Typical Rates of Seafloor Spreading Are Approximately

Typical Rates of Seafloor Spreading Are Approximately

4.five Divergent Plate Boundaries

Modified from “Physical Geology” by Steven Earle*

are spreading boundaries, where new is created to fill in the space as the plates motion autonomously. Most divergent boundaries are located forth mid-ocean oceanic ridges (although some are on land). The

organization is a giant undersea mountain range, and is the largest geological characteristic on Globe; at 65,000 km long and about 1000 km wide, it covers 23% of Earth’s surface (Effigy 4.5.one). Because the new chaff formed at the plate purlieus is warmer than the surrounding crust, it has a lower density so it sits higher on the , creating the mountain chain. Running downwards the middle of the mid-ocean ridge is a

25-50 km wide and 1 km deep. Although oceanic spreading ridges announced to be curved features on World’s surface, in fact the ridges are equanimous of a series of directly-line segments, offset at intervals by faults perpendicular to the ridge, called
. These transform faults make the mid-ocean ridge organisation look similar a giant zipper on the seafloor (Effigy 4.5.ii). Equally nosotros will come across in section four.vii, movements forth transform faults between 2 adjacent ridge segments are responsible for many earthquakes.

Figure 4.5.1
Ocean floor topography. The mid-ocean ridge organisation tin exist seen equally the light bluish chain of mountains running throughout the oceans (http://www.ngdc.noaa.gov/mgg/epitome/mggd.gif).
Figure 4.5.2
Closeup of the mid-Atlantic ridge system, showing transform faults perpendicular to the ridge centrality. Arrows indicate the direction of plate motion on either side of the error (USGS, Public domain, via Wikimedia Eatables).

The crustal material created at a spreading boundary is always in character; in other words, it is igneous rock (eastward.g., or gabbro, rich in ferromagnesian minerals), forming from derived from partial melting of the acquired by decompression every bit hot drape stone from depth is moved toward the surface (Figure 4.5.3). The triangular zone of partial melting well-nigh the ridge crest is approximately lx 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 fourth dimension, the igneous rock of the oceanic crust gets covered with layers of , which eventually become sedimentary rock.

Figure 4.5.1
Figure 4.5.3
Machinery for divergent plate boundaries. The region in the outlined rectangle correspond the mid-ocean ridge (Steven Earle, “Concrete Geology”).

Spreading is hypothesized to start inside a continental area with upward-warping or doming of crust related to an underlying or series of mantle plumes. The buoyancy of the pall plume material creates a dome inside the crust, causing it to fracture. When a series of mantle plumes exists beneath a big continent, the resulting rifts may align and lead to the germination of a (such as the nowadays-day Great Rift Valley in eastern Africa). It is suggested that this type of valley eventually develops into a linear sea (such every bit the present-24-hour interval Red Sea), and finally into an ocean (such equally the Atlantic). It is probable that as many as 20 mantle plumes, many of which still exist, were responsible for the initiation of the rifting of along what is now the mid-Atlantic ridge.

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

ane. Age of the chaff:

Comparison the ages of the oceanic crust near a mid-bounding main ridge shows that the crust is youngest right at the spreading center, and gets progressively older as you motion away from the divergent boundary in either management, aging approximately 1 million years for every 20-forty km from the ridge. Furthermore, the pattern of chaff historic period is fairly symmetrical on either side of the ridge (Figure iv.five.4).

The oldest oceanic chaff is around 280 in the eastern Mediterranean, and the oldest parts of the open sea are around 180 Ma on either side of the due north Atlantic. It may be surprising, because that parts of the are shut to 4,000 Ma erstwhile, that the oldest seafloor is less than 300 Ma. Of form, the reason for this is that all seafloor older than that has been either (see department 4.6) or pushed up to become function of the continental crust. Equally one would expect, the oceanic chaff is very immature near the spreading ridges (Figure 4.5.4), and at that place are obvious differences in the rate of ocean-floor spreading along dissimilar ridges. The ridges in the Pacific and southeastern Indian Oceans have broad 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 than slowly (less than 2 cm/year on each side in some areas).

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Figure 4.5.3
Figure 4.5.four
Age of the oceanic crust (http://world wide web.ngdc.noaa.gov/mgg/ocean_age/information/2008/paradigm/age_oceanic_lith.jpg).

2. Sediment thickness:

With the evolution of seismic reflection sounding (similar to echo sounding described in department 1.iv) it became possible to
see through
the seafloor sediments and map the boulder topography and crustal thickness. Hence sediment thicknesses could be mapped, and information technology was presently discovered that although the sediments were up to several thousands of meters thick virtually the continents, they were relatively sparse — or even non-existent — in the ocean ridge areas (Figure four.5.five). 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 information technology has had to accumulate sediment, and the thicker the sediment layer. Additionally, the bottom layers of sediment are older the farther you become from the ridge, indicating that they were deposited on the chaff long agone when the crust was start formed at the ridge.

Effigy 4.v.5
Seafloor sediment thickness (Modified from https://www.ngdc.noaa.gov/mgg/sedthick/).

3. Estrus flow:

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

4. Magnetic reversals:

In section 4.ii we saw that rocks could retain magnetic data that they acquired when they were formed. Withal, World’due south magnetic field is non stable over geological fourth dimension. For reasons that are not completely understood, the magnetic field decays periodically and and then becomes re-established. When it does re-establish, information technology may exist oriented the way it was before the decay, or information technology 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 merely regular. The shortest ones that geologists have been able to define lasted just a few thousand years, and the longest one was more than 30 one thousand thousand years, during the (Effigy 4.5.half dozen). The present “normal” upshot has persisted for near 780,000 years.

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Effigy iv.5.half-dozen
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).
Effigy 4.five.7
Pattern of magnetic anomalies in oceanic crust in the Pacific northwest (Steven Earle, “Concrete Geology”).

Kickoff in the 1950s, scientists started using magnetometer readings when studying body of water flooring topography. The beginning 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.vii). Subsequent studies elsewhere in the ocean too observed these magnetic anomalies, and most importantly, the fact that the magnetic patterns are symmetrical with respect to sea ridges. In the 1960s, in what would become known as the Vine-Matthews-Morley (VMM) hypothesis, information technology was proposed that the patterns associated with ridges were related to the magnetic reversals, and that oceanic crust created from cooling basalt during a
event would accept polarity aligned with the present magnetic field, and thus would produce a positive anomaly (a black stripe on the ocean-floor magnetic map), whereas oceanic crust created during a
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 unlike ridges. This process is illustrated in Effigy iv.5.8. New crust is formed (panel a) and takes on the existing normal magnetic polarity. Over fourth dimension, every bit the plates continue to diverge, the magnetic polarity reverses, and new chaff formed at the ridge now takes on the reversed polarity (white stripes in Figure 4.5.8). In panel b, the poles take reverted to normal, so in one case over again the new crust shows normal polarity earlier moving abroad from the ridge. Somewhen, this creates a series of parallel, alternate bands of reversals, symmetrical around the spreading center (console c).

Figure four.5.eight
Germination of alternating patterns of magnetic polarity along a mid-ocean ridge (Steven Earle, “Physical Geology”).

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

Typical Rates of Seafloor Spreading Are Approximately

Source: https://rwu.pressbooks.pub/webboceanography/chapter/4-5-divergent-plate-boundaries/