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4.5: Convergent Plate Boundaries - Geosciences


Convergent Plate Boundaries

Convergent Plates move together and collide so you have compressional forces. They are associated with active margins—locations where mountain building is occurring, resulting in numerous earthquakes and andesite (explosive) volcanoes.

A subduction zone is a plate boundary along which one plate of the Earth’s outer shell descends (subducts) at an angle beneath another (Figure 4.14). A subduction zone is usually marked by a deep trench on the sea floor. An example is the Cascadia Subduction Zone offshore of Washington, Oregon, and northern California (see Figure 4.19 below). Most tsunamis are generated by subduction-zone-related earthquakes.

Figure 4.14 illustrates how earthquake data reveals the geometry of a subduction zone. This diagram show the location and intensity of earthquakes over a period of time in the vicinity of the Tonga Islands in the South Pacific Ocean. A deep ocean trench runs along the southeast side of the island chain. Earthquake data shows that a major fault system descends at an angle, extending eastward beneath the Tonga Island and extends of hundreds of kilometers at a steep angle deep into the upper mantle (asthenosphere) where it is presumed that earthquakes cease because rocks are too hot and under intense pressure that it easier for them to fold and flow plastically than to fracture as brittle rock. The earthquake data suggests that the eastern edge of the Australian Plate is being over run by the western edge of the Pacific Plate, and that rocks of the Australian Plate are descending into the upper mantle.


Figure 4.14. Earthquake data reveals the geometry of a subduction zone in the region of Tonga.

Three types of Convergent Plate Boundaries: OC/CC, OC/OC, & CC/CC

Three types of convergent plate boundaries are recognized: .
a) Subduction of ocean crust (OC) beneath continental crust (CC)
b) Subduction of ocean crust (OC) beneath ocean crust (OC)
c) Continental Collisions: continental crust (CC) colliding with continental crust (CC).

A) Subduction of ocean crust (OC) beneath continental crust (CC).

• Denser, thinner OC is pushed or subducted beneath less dense and more buoyant CC.
• A chain of volcanoes formed, called a continental volcanic arc.
• Subduction produces both deep and shallow focus earthquakes (with tsunami potential); the largest ever--9.5 magnitude in Peru/Chile Trench in 1960.
• Volcanoes of the andesite (explosive) type. Examples include the Andes and the Cascade Range , etc.
• Deep trenches form around continents margins. Trenches are especially well developed in regions far away from spreading centers (where the ocean crust is old, cold, and denser, and therefore sinks more rapidly).
• Subduction reduces amount of (and destroys) OC.
• Rates of subduction are up to 15 cm/yr in the active margins of the Pacific Basin.

Examples:
• Andes in South America (Figure4-15)
• Cascades in United States (include such volcanoes as Mt. St. Helens, Mt. Rainier, Mount Shasta, Crater Lake and many others)

B) Subduction of ocean crust (OC) beneath ocean crust (OC).

• Many similar features as above [OC/CC].
• Denser, older, cooler OC is pushed or subducted beneath less dense, warmer, younger OC.
• Forms island volcanic arcs.
• Deep and shallow (tsunami potential) focus earthquakes
• Volcanoes not as explosive as above with OC/CC, as there is no mixing of CC rocks (called granites). Volcanic rocks are mostly basaltic in composition.
• Subduction reduces amount of (destroys) OC.

Examples:
Japan, Tonga Islands, and Aleutian Islands (Alaska)(Figures 4-16 and 4-17)

C) Continental Collisions: continental crust (CC) colliding with continental crust (CC)

When continents collide with other continental landmasses:
• Neither of the CC are subducted,
• Both are very buoyant and want to "float" or ride high.
• This is where you form the very large mountain chains.
• Mountain building occurs with lots of earthquakes; massive erosion also occurs.

Examples
Himalayas (India) beginning 45 million years ago) (Figure 4.18)
Alps Mountains are being pushed up by collisions between Africa (and Italian Peninsula) with Europe.
Appalachians Mountains in the eastern United States (formed when North America collided with Africa about 350-400 million years ago (before the Atlantic Ocean opened later).


Figure 4.18. Migration of "India" away from ancient Pangaea has led to the collision of continental land masses resulting in the rise of the Himalayan Mountains. In this region, the continental crust on both sides of the plate boundary are too light to sink into the mantle.


4.6 Convergent Plate Boundaries

Convergent boundaries , where two plates are moving toward each other, are of three types, depending on the type of crust present on either side of the boundary — oceanic or continental . The types are ocean-ocean, ocean-continent, and continent-continent.

At an ocean-ocean convergent boundary, one of the plates (oceanic crust and lithospheric mantle) is pushed, or subducted , under the other (Figure 4.6.1). Often it is the older and colder plate that is denser and subducts beneath the younger and warmer plate. There is commonly an ocean trench along the boundary as the crust bends downwards. The subducted lithosphere descends into the hot mantle at a relatively shallow angle close to the subduction zone, but at steeper angles farther down (up to about 45°). The significant volume of water within the subducting material is released as the subducting crust is heated. It mixes with the overlying mantle, and the addition of water to the hot mantle lowers the crust’s melting point and leads to the formation of magma (flux melting). The magma, which is lighter than the surrounding mantle material, rises through the mantle and the overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands known as an island arc . A mature island arc develops into a chain of relatively large islands (such as Japan or Indonesia) as more and more volcanic material is extruded and sedimentary rocks accumulate around the islands. Earthquakes occur relatively deep below the seafloor, where the subducting crust moves against the overriding crust.

Figure 4.6.1 A trench and volcanic island formed from an ocean-ocean convergent zone (Steven Earle, “Physical Geology”).

Examples of ocean-ocean convergent zones are subduction of the Pacific Plate south of Alaska (creating the Aleutian Islands) and under the Philippine Plate, where it creates the Marianas Trench, the deepest part of the ocean.

At an ocean-continent convergent boundary, the denser oceanic plate is pushed under the less dense continental plate in the same manner as at an ocean-ocean boundary. Sediment that has accumulated on the seafloor is thrust up into an accretionary wedge, and compression leads to thrusting within the continental plate (Figure 4.6.2). The magma produced adjacent to the subduction zone rises to the base of the continental crust and leads to partial melting of the crustal rock. The resulting magma ascends through the crust, producing a mountain chain with many volcanoes. As with an ocean-ocean boundary, the subducting crust can produce a deep trench running parallel to the coastline.

Figure 4.6.2 A trench and volcanic mountains formed from an ocean-continent convergent zone (Steven Earle, “Physical Geology”).

Examples of ocean-continent convergent boundaries are subduction of the Nazca Plate under South America (which has created the Andes Mountains and the Peru Trench) and subduction of the Juan de Fuca Plate under North America (creating the Cascade Range).

A continent-continent collision occurs when a continent or large island that has been moved along with subducting oceanic crust collides with another continent (Figure 4.6.3). The colliding continental material will not be subducted because it is too light (i.e., because it is composed largely of light continental rocks), but the root of the oceanic plate will eventually break off and sink into the mantle. There is tremendous deformation of the pre-existing continental rocks, forcing the material upwards and creating mountains.

Figure 4.6.3 Mountains formed from a continent-continent convergent zone (Steven Earle, “Physical Geology”).

Examples of continent-continent convergent boundaries are the collision of the India Plate with the Eurasian Plate, creating the Himalaya Mountains, and the collision of the African Plate with the Eurasian Plate, creating the series of ranges extending from the Alps in Europe to the Zagros Mountains in Iran.

*”Physical Geology” by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http://open.bccampus.ca

a plate boundary at which the two plates are moving towards each other (4.6)

the Earth’s crust underlying the oceans (as opposed to continental crust) (3.2)

the Earth’s crust underlying the continents (as opposed to ocean crust) (3.2)

when part of a plate is forced beneath another plate along a subduction zone (4.3)

the rigid outer part of the Earth, including the crust and the mantle down to a depth of about 100 km (3.2)

long chains of volcanic islands found along convergent tectonic plate boundaries (4.6)


4.7 Transform Plate Boundaries

Transform boundaries exist where one plate slides past another without production or destruction of crustal material. As explained in section 4.5, most transform faults connect segments of mid-ocean ridges and are thus ocean-ocean plate boundaries. Some transform faults connect continental parts of plates. An example is the San Andreas Fault, which connects the southern end of the Juan de Fuca Ridge with the northern end of the East Pacific Rise (ridge) in the Gulf of California (Figure 4.7.1). The part of California west of the San Andreas Fault and all of Baja California are on the Pacific Plate. Transform faults do not just connect divergent boundaries . For example, the Queen Charlotte Fault connects the north end of the Juan de Fuca Ridge, starting at the north end of Vancouver Island, to the Aleutian subduction zone .

Figure 4.7.1 Transform faults along the U.S. west coast (Steven Earle, “Physical Geology”).

As we will see in the next section, earthquakes are common along transform faults, as the two plates slide past each other.

*”Physical Geology” by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http://open.bccampus.ca

a type of fault in which two pieces of crust slide past one another (4.5)

a plate boundary at which the two plates are moving away from each other (4.5)

the sloping region along which a tectonic plate descends into the mantle beneath another plate (4.6)


Which plate boundaries produce earthquakes?

Click to see complete answer. Just so, what plate boundaries cause earthquakes?

Movement in narrow zones along plate boundaries causes most earthquakes. Most seismic activity occurs at three types of plate boundaries&mdashdivergent, convergent, and transform. As the plates move past each other, they sometimes get caught and pressure builds up.

Likewise, how do divergent boundaries create earthquakes? Earthquakes at divergent plate boundaries occur as new crust is created and other crust is pushed apart. This causes the crust to crack and form faults where earthquakes occur. Most earthquakes at divergent plate boundaries occur at mid-ocean ridges where two pieces of oceanic crust are moving away from each other.

Accordingly, do earthquakes occur at divergent plate boundaries?

Divergent boundaries are those at which crustal plates move away from each other, such as at midoceanic ridges. Divergent faults and rift valleys within a continental mass also host shallow-focus earthquakes. Shallow-focus earthquakes occur along transform boundaries where two plates move past each other.

Why do convergent plates cause earthquakes?

Earthquakes at convergent plate boundaries mark the motions of subducting lithosphere as it plunges through the mantle (Figure below). Eventually the plate heats up enough deform plastically and earthquakes stop. Convergent plate boundaries produce earthquakes all around the Pacific Ocean basin.


4.8 Earthquakes and Plate Tectonics

An earthquake is the shaking caused by the rupture (breaking) and subsequent displacement of rocks (one body of rock moving with respect to another) beneath Earth’s surface.

A body of rock that is under stress becomes deformed. When the rock can no longer withstand the deformation, it breaks and the two sides slide past each other. Because most rock is strong (unlike loose sand, for example), it can withstand a significant amount of deformation without breaking. But every rock has a deformation limit and will rupture (break) once that limit is reached. At that point, in the case of rocks within the crust , the rock breaks and there is displacement along the rupture surface. The magnitude of the earthquake depends on the extent of the area that breaks (the area of the rupture surface) and the average amount of displacement (sliding).

Most earthquakes take place near plate boundaries, but not necessarily right on a boundary, and not necessarily even on a pre-existing fault. The distribution of earthquakes across the globe is shown in Figure 4.8.1. It is relatively easy to see the relationships between earthquakes and the plate boundaries. Along divergent boundaries like the mid-Atlantic ridge and the East Pacific Rise, earthquakes are common, but restricted to a narrow zone close to the ridge, and consistently at less than 30 km depth. Shallow earthquakes are also common along transform faults , such as the San Andreas Fault. Along subduction zones earthquakes are very abundant, and they are increasingly deep on the landward side of the subduction zone.

Figure 4.8.1 Global distribution of earthquakes. Red dots indicate shallow earthquakes (<33 km deep), green and blue indicate deep earthquakes (Steven Earle, “Physical Geology”).

Earthquakes are also relatively common at a few intraplate locations. Some are related to the buildup of stress due to continental rifting or the transfer of stress from other regions, and some are not well understood. Examples of intraplate earthquake regions include the Great Rift Valley area of Africa, the Tibet region of China, and the Lake Baikal area of Russia.

Earthquakes at Divergent and Transform Boundaries

Figure 4.8.2 provides a closer look at magnitude (M) 4 and larger earthquakes in an area of divergent boundaries in the mid-Atlantic region near the equator. Here, as we saw in section 4.5, the segments of the mid-Atlantic ridge are offset by some long transform faults . Most of the earthquakes are located along the transform faults, rather than along the spreading segments, although there are clusters of earthquakes at some of the ridge-transform boundaries. Some earthquakes do occur on spreading ridges, but they tend to be small and infrequent because of the relatively high rock temperatures in the areas where spreading is taking place. Earthquakes along divergent and transform boundaries tend to be shallow, as the crust is not very thick.

Figure 4.8.2 Earthquake activity along the mid Atlantic ridge (Steven Earle, “Physical Geology”).

Earthquakes at Convergent Boundaries

The distribution and depths of earthquakes in the North Pacific are shown in Figure 4.8.3. In this region, the Pacific Plate is subducting beneath the North America Plate, creating the Aleutian Trench and the Aleutian Islands. Shallow earthquakes are common along the trench, but there is also significant earthquake activity extending down several hundred kilometers, as the subducting plate continues to interact at depth with the overriding plate. The earthquakes get deeper with distance from the trench note in the left panel in Figure 4.8.3 that as you move along the transect from point a to point b, there is a trend of increasing earthquake depth. This reveals that it is the Pacific Plate that is moving northwards and being subducted.

Figure 4.8.3 Earthquake activity along a convergent boundary at the Aleutian Islands. Red dots indicate shallow earthquakes, green and blue indicate deeper earthquakes (Steven Earle, “Physical Geology”).

The distribution of earthquakes in the area of the India-Eurasia plate boundary is shown in Figure 4.8.4. This is a continent-continent convergent boundary, and it is generally assumed that although the India Plate continues to move north toward the Asia Plate, there is no actual subduction taking place. There are transform faults on either side of the India Plate in this area.

Figure 4.8.4 The distribution of earthquakes in the area of the India-Eurasia plate boundary (Steven Earle, “Physical Geology”).

The entire northern India and southern Asia region is very seismically active. Earthquakes are common in northern India, Nepal, Bhutan, Bangladesh and adjacent parts of China, and throughout Pakistan and Afghanistan. Many of the earthquakes are related to the transform faults on either side of the India Plate, and most of the others are related to the significant tectonic squeezing caused by the continued convergence of the India and Asia Plates. That squeezing has caused the Asia Plate to be thrust over top of the India Plate, building the Himalayas and the Tibet Plateau to enormous heights.

*”Physical Geology” by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http://open.bccampus.ca

the uppermost layer of the Earth, ranging in thickness from about 5 km (in the oceans) to over 50 km (on the continents) (3.2)

a plate boundary at which the two plates are moving away from each other (4.5)

a type of fault in which two pieces of crust slide past one another (4.5)

the sloping region along which a tectonic plate descends into the mantle beneath another plate (4.6)


Contents

Subduction zones are areas where one lithospheric plate slides beneath another at a convergent boundary due to lithospheric density differences. These plates dip at an average of 45° but can vary. Subduction zones are often marked by an abundance of earthquakes, the result of internal deformation of the plate, convergence with the opposing plate, and bending at the oceanic trench. Earthquakes have been detected to a depth of 670 km (416 mi). The relatively cold and dense subducting plates are pulled into the mantle and help drive mantle convection. [6]

In collisions between two oceanic plates, the cooler, denser oceanic lithosphere sinks beneath the warmer, less dense oceanic lithosphere. As the slab sinks deeper into the mantle, it releases water from dehydration of hydrous minerals in the oceanic crust. This water reduces the melting temperature of rocks in the asthenosphere and causes partial melting. Partial melt will travel up through the asthenosphere, eventually, reach the surface, and form volcanic island arcs.

When oceanic lithosphere and continental lithosphere collide, the dense oceanic lithosphere subducts beneath the less dense continental lithosphere. An accretionary wedge forms on the continental crust as deep-sea sediments and oceanic crust are scraped from the oceanic plate. Volcanic arcs form on continental lithosphere as the result of partial melting due to dehydration of the hydrous minerals of the subducting slab.

Some lithospheric plates consist of both continental and oceanic crust. Subduction initiates as oceanic lithosphere slides beneath continental crust. As the oceanic lithosphere subducts to greater depths, the attached continental crust is pulled closer to the subduction zone. Once the continental lithosphere reaches the subduction zone, subduction processes are altered, since continental lithosphere is more buoyant and resists subduction beneath other continental lithosphere. A small portion of the continental crust may be subducted until the slab breaks, allowing the oceanic lithosphere to continue subducting, hot asthenosphere to rise and fill the void, and the continental lithosphere to rebound. [7] Evidence of this continental rebound includes ultrahigh pressure metamorphic rocks, which form at depths of 90 to 125 km (56 to 78 mi), that are exposed at the surface. [8]

The oceanic crust contains hydrated minerals such as the amphibole and mica groups. During subduction, oceanic lithosphere is heated and metamorphosed, causing breakdown of these hydrous minerals, which releases water into the asthenosphere. The release of water into the asthenosphere leads to partial melting. Partial melting allows the rise of more buoyant, hot material and can lead to volcanism at the surface and emplacement of plutons in the subsurface. [9] These processes which generate magma are not entirely understood. [10]

Where these magmas reach the surface they create volcanic arcs. Volcanic arcs can form as island arc chains or as arcs on continental crust. Three magma series of volcanic rocks are found in association with arcs. The chemically reduced tholeiitic magma series is most characteristic of oceanic volcanic arcs, though this is also found in continental volcanic arcs above rapid subduction (>7 cm/year). This series is relatively low in potassium. The more oxidized calc-alkaline series, which is moderately enriched in potassium and incompatible elements, is characteristic of continental volcanic arcs. The alkaline magma series (highly enriched in potassium) is sometimes present in the deeper continental interior. The shoshonite series, which is extremely high in potassium, is rare but sometimes is found in volcanic arcs. [5] The andesite member of each series is typically most abundant, [11] and the transition from basaltic volcanism of the deep Pacific basin to andesitic volcanism in the surrounding volcanic arcs has been called the andesite line. [12] [13]

Back arc basins form behind a volcanic arc and are associated with extensional tectonics and high heat flow, often being home to seafloor spreading centers. These spreading centers are like mid ocean ridges, though the magma composition of back arc basins is generally more varied and contains a higher water content than mid ocean ridge magmas. [14] Back arc basins are often characterized by thin, hot lithosphere. Opening of back arc basins may arise from movement of hot asthenosphere into lithosphere, causing extension. [15]

Oceanic trenches are narrow topographic lows that mark convergent boundaries or subduction zones. Oceanic trenches average 50 to 100 km (31 to 62 mi) wide and can be several thousand kilometers long. Oceanic trenches form as a result of bending of the subducting slab. Depth of oceanic trenches seems to be controlled by age of the oceanic lithosphere being subducted. [5] Sediment fill in oceanic trenches varies and generally depends on abundance of sediment input from surrounding areas. An oceanic trench, the Mariana Trench, is the deepest point of the ocean at a depth of approximately 11,000 m (36,089 ft).

Earthquakes are common along convergent boundaries. A region of high earthquake activity, the Wadati-Benioff zone, generally dips 45° and marks the subducting plate. Earthquakes will occur to a depth of 670 km (416 mi) along the Wadati-Benioff margin.

Both compressional and extensional forces act along convergent boundaries. On the inner walls of trenches, compressional faulting or reverse faulting occurs due to the relative motion of the two plates. Reverse faulting scrapes off ocean sediment and leads to the formation of an accretionary wedge. Reverse faulting can lead to megathrust earthquakes. Tensional or normal faulting occurs on the outer wall of the trench, likely due to bending of the downgoing slab. [16]

A megathrust earthquake can produce sudden vertical displacement of a large area of ocean floor. This in turn generates a tsunami. [17]

Some of the deadliest natural disasters have occurred due to convergent boundary processes. The 2004 Indian Ocean earthquake and tsunami was triggered by a megathrust earthquake along the convergent boundary of the Indian plate and Burma microplate and killed over 200,000 people. The 2011 tsunami off the coast of Japan, which caused 16,000 deaths and did US$360 billion in damage, was caused by a magnitude 9 megathrust earthquake along the convergent boundary of the Eurasian plate and Pacific Plate.

Accretionary wedges (also called accretionary prisms) form as sediment is scraped from the subducting lithosphere and emplaced against the overriding lithosphere. These sediments include igneous crust, turbidite sediments, and pelagic sediments. Imbricate thrust faulting along a basal decollement surface occurs in accretionary wedges as forces continue to compress and fault these newly added sediments. [5] The continued faulting of the accretionary wedge leads to overall thickening of the wedge. [18] Seafloor topography plays some role in accretion, especially emplacement of igneous crust. [19]


4.5 Divergent Plate Boundaries

Divergent boundaries are spreading boundaries, where new oceanic crust is created to fill in the space as the plates move apart. Most divergent boundaries are located along mid-ocean oceanic ridges (although some are on land). The 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 Earth’s surface (Figure 4.5.1). Because the new crust formed at the plate boundary is warmer than the surrounding crust, it has a lower density so it sits higher on the mantle , creating the mountain chain. Running down the middle of the mid-ocean ridge is a rift valley 25-50 km wide and 1 km deep. Although oceanic spreading ridges appear to be curved features on Earth’s surface, in fact the ridges are composed of a series of straight-line segments, offset at intervals by faults perpendicular to the ridge, called transform faults . These transform faults make the mid-ocean ridge system look like a giant zipper on the seafloor (Figure 4.5.2). As we will see in section 4.7, movements along transform faults between two adjacent ridge segments are responsible for many earthquakes.

Figure 4.5.1 Ocean floor topography. The mid-ocean ridge system can be seen as the light blue chain of mountains running throughout the oceans (http://www.ngdc.noaa.gov/mgg/image/mggd.gif). Figure 4.5.2 Closeup of the mid-Atlantic ridge system, showing transform faults perpendicular to the ridge axis. Arrows indicate the direction of plate motion on either side of the fault (USGS, Public domain, via Wikimedia Commons).

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 4.5.3). 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 magma oozes out onto the seafloor to form pillow basalts, breccias (fragmented basaltic rock), and flows, interbedded in some cases with limestone or chert. Over time, the igneous rock of the oceanic crust gets covered with layers of sediment , which eventually become sedimentary rock.

Figure 4.5.3 Mechanism for divergent plate boundaries. The region in the outlined rectangle represent the mid-ocean ridge (Steven Earle, “Physical Geology”).

Spreading is hypothesized to start within a continental area with up-warping or doming of crust related to an underlying mantle plume or series of mantle plumes. The buoyancy of the mantle plume material creates a dome within the crust, causing it to fracture. When a series of mantle plumes exists beneath a large continent, the resulting rifts may align and lead to the formation of a rift valley (such as the present-day Great Rift Valley in eastern Africa). It is suggested that this type of valley eventually develops into a linear sea (such as the present-day Red Sea), and finally into an ocean (such as the Atlantic). 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.

There are multiple lines of evidence demonstrating that new oceanic crust is forming at these seafloor spreading centers:

1. Age of the crust:

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. Furthermore, the pattern of crust age is fairly symmetrical on either side of the ridge (Figure 4.5.4).

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. It may be surprising, considering that parts of the continental crust are close to 4,000 Ma old, that the oldest seafloor is less than 300 Ma. 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. As one would expect, the oceanic crust is very young near the spreading ridges (Figure 4.5.4), and there are obvious differences in the rate of sea-floor spreading along different ridges. 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).

Figure 4.5.4 Age of the oceanic crust (http://www.ngdc.noaa.gov/mgg/ocean_age/data/2008/image/age_oceanic_lith.jpg).

2. Sediment thickness:

With the development of seismic reflection sounding (similar to echo sounding described in section 1.4) it became possible to see through the seafloor sediments and map the bedrock topography and crustal thickness. Hence sediment thicknesses could be mapped, and it was soon discovered that although the sediments were up to several thousands of meters thick near the continents, they were relatively thin — or even non-existent — in the ocean ridge areas (Figure 4.5.5). 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. 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.

Figure 4.5.5 Seafloor sediment thickness (Modified from https://www.ngdc.noaa.gov/mgg/sedthick/).

3. Heat flow:

Measurements of rates of heat flow through the ocean floor revealed that the rates are higher than average (about 8x higher) along the ridges, and lower than average in the trench areas (about 1/20th of the average). The areas of high heat flow are correlated with upward convection of hot mantle material as new crust is formed, and the areas of low heat flow are correlated with downward convection at subduction zones .

4. Magnetic reversals:

In section 4.2 we saw that rocks could retain magnetic information that they acquired when they were formed. However, Earth’s magnetic field is not stable over geological time. For reasons that are not completely understood, the magnetic field decays periodically and then becomes re-established. 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. During periods of reversed polarity, a compass would point south instead of north. Over the past 250 Ma, there have a few hundred magnetic field reversals, and their timing has been anything but regular. The shortest ones that geologists have been able to define lasted only a few thousand years, and the longest one was more than 30 million years, during the Cretaceous (Figure 4.5.6). The present “normal” event has persisted for about 780,000 years.

Figure 4.5.6 Magnetic field reversal chronology for the past 170 Ma (Steven Earle after: http://upload.wikimedia.org/wikipedia/en/c/c0/Geomagnetic_polarity_0-169_Ma.svg). Figure 4.5.7 Pattern of magnetic anomalies in oceanic crust in the Pacific northwest (Steven Earle, “Physical Geology”).

Beginning in the 1950s, scientists started using magnetometer readings when studying ocean floor topography. The first comprehensive magnetic data set was compiled in 1958 for an area off the coast of British Columbia and Washington State. This survey revealed a mysterious pattern of alternating stripes of low and high magnetic intensity in sea-floor rocks (Figure 4.5.7). Subsequent studies elsewhere in the ocean also observed these magnetic anomalies, and most importantly, the fact that the magnetic patterns are symmetrical with respect to ocean ridges. 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). The widths of the anomalies varied according to the spreading rates characteristic of the different ridges. This process is illustrated in Figure 4.5.8. New crust is formed (panel a) and takes on the existing normal magnetic polarity. Over time, as the plates continue to diverge, the magnetic polarity reverses, and new crust formed at the ridge now takes on the reversed polarity (white stripes in Figure 4.5.8). In panel b, the poles have reverted to normal, so once again the new crust shows normal polarity before moving away from the ridge. Eventually, this creates a series of parallel, alternating bands of reversals, symmetrical around the spreading center (panel c).

Figure 4.5.8 Formation of alternating patterns of magnetic polarity along a mid-ocean ridge (Steven Earle, “Physical Geology”).

a plate boundary at which the two plates are moving away from each other (4.5)


25 4.1 Plate Tectonics and Volcanism

The relationships between plate tectonics and volcanism are shown on Figure 4.3. As summarized in Chapter 3, magma is formed at three main plate-tectonic settings: divergent boundaries (decompression melting), convergent boundaries (flux melting), and mantle plumes (decompression melting).

Figure 4.3 The plate-tectonic settings of common types of volcanism. Composite volcanoes form at subduction zones, either on ocean-ocean convergent boundaries (left) or ocean-continent convergent boundaries (right). Both shield volcanoes and cinder cones form in areas of continental rifting. Shield volcanoes form above mantle plumes, but can also form at other tectonic settings. Sea-floor volcanism can take place at divergent boundaries, mantle plumes and ocean-ocean-convergent boundaries. [SE, after USGS (http://pubs.usgs.gov/gip/dynamic/Vigil.html)]

The mantle and crustal processes that take place in areas of volcanism are illustrated in Figure 4.4. At a spreading ridge, hot mantle rock moves slowly upward by convection (cm/year), and within about 60 km of the surface, partial melting starts because of decompression. Over the triangular area shown in Figure 4.4a, about 10% of the ultramafic mantle rock melts, producing mafic magma that moves upward toward the axis of spreading (where the two plates are moving away from each other). The magma fills vertical fractures produced by the spreading and spills out onto the sea floor to form basaltic pillows (more on that later) and lava flows. There is spreading-ridge volcanism taking place about 200 km offshore from the west coast of Vancouver Island.

Exercises

Exercise 4.1 How Thick Is the Oceanic Crust?

Figure 4.4a shows a triangular zone about 60 km thick within this zone, approximately 10% of the mantle rock melts to form oceanic crust. Based on this information, approximately how thick do you think the resulting oceanic crust should be?

/> Figure 4.4 The processes that lead to volcanism in the three main volcanic settings on Earth: (a) volcanism related to plate divergence, (b) volcanism at an ocean-continent boundary*, and (c) volcanism related to a mantle plume. [SE, after USGS (http://pubs.usgs.gov/gip/dynamic/Vigil.html)]
*Similar processes take place at an ocean-ocean convergent boundary.

At an ocean-continent or ocean-ocean [1] convergent boundary, oceanic crust is pushed far down into the mantle (Figure 4.4b). It is heated up, and while there isn’t enough heat to melt the subducting crust, there is enough to force the water out of some of its minerals. This water rises into the overlying mantle where it contributes to flux melting of the mantle rock. The mafic magma produced rises through the mantle to the base of the crust. There it contributes to partial melting of crustal rock, and thus it assimilates much more felsic material. That magma, now intermediate in composition, continues to rise and assimilate crustal material in the upper part of the crust, it accumulates into plutons. From time to time, the magma from the plutons rises toward surface, leading to volcanic eruptions. Mt. Garibaldi (Figures 4.1 and 4.2) is an example of subduction-related volcanism.

A mantle plume is an ascending column of hot rock (not magma) that originates deep in the mantle, possibly just above the core-mantle boundary. Mantle plumes are thought to rise at approximately 10 times the rate of mantle convection. The ascending column may be on the order of kilometres to tens of kilometres across, but near the surface it spreads out to create a mushroom-style head that is several tens to over 100 kilometres across. Near the base of the lithosphere (the rigid part of the mantle), the mantle plume (and possibly some of the surrounding mantle material) partially melts to form mafic magma that rises to feed volcanoes. Since most mantle plumes are beneath the oceans, the early stages of volcanism typically take place on the sea floor. Over time, islands may form like those in Hawaii.

Volcanism in northwestern B.C. (Figures 4.5 and 4.6) is related to continental rifting. This area is not at a divergent or convergent boundary, and there is no evidence of an underlying mantle plume. The crust of northwestern B.C. is being stressed by the northward movement of the Pacific Plate against the North America Plate, and the resulting crustal fracturing provides a conduit for the flow of magma from the mantle. This may be an early stage of continental rifting, such as that found in eastern Africa.

Figure 4.5 Volcanoes and volcanic fields in the Northern Cordillera Volcanic Province, B.C. (base map from Wikipedia (http://commons.wikimedia.org/wiki/File:South-West_Canada.jpg). Volcanic locations from Edwards, B. & Russell, J. (2000). Distribution, nature, and origin of Neogene-Quaternary magmatism in the northern Cordilleran volcanic province, Canada. Geological Society of America Bulletin. pp. 1280-1293[SE]Cordillera Volcanic Province, B.C. Figure 4.6 Volcanic rock at the Tseax River area, northwestern B.C. [SE]


Convergent Plate Boundary Development

Subduction

Where tectonic plates converge, the one with thin oceanic crust subducts beneath the one capped by thick continental crust. A subduction zone consists of material scraped off the ocean floor near the coast (accretionary wedge) and a chain of volcanoes farther inland (volcanic arc).

Terrane Accretion

Oceanic islands and continental fragments approaching the subduction zone are too thick and buoyant to subduct. Instead, they attach to the edge of the continent as accreted terranes.

Continental Collision

Continents collide where subduction completely closes an ocean. The buoyant continental crust lifts up a broad region known as a collisional mountain range.

Images above modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172

Continents grow outward as volcanic islands and continental fragments enter a subduction zone and attach to the edge of the continent. Examples of such accreted terranes are found in NPS sites in southern Alaska and northern Washington State. Sometimes plate convergence closes an entire ocean. The crusts of the continents are too thick and buoyant to subduct, forming a collisional mountain range, such as the Appalachian/Ouachita/Marathon chain in the eastern United States and the Brooks Range in northern Alaska.


4.5: Convergent Plate Boundaries - Geosciences

The relationships between plate tectonics and volcanism are shown on Figure 4.3. As summarized in Chapter 3, magma is formed at three main plate-tectonic settings: divergent boundaries (decompression melting), convergent boundaries (flux melting), and mantle plumes (decompression melting).

Figure 4.3 The plate-tectonic settings of common types of volcanism. Composite volcanoes form at subduction zones, either on ocean-ocean convergent boundaries (left) or ocean-continent convergent boundaries (right). Both shield volcanoes and cinder cones form in areas of continental rifting. Shield volcanoes form above mantle plumes, but can also form at other tectonic settings. Sea-floor volcanism can take place at divergent boundaries, mantle plumes and ocean-ocean-convergent boundaries. [SE, after USGS (http://pubs.usgs.gov/gip/dynamic/Vigil.html)]

The mantle and crustal processes that take place in areas of volcanism are illustrated in Figure 4.4. At a spreading ridge, hot mantle rock moves slowly upward by convection (cm/year), and within about 60 km of the surface, partial melting starts because of decompression. Over the triangular area shown in Figure 4.4a, about 10% of the ultramafic mantle rock melts, producing mafic magma that moves upward toward the axis of spreading (where the two plates are moving away from each other). The magma fills vertical fractures produced by the spreading and spills out onto the sea floor to form basaltic pillows (more on that later) and lava flows. There is spreading-ridge volcanism taking place about 200 km offshore from the west coast of Vancouver Island.

Exercises

Exercise 4.1 How Thick Is the Oceanic Crust?

Figure 4.4a shows a triangular zone about 60 km thick within this zone, approximately 10% of the mantle rock melts to form oceanic crust. Based on this information, approximately how thick do you think the resulting oceanic crust should be?

Figure 4.4 The processes that lead to volcanism in the three main volcanic settings on Earth: (a) volcanism related to plate divergence, (b) volcanism at an ocean-continent boundary*, and (c) volcanism related to a mantle plume. [SE, after USGS (http://pubs.usgs.gov/gip/dynamic/Vigil.html)]
*Similar processes take place at an ocean-ocean convergent boundary.

At an ocean-continent or ocean-ocean [1] convergent boundary, oceanic crust is pushed far down into the mantle (Figure 4.4b). It is heated up, and while there isn’t enough heat to melt the subducting crust, there is enough to force the water out of some of its minerals. This water rises into the overlying mantle where it contributes to flux melting of the mantle rock. The mafic magma produced rises through the mantle to the base of the crust. There it contributes to partial melting of crustal rock, and thus it assimilates much more felsic material. That magma, now intermediate in composition, continues to rise and assimilate crustal material in the upper part of the crust, it accumulates into plutons. From time to time, the magma from the plutons rises toward surface, leading to volcanic eruptions. Mt. Garibaldi (Figures 4.1 and 4.2) is an example of subduction-related volcanism.

A mantle plume is an ascending column of hot rock (not magma) that originates deep in the mantle, possibly just above the core-mantle boundary. Mantle plumes are thought to rise at approximately 10 times the rate of mantle convection. The ascending column may be on the order of kilometres to tens of kilometres across, but near the surface it spreads out to create a mushroom-style head that is several tens to over 100 kilometres across. Near the base of the lithosphere (the rigid part of the mantle), the mantle plume (and possibly some of the surrounding mantle material) partially melts to form mafic magma that rises to feed volcanoes. Since most mantle plumes are beneath the oceans, the early stages of volcanism typically take place on the sea floor. Over time, islands may form like those in Hawaii.


Watch the video: Plate Tectonics Educational Parody of Whistle by Flo Rida (October 2021).