2.4: Types of Plate Boundaries and the Features they Create

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 \(\PageIndex{1}\)). 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 \(\PageIndex{2}\)). As we will see later, movements along transform faults between two adjacent ridge segments are responsible for many earthquakes.

 

Figure \(\PageIndex{1}\) Ocean floor topography. The mid-ocean ridge system can be seen as the light blue chain of mountains running throughout the oceans. The Mid-Atlantic Ridge is the long "spine" of light blue located in the middle of the Atlantic Ocean. (http://www.ngdc.noaa.gov/mgg/image/mggd.gif).

 

Figure \(\PageIndex{2}\) 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. This magma oozes out onto the seafloor and over time is covered with layers of sediment. 

 

Figure \(\PageIndex{3}\) 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 \(\PageIndex{4}\)).

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 \(\PageIndex{4}\)), 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 \(\PageIndex{4}\) Age of the oceanic crust (http://www.ngdc.noaa.gov/mgg/ocean_a...eanic_lith.jpg).

 

2. Sediment thickness:

With the development of seismic reflection sounding (similar to echo sounding described in earlier sections) 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 \(\PageIndex{5}\)). 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 \(\PageIndex{5}\) 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 \(\PageIndex{6}\)). The present “normal” event has persisted for about 780,000 years.

Figure \(\PageIndex{6}\) Magnetic field reversal chronology for the past 170 Ma (Steven Earle after: http://upload.wikimedia.org/wikipedi...y_0-169_Ma.svg).

Figure \(\PageIndex{7}\) 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 \(\PageIndex{7}\)). 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 \(\PageIndex{8}\). 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 \(\PageIndex{8}\)). 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 \(\PageIndex{8}\) Formation of alternating patterns of magnetic polarity along a mid-ocean ridge (Steven Earle, “Physical Geology”).

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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.

 

Ocean-Ocean Convergence and the Formation of Trenches and Volcanoes

At an ocean-ocean convergent boundary, one of the plates (oceanic crust and lithospheric mantle) is pushed, or subducted, under the other (Figure \(\PageIndex{9}\)). 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 \(\PageIndex{9}\) 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 (Figure \(\PageIndex{10}\)), the deepest part of the ocean

 

Figure \(\PageIndex{10}\) The Mariana Trench, the deepest part of the ocean, is shown in dark blue. This trench is the result of an Ocean-Ocean Convergence. SRTM bathymetry map of the Mariana trench region by astrobob is licensed as CC-BY 4.0

 

Ocean-Continent Convergence and the Formation of Trenches and Volcanoes

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 \(\PageIndex{11}\)). 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 \(\PageIndex{11}\) 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).

 

Continent-Continent Convergence and the Creation of Mountains

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 \(\PageIndex{12}\)). 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 \(\PageIndex{12}\) 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. The Rocky Mountains in North America are also a result of continent-continent collisions.

 

Figure \(\PageIndex{13}\) The Appalachian Mountains resulted from several continent-continent collisions occurring millions of years ago. Rendered landform configuration of the Appalachian mountains and the US east coast region by maps-for-free.com is licensed under the public domain.

 

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 \(\PageIndex{1}\)). 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 \(\PageIndex{1}\) 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.

 

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