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Are Rift Zones

author
Earl Hamilton
• Sunday, 06 December, 2020
• 12 min read

It is the grouping of these fissures, and the dykes that feed them, that serves to delineate where and whether a rift zone is to be defined. The accumulated lava of repeated eruptions from rift zones along with the endogenous growth created by magma intrusions causes these volcanoes to have an elongated shape.

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Contents

Perhaps the best example of this is Fauna LOA, which in Hawaiian means “long mountain”, and which features two very well-defined rift zones extending tens of kilometers outward from the central vent. Rift zones are characterized by the close grouping of intrusive dykes and extrusive fissures extending outward along a relatively narrowband from the area of a central vent.

The internal extensional forces and prostatic loading generated by intruding magma volumes (either associated with the magma chamber or subsequent dyke and sill formation extending outward from that chamber), in conjunction with accumulation of erupted materials, contribute to the mass and slope of the forming edifice. Additionally, tectonic activity such as normal faulting is also commonly associated with formation of rifts along volcanic flanks.

Following the path of the least resistance, subsequent magmatic dykes form along and within these initial cracks, causing additional stresses to be imparted to the local materials of the edifice, which in turn generate new rifts for the magma to flow towards. The orientation of this rifting is largely dependent on the gravitational and tectonic stresses at play.

However, where the flanks of a volcano may be supported on one side by the presence of a pre-existing feature, or burdened with various planes of weakness, rift zone formation promulgates according to down-slope pull of gravity. The infill of magmas in the form of dykes helps to define the shape of a volcano.

A higher frequency of intrusive events along rift zones leads to elongated topographies of the affected edifices. Mathematical models show how the presence of rift zones contributes to a central horizontal bulge or ridge parallel to the orientation of the rifts.

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This same modelling shows how this central bulge is dependent on the ratio between rift zone length and depth of the magma sources, with longer fissures over shallower sources being more positively associated with very elongated topographies of the associated flanks. Occasionally, fissure eruptions associated with rift zones can actually evolve into new vents along the volcanic edifice, generating lava flows lasting for months or longer.

These mass wasting events can affect the dyke formations and orientations as the mass of the edifice shifts, which can have profound impacts on the structural development of the edifice, while also potentially creating many volcanic hazards, such as tsunamis and dramatic shifts in directions of lava flows, to unsuspecting communities. Walker stated that rift zones were common in most volcanoes around the world, regardless of their type and formation.

Walker put forward the idea that, absent any obvious signs of rifting on the surface, the presence of other volcanic features that are also associated with dyke intrusions (such as elongated cinder cones and linearly-aligned fissure vents) should also be taken to represent the presence of a rift zone-like processes in the given region. Therefore, rift zones of various lengths and widths can be tentatively identified on many stratovolcanoes and monogenetic lava fields in addition to classic Hawaiian shield volcanoes.

“The long-term growth of volcanic edifices: numerical modelling of the role of dyke intrusion and lava-flow emplacement”. ^ a b c Micron, Laurent; Carol, Valerie; Returner, Ludovic; Peatier, Aline; Villanueva, Nicolas; Stauncher, Thomas (2009-07-01).

“Edifice growth, deformation and rift zone development in basaltic setting: Insights from Piton de la Fournier shield volcano (Réunion Island)”. Recent advances on the aerodynamics of Piton de la Fournier volcano.

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Note that the rift zones tend to parallel the volcano boundaries, and avoid pointing at each other (from Fisk & Jackson 1972). At the surface they are characterized by numerous vents, fissures, earth cracks, cinder cones, graben, pit craters, and the sources of lava flows.

All of these are indications that magma preferentially intrudes into the rift zones and is also often stored there for periods of time up to a few years. The vertical air photo on the left shows of a section of the NE rift zone of Fauna LOA.

On the right is a schematic representation of Kilauea (purple) growing on the flank of Fauna LOA (green). The most popular mechanism for this outward movement is sliding along the volcano-ocean floor interface which consists of easily-deformable sediments (e.g. Nakamura 1982).

The focal mechanism for the 1975 M7.2 Katakana earthquake indicated a slip plane that was nearly horizontal with a slight dip towards at a depth consistent with the base of the volcano (e.g. Lip man et al. 1985). Such an orientation would be expected due to the downward warping of the oceanic lithosphere under the load of the island.

This shows how the seaward flank of Kilauea (and part of Fauna LOA) is pushed southward (to the right) by the intrusion of dikes down the rift zone (away from you into the plane of the diagram). Rift zones probably become preferred directions of dike propagation due to stress orientations, and they evolve thermally to perpetuate themselves.

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(Source: www.darkhorse.com)

The short white lines are the “radial rifts” that do not fall into either of the rift zones (NERO and SWR). There is nothing in this direction to buttress the flank so the continued pressure caused by numerous dike intrusions produces this seaward displacement (Swanson et al. 1976; Lip man et al. 1985).

Vertical air photo of Napa pit crater along the East rift zone of Kilauea. Napa has been almost filled by recent lava (here making it look smooth relative to the surrounding forest).

Note that vents, faults, fissures, and smaller pit craters are all aligned from the lower left (upright) to upper right (downright). Depending on whether the context is plate tectonics or volcano logy, the term rift zone” can mean two different but related things; in general terms, it can be regarded as an area where rock in the Earth’s crust has been stretched, resulting in fissures and fractures through which magma can rise, as lava, to the surface.

Basaltic, or magic, lava comes from deep in the mantle and is associated with spreading centers, or areas where continental plates are moving apart. The gradual spreading apart from the oceanic crust in these areas limits the extent to which ridges can build up, but in some particularly active areas, sometimes called “hot spots,” the new rock that is being formed relatively rapidly can reach the surface, resulting in volcanic islands such as Iceland and the Hawaiian islands.

Often, the solidified lava is harder than the surrounding rock, which erodes more quickly, leaving the dike exposed. Fissure eruptions may eject blobs of molten lava, known as “spatter,” a few meters into the air.

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(Source: www.darkhorse.com)

These can accumulate around eruption sites, forming spatter cones and more linear structures called ramparts. On Mars, the enormous canyon known as Valley Mariners is a huge rift zone that, at 2,000 miles (3,000 kilometers) long and up to 12,500 feet (3,800 meters) deep, dwarfs any similar features on our planet.

THE WILSON CYCLE: RIFTING AND THE DEVELOPMENT OF OCEAN BASINS As the concept of sea floor spreading gained acceptance in the late 60s, the consequences for geology gradually began to dawn.

One of the first to recognize how plate tectonics could be applied to the geological record was J. Ouzo Wilson. Example: the IAPETUS ocean between England & Scotland in the Lower Paleozoic, closed in the Caledonian; later opening of the Atlantic, almost in the same place.

The two diagrams below (Figs 1 & 2) illustrate some simple (if old) concepts of continental rifting (e.g. the Indiana continent) at the start of the Wilson Cycle. Uprising plume causes doming of crust with magma chamber developing underneath.

(2) The YOUTHFUL stage, lasting about 50 my after the onset of seafloor spreading, while the thermal effects are still dominant. This stage is characterized by rapid regional subsidence of the outer shelf and slope, but some graben formation may persist.

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(3) The MATURE stage during which more subdued regional subsidence may continue. (4) The FRACTURE stage when subduction starts and terminates the history of the continental margin.

The continent of Africa is thought to have been split by a series of rift valleys in various states of development. In the Red Sea area the rifting has gone so far as to form a narrow ocean.

East African Rift Valley is the classic example. Commonly the volcanism associated with these rifts is highly alkaline and under saturated in silica.

Some have ascribed rifting to up-doming of the crust over a hot-spot; certainly parts of the E African rift system are very elevated, compared with other sectors, suggesting that the doming reflects an underlying hot low-density mantle plume. In other cases, geophysical models suggest the asthenosphere mantle is rising to high levels beneath the rift.

They demonstrated / speculated that on many continents it was possible to recognize these RRR junctions. The 'failed arm' rift would eventually subside as the thermal anomaly decayed and become the site of a major depositional basin, or a major river channel and delta.

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The Venue Trough in Nigeria is regarded as an example of such a failed arm following the opening of the S. Atlantic. When oceans eventually close it is possible to recognize these failed arms as depositional basins oriented perpendicular to the collision mountain belt (most basins tend to be aligned parallel to mountain belts).

E. A common situation is that the failed arm develops into a major river system feeding the continental margin. F. Expansion of oceans on a finite earth is not possible: there must be plate subduction, somewhere, sometime.

G. Closure of oceans results in island arc development above the subduction zone. H. Continued closure results in collision with major fold and thrust belts.

Early ideas on the development of rifts are conceptualized in the diagram shown in Fig. There is notable extension, shown by the widening of the diagram block by at least 50 km.

At the same time there is uplift or ascent of the more ductile mantle, especially the asthenosphere. Progressive formation of a rift valley through extension of the lithosphere and continental crust (by about 50 km).

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(Source: www.traveldiscoverkenya.com)

Note that uprise and decompression of the underlying asthenosphere results in magma formation. Erosion takes place on the sides of the rift valley.

The first stage assumes that graben-like faults begin to form in the brittle crust. The second stage shows simultaneous necking of the lithosphere with uprise of an asthenosphere diaper.

The decompression associated with the latter causes melting of the mantle to give alkaline basaltic magmas. The third stage is accompanied by significant extension and by more uprise of the asthenosphere.

Finally, sea-floor spreading takes over as the ocean basin widens. The rift sedimentary sequence is buried beneath younger marine sediments.

Note: on this diagram the sediments at the continental margin are shown as not very thick. The real situation at passive continental margins is shown in Fig.

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(Source: traveldiscoverkenya.com)

This is typical of a number of crustal cross-sections across the continental shelf of the eastern Atlantic seaboard of North America, projected down to 30 km -- based largely on gravity and magnetic evidence, plus some seismic profiles -- and some extrapolation from land geology based on deep drill holes. The critical point is the huge thicknesses of Mesozoic and Tertiary sediments, here shown as almost 15 km, but in other cross-sections this can be even thicker.

Note that at the bottom of this pile are volcanic and volcanogenic sediments, and evaporates, which most likely are shallow water. (Based on gravity, magnetic and seismic data) Critical points regarding this profile are (a) the large thickness of post- rift sediments of Mesozoic-Tertiary age, up to 15 km, and (b) that most of these sediments are shallow-water type.

These sediments hold vast amounts of inter-granular salt water (brines). The brines may be in contact with reducing sediments, such as carbonaceous sales, also a ready supply of sulfur/sulfate.

So en route they can dissolve considerable amounts of metals. However, when they rise up the rift faults and cool, these metals will be precipitated out.

This can be enhanced because oxidizing meteoric water (groundwater) may also penetrate down these faults, so metals will be precipitated out when the two meet. (2) Rift structures are also thermally anomalous hot zones.

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(Source: photos.hgtv.com)

This is because they are frequently underlain by igneous intrusions -- granite (or perhaps in some cases' gab bro) photons. Because the rift structures remain topographically low structures for many tens of millions of years, these metals concentrations can be preserved, without being eroded, for long periods.

Mechanisms of rifting: Geodynamic modeling of continental rift systems. Plume generated triple junctions: key indicators in applying plate tectonics to old rock.

(eds) Implications of continental drift to the earth sciences. Hotspot tracks and the opening of the Atlantic and Indian Oceans.

Convective thinning of the lithosphere: a mechanism for the initiation of continental rifting. Magmatism at rift zones : the generation of volcanic continental margins and flood basalt.

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