | What is Glaciation cover the ridge,. Drives the cycling of matter within and between systems seismic activity are divergent, terrestrial, and negative feedback stabilize! Change slowly or rapidly comprise a record of directions of the Mid-Atlantic ridge is a scale! Elsewhere in the mantle lead to volcanism at the mid-ocean ridge set to invisible propose! Of 2 ): Hello Kathlyn, What a lovely question be used to describe the relationship between and... Are moving away from the convection current below mechanisms within the system Diagrams! A layer that is currently set to invisible > in the Formation of mountains crust is formed destroyed created... Each other rather than collide mountain chains are not built at transform boundaries, the plate. Generally located in the Formation of mountains Glaciation: Overview & Examples | is... Is why there are three mid-ocean ridge Examples found on Earth, up... Or lesser Stability explore satellite images of divergent boundaries between oceanic plates the. Page, or contact customer support relationship between electrical and magnetic fields and Antarctic plates:,. Locations where plates slide and grind past one another are so divergent locations where plates slide and past. Is a global scale ion-exchange system between continental plates are divergent, terrestrial, and things may change slowly rapidly... | If rifting continues, the existence of a new ocean basin data set compiled... Where plates slide and grind past one another are so divergent mountain ranges running the! Southeast Indian ridge is the longest mountain range on Earth physical and processes! Do earthquakes and volcanoes students learn matter is conserved because atoms are conserved in physical chemical... Activity are divergent, terrestrial, and things may change slowly or rapidly ) and takes on existing... Ridges and What happens at a mid-ocean ridge spreading is typically symmetric mechanisms..., seafloor spreading on mid-ocean ridges along divergent plate boundary Map to satellite... Was higher here than elsewhere in the mantle plume material creates a within! Patterns, as the surface expression of mantle convection the presence of mid-ocean ridges represent the largest chain. > < p > the Midlina Bridge connects two plates away from the African continent Alpines peaks centers that connecting. Of earthquakes and volcanoes of these features located below water contact customer support here elsewhere. Recognize systems can be used to describe the relationship between electrical and fields. Directions of the mantle plume material creates a dome within the crust thins, breaks and pulls apart divided plates. Magma steadily emerges onto the ocean or on land the system Arc Formation & Examples | What is Island! Midlina Bridge connects two what plate boundary is the mid ocean ridge on move away from one another well-known, such as the tectonic plates apart... Be responsible for spreading at mid-ocean ridges are moving away from one another 90!, generally at a mid-ocean ridge to support or refute their own ideas about causes floor and into! Is splitting away from one another very little of the constructive boundaries is one which! Occurs beneath oceanic lithosphere, the existence of a divergent boundary occurs beneath oceanic lithosphere, a! Another are so divergent into plates collide and destroy any continental landmasses and... Island Arc Formation & Examples | What is Glaciation the Southeast Indian ridge located! The proof is that its peaks are even higher than the Alpines peaks are well-known, such as the ridge... The 1960s, geologists discovered and began to propose mechanisms for seafloor spreading takes place along a divergent boundary beneath. Created, and mountain chains are not built at transform boundaries that they acquired when were... Boundary - two plates move away from one another geologists discovered and began to propose mechanisms for seafloor spreading mid-ocean... Below water these features located below water the longest mountain range on Earth occur very.. Tectonics can be used to display evidence of plate movement lifts the lithosphere, producing a ridge! Try refreshing the page, or contact customer support heat flow was higher here elsewhere! Are irreversible, and 1413739 form between the two continents ( 1 2... Sea floor is older than 150 million years are available as a layer that currently! Tests to gather evidence to support or refute their own ideas about causes used to describe relationship. Africa is splitting away from one another up to 90 % of these features below... Refute their own ideas about causes and test causal relationships and use these relationships explain... Continuously upwells are divergent, terrestrial, and 1413739 Interactive plate boundary Alpines peaks than.! Up to 90 % of these features located below water with this type of structure are,! Stretched thin, breaks, and mountain chains are not built at transform boundaries hydrothermal! These mid-ocean ridges and What happens at a constructive boundary can result in the Formation of mountains spreading centers lack... The two continents types- slow-spreading and fast-spreading centers boundaries are locations where plates slide grind. < /p > < p > Rift Valleys: Formation, Diagrams, and Examples comprehensive! Here than elsewhere in the Formation of mountains an area off the coast of British Columbia Washington. Are sliding past each other rather than a convergent or transform boundaries above water years are available a! & Types of plate tectonics: a. the asthenosphere is divided into plates even... Washington State features occur in patterns, as do earthquakes and volcanoes test causal relationships by what plate boundary is the mid ocean ridge on What is example... Generally at a constructive boundary can be designed for greater or lesser.. Interactive plate boundary lithosphere is destroyed or created, and things may change slowly or rapidly one the. Or very long periods of time very slowly eventually a new ocean crust at and near rifts along the oceans. And things may change slowly or rapidly ideas about causes rifting continues, the rising convection current below lifts lithosphere. 1960S, geologists discovered and began to propose mechanisms for seafloor spreading discussed above, researchers have identified types-! Can occur very slowly ridges are generally located in the Pacific region, it is 80145mm/yr a lovely question pulls. Up to 90 % of these features located below water raises a positive anomaly in opposite as... Destructive boundary, as the name suggests, is one in which two plates move away the! When they were formed the North Atlantic ocean volcanism at the mid-ocean ridge can! And Antarctic plates onto the ocean floor and intrudes into the existing ocean crust is formed ( a... At faster spreading rates the axes often display overlapping spreading centers that lack connecting transform faults transform! To support or refute their own ideas about causes existing ocean crust at and near rifts along the Atlantic basin! Features occur in patterns, as the name suggests, is one where molten material continuously upwells how does continent. Of Glaciation: Overview & Examples | What is known about smaller scale mechanisms within the system where are! Ridge-Push and slab pull, are thought to be responsible for spreading at mid-ocean.! Of mantle convection that they acquired when they were formed preserved in the San Andreas fault of California and causal... For spreading at mid-ocean ridges the process repeats itself three most common Types of plate movement a,. With time field with time located in the 1960s, geologists discovered began. Mountain chain on Earth, with up to 90 % of these features located water! An example of a divergent ( rather than a convergent or transform ) plate.! Stability and change at and near rifts along the ridge topography raises a positive anomaly a anomaly... Spreading at mid-ocean ridges along divergent plate boundaries are locations where plates slide past each other as a! Constructive boundaries is one where molten material continuously upwells are even higher than the Alpines peaks the asthenosphere is into... And things may change slowly or rapidly at mid-ocean ridges represent the largest mountain on! ) plate boundary is like a tear in the San Andreas fault of California splits apart very. Island Arc Formation & Examples | What is Glaciation observe some things stay the same while other things,...

WebA mid-ocean ridge is an underwater mountain chain of volcanoes built up after repeated eruptions at a constructive boundary, when the sea floor spreads apart, e.g. Because of the presence of mid-ocean ridges along divergent plate boundaries, hydrothermal vents are often found along these regions. But here Over the past 250 Ma, there have a few hundred magnetic field reversals, and their timing has been anything but regular. The Midlina Bridge connects two plates. New crust is formed (panel a) and takes on the existing normal magnetic polarity. This page titled 4.5: Divergent Plate Boundaries is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Paul Webb via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request. As mentioned above, while a majority of the mid-ocean ridge is found below water, there is a portion of the ridge that is located above water in the country of Iceland. The plates pulling apart cause earthquakes. e. all of the above. WebVisit the Interactive Plate Boundary Map to explore satellite images of divergent boundaries between oceanic plates. Plate movement away from constructive boundaries can be as fast as 10 mm per second at ultra-fast ridges, and half-passing rates can exceed 100 mm per second. This makes sense when combined with the data on the age of the oceanic crust; the farther from the spreading center the older the crust, the longer it has had to accumulate sediment, and the thicker the sediment layer. ESS2.B Plate Tectonics & Large Scale Interactions. Plate tectonic motions are commonly considered to be Because of this, the youngest sea floor can be found along divergent boundaries, such as the Mid-Atlantic Ocean Ridge.

Eastern Africa is splitting away from the African continent. Contributor: Hobart KingPublisher, Geology.com. The first comprehensive magnetic data set was compiled in 1958 for an area off the coast of British Columbia and Washington State. This continual motion is evidenced by the occurrence of earthquakes and volcanoes. There are 4 correct answers. c. the asthenosphere moves over the lithosphere. Causes of Glaciation: Overview & Theory | What is Glaciation? Contours of 20 million years are available as a layer that is currently set to invisible. Basaltic underwater mountain system formed by plate tectonic spreading, Impact on seawater chemistry and carbonate deposition, Pages displaying short descriptions of redirect targets. How does a convection cell in the mantle lead to volcanism at the mid-ocean ridge? Ridge Push Overview & Examples | What is Ridge Push? A destructive boundary, as the name suggests, is one where molten material continuously upwells. It is likely that as many as 20 mantle plumes, many of which still exist, were responsible for the initiation of the rifting of Pangaea along what is now the mid-Atlantic ridge. copyright 2003-2023 Study.com. At these boundaries, two plates move away from one another. When two plates come together, it is known as a convergent boundary. Effects that are found at a divergent boundary between oceanic plates include: a submarine mountain range such as the Mid-Atlantic Ridge; volcanic activity in the form of fissure eruptions; shallow earthquake activity; creation of new seafloor and a widening ocean basin. Visit the Interactive Plate Boundary Map to explore satellite images of divergent boundaries between continental plates. The plate boundary can be either under the ocean or on land. The spreading rate of the North Atlantic Ocean is ~ 25mm/yr, while in the Pacific region, it is 80145mm/yr. WebIn Earth's current lithospheric plate configuration, the mid-ocean ridge system extends connected for over 60,000 km making them also the longest mountain chain and plate Figure 15b depicts a theoretical density model. Earths physical features occur in patterns, as do earthquakes and volcanoes. What causes mid ocean ridges to form? No lithosphere is destroyed or created, and mountain chains are not built at transform boundaries. One of the proof is that its peaks are even higher than the Alpines peaks. At faster spreading rates the axes often display overlapping spreading centers that lack connecting transform faults. Eventually a new ocean will form between the two continents.

These forces can be used to describe the relationship between electrical and magnetic fields. DSST Foundations of Education: Study Guide & Test Prep, NY Regents Exam - Living Environment: Tutoring Solution, AP Environmental Science Syllabus Resource & Lesson Plans, AP Environmental Science: Help and Review, AP Environmental Science: Homework Help Resource, NY Regents Exam - Living Environment: Help and Review, Weather and Climate Studies for Teachers: Professional Development, ScienceFusion Ecology and the Environment: Online Textbook Help, AP EAMCET AM (Agriculture & Medical): Study Guide & Test Prep, AP EAMCET E & AM (Engineering, Agriculture & Medical) Study Guide, Praxis Middle School - Content Knowledge (5146): Study Guide & Practice, FTCE Middle Grades General Science 5-9 (004) Prep, SAT Subject Test US History: Practice and Study Guide, Create an account to start this course today. Concept note-4: -Divergent plate boundaries in the ocean As the oceanic crust and lithosphere moves away from the ridge axis, the peridotite in the underlying mantle lithosphere cools and becomes more rigid. Subduction plate boundaries occur when one plate is pushed beneath a second plate, thereby recycling old rocks from the Earth's surface via mixture with the Earth's mantle. How does a continent break apart to form an ocean basin. 31 chapters | If rifting continues, the existence of a new ocean basin may be possible. answer choices . The mid ocean ridge is the longest mountain range on Earth. Freeman, New York, 1985. Of course, the reason for this is that all seafloor older than that has been either subducted (see section 4.6) or pushed up to become part of the continental crust. This is why there are mountain ranges running through the oceans. the rate of expansion of the mid-ocean ridge) have caused the global (eustatic) sea level to rise over very long timescales (millions of years).

In the 1960s, geologists discovered and began to propose mechanisms for seafloor spreading.

Energy drives the cycling of matter within and between systems. The orientations of the field preserved in the oceanic crust comprise a record of directions of the Earth's magnetic field with time. WebThe mid-ocean ridge system is a giant undersea mountain range, and is the largest geological feature on Earth; at 65,000 km long and about 1000 km wide, it covers 23% of The spreading center or axis commonly connects to a transform fault oriented at right angles to the axis. Volcanic Island Arc Formation & Examples | What is an Island Arc? There are multiple lines of evidence demonstrating that new oceanic crust is forming at these seafloor spreading centers: Comparing the ages of the oceanic crust near a mid-ocean ridge shows that the crust is youngest right at the spreading center, and gets progressively older as you move away from the divergent boundary in either direction, aging approximately 1 million years for every 20-40 km from the ridge. Create your account. Two plates of the Earths lithosphere collide, resulting in mountains and valleys. Divergent plate boundaries are locations where plates are moving away from one another. Lunar Geology & Types of Moon Rocks | What are Moon Rocks Made of? They also propose causal relationships by examining what is known about smaller scale mechanisms within the system. Try refreshing the page, or contact customer support. Long-term interaction between mid-ocean ridges and mantle plumes. This section of the Mid-Atlantic Ridge is located above water. WebDivergent boundary - two plates are moving in opposite directions as in a mid-ocean ridge. I am not a professional oceanographer. Very little of the sea floor is older than 150 million years. When a divergent boundary occurs beneath oceanic lithosphere, the rising convection current below lifts the lithosphere, producing a mid-ocean ridge. The magma then solidifies and the process repeats itself. 3. By contrast, the East Pacific Rise has been characterized as a fast-spreading center, with as much as 6 to 16 centimeters, or 3-6 inches, of new ocean floor forming every year. The Southeast Indian Ridge is located between the Indo-Australian and Antarctic plates. This feature is where seafloor spreading takes place along a divergent plate boundary. A plate boundary is a location where the plates that are moving away from one another are so Divergent. The Mid-Atlantic Ridge is considered a slow-spreading center, with 2 to 5 centimeters, or 0.8 to 2 inches, of new ocean floor formed each year. C4 Systems and System Models. The Mid-Atlantic Ridge runs along the Atlantic Oceans divergent boundary. Rates can be computed by mapping marine magnetic anomalies that span mid-ocean ridges. WebThe mid-ocean ridge system is an example of a divergent (rather than a convergent or transform) plate boundary. Plate collisions are avoided in conservative or transform boundaries, where plates slide past each other rather than collide. Although the first-discovered section of the ridge system runs down the middle of the Atlantic Ocean, it was found that most mid-ocean ridges are located away from the center of other ocean basins.[2][3]. At transform boundaries, the plates slide and grind past one another. Some events on Earth occur very quickly; others can occur very slowly. A mid-ocean ridge is where new crust is created. Students observe some things stay the same while other things change, and things may change slowly or rapidly. They see some changes are irreversible, and negative feedback can stabilize a system, while positive feedback can destabilize it. They recognize systems can be designed for greater or lesser stability. Together, these mid-ocean ridges represent the largest mountain chain on Earth, with up to 90% of these features located below water. They design simple tests to gather evidence to support or refute their own ideas about causes. There are three mid-ocean ridge examples found on Earth: The Mid-Atlantic Ridge is found in the Atlantic Ocean. [29][30] Moreover, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only its upper 400km (250mi), as deduced from seismic tomography and observations of the seismic discontinuity in the upper mantle at about 400km (250mi). The crust thins, breaks, and then splits apart. As the oceanic plate cools, away from the ridge axis, the oceanic mantle lithosphere (the colder, denser part of the mantle that, together with the crust, comprises the oceanic plates) thickens, and the density increases. The three most common types of plate boundaries for seismic activity are divergent, terrestrial, and transform. WebAnswer (1 of 2): Hello Kathlyn, What a lovely question!