670 nature geoscience | VOL 3 | OCTOBER 2010 | www.nature.com/naturegeoscience

news & views

at continental rift zones, the continental lithosphere is stretched and heated in response to either regional pulling

forces from plate tectonics or upwelling within the underlying mantle. Over millions of years, the stretching and heating concentrate in narrow regions within the ever-broadening rift zone. Eventually, the continental lithosphere ruptures, seafloor spreading commences and new oceanic lithosphere is formed. The wide zones of stretched and heated lithosphere on either side of the new oceanic spreading centre cool, subside and accumulate sediments; they become passive margins (Fig. 1). Passive margins were traditionally considered to be tectonically inactive, with magmatism and active faulting restricted to the new mid-ocean ridge. But this need not always be the case: writing in Nature Geoscience, Pallister et al.1 document a brief, intense period of magma intrusion and surface faulting in the Harrat Lunayyir lava field, Saudi Arabia, which covers part of the eastern passive margin shouldering the Red Sea rift, indicating that potentially damaging volcanic eruptions and earthquakes can and will occur along passive margins.

In some continental rifts, the initial stages of plate stretching and faulting are accompanied by magmatism and the newly emplaced volcanic material can accommodate some of the plate extension. In other rifts, volcanism only commences during later stages, after the continental plate has ruptured and the sea floor has started to spread2. Surface volcanism is often spectacular and easily seen, but the intrusion of magma in the form of dykes is much harder to observe: the magma rarely reaches the surface, the magnitude of earthquakes generated by the volcanic intrusion are commonly below the detection threshold of the global seismic network, and surface faulting may be minor. Thus, the volume of magma trapped within the plate, and its role in achieving the plate stretching, remains poorly constrained. Furthermore, it is unclear how the underlying mantle affects the distribution of magmatism after the plate has ruptured, or how strain is distributed in the plate after the onset of seafloor spreading.

Pallister et al.1 present seismic, geodetic and field observations that document the intrusion of a volcanic dyke in the Harrat Lunayyir lava field in 2009. Their seismic

and satellite geodesy data document ongoing deformation four million years after the onset of seafloor spreading. That is, the passive margin is actively undergoing extension through the intrusion of long, thin sheets of magma in the crust and the formation of faults above these dykes. Dykes transport magma from pressurized reservoirs located at depth in the crust, and they are generally emplaced roughly perpendicular to the regional direction of stress3. Pallister et al.1 attribute the Harrat Lunayyir volcanism and faulting to the production of magma and the concentration of stress above a deep, hot zone within the mantle, which flows away from the centre of the Red Sea.

Only a few large-volume dyke intrusions have been observed within rift zones, and the intrusion of magma is often regarded as a minor contribution to plate stretching. But in 2005, intense and ongoing dyke intrusions commenced in the southern-most Red Sea rift in Afar, southwest of Harrat Lunayyir. These intrusions alerted geoscientists to the attendant seismic and volcanic hazards within active rift zones4,5. The characteristics of the 2009 Harrat Lunayyir dyke intrusion event bear striking similarities to the Afar dyke intrusions. Specifically, the dimensions of the Harrat Lunayyir dyke — about 10 km in length, 1 m in width and 5 km in height — match those observed in Afar6,7, despite the profound differences in tectonic setting. Likewise, both the Harrat Lunayyir and Afar dykes are marked by earthquakes with unusually low frequencies, interspersed with more usual tectonic earthquakes. Pallister et al.1 suggest that these unusual seismic signals result from superposed tensile opening and shear failure as magma pulses into brittle rocks. Seismic and geodetic monitoring of the subtle signals of dyke intrusions along rifts and passive margins promises to improve forward predictive models of dyke intrusion7,8.

The observation of active extension by means of dyke intrusion along the flanks of the Red Sea, nearly 200 km from the oceanic spreading centre, redefines passive margins. The 2009 dyke intrusion event could signal sporadic, but distributed, extension and

geoDynamicS

active passive marginsPassive margins were thought to be tectonically inactive. Documentation of a volcanic dyke intrusion along the eastern flank of the Red Sea rift proves this plate tectonic tenet wrong, however, with implications for hazard assessments in these regions.

cynthia ebinger and manahloh Belachew

Red SeaPassive margin

Volcanically activepassive margin Harrat Lunayyir

Continental crust

Seafloor spreadingcentre

Oceanic crust

June 2009 dyke

Far-field force

Mantle lithosphere

African plate Arabian plateMagma source?

Asthenosphere

?

Mantle hot zone?

?

?

2009 dyke30 to 0 Myr–old dyke30 to 0 Myr–old sillNormal fault

Figure 1 | Schematic diagram of the Red Sea rift. On either side of the spreading ridge, the continental lithosphere has been stretched and thinned, and hot mantle rises to fill the space created by the thinning. Active dyke intrusion and faulting are expected at or near the seafloor spreading ridge, whereas the flanking stretched continental lithosphere was thought to be inactive. Contrary to this assumption, Pallister et al.1 document dyke intrusion in Harrat Lunayyir, about 200 km away from the active spreading centre, along the eastern passive margin of the rift.

ngeo_N&V_OCT10.indd 670 21/9/10 10:39:46

© 20 Macmillan Publishers Limited. All rights reserved10

nature geoscience | VOL 3 | OCTOBER 2010 | www.nature.com/naturegeoscience 671

news & views

magma intrusion across a region about 200 km wide on either side of the central oceanic spreading ridge. Alternatively, the dyke intrusion could be tied to the presence of a long, narrow ridge of hot mantle rocks that are upwelling and generating melt further east9,10 (Fig. 1). The data from Harrat Lunayyir cannot help with the differentiation between these models, but they clearly reveal the need for a revision to current models of the distribution of faulting and magmatism at continental rupture.

The 2009 Harrat Lunayyir seismo-volcanic crisis documented by Pallister et al.1 indicates that volcanism and extension are occurring

along the eastern flank of the Red Sea rift, and provides a warning to communities along the Red Sea, the Gulf of Aden and in other youthful rift margins worldwide. Magmatic processes are a substantial natural hazard that requires investment in earthquake monitoring not only within the rifts, but also along their margins. ❐

Cynthia Ebinger and Manahloh Belachew are at the University of Rochester, Department of Earth and Environmental Sciences, Rochester, New York 14627, USA. e-mail: cebinger@ur.rochester.edu; manahloh@earth.rochester.edu

References1. Pallister, J. et al. Nature Geosci. 3, 705–712 (2010).2. Buck, W. R. in Rheology and Deformation of the Lithosphere at

Continental Margins (eds Karner, G., Taylor, B., Driscoll, N. & Kohlstedt, B.) 92–137 (Columbia Univ. Press, 2004).

3. Delaney, P., Pollard, D., Ziony, J. & McKee, E. J. Geophys. Res. 91, 4920–4938 (1986).

4. Wright, T. J. et al. Nature 442, 291–294 (2006).5. Ayele, A. et al. Earth Planet. Sci. Lett. 255, 177–187 (2007).6. Ebinger, C. et al. Annu Rev. Earth Planet. Sci. 38, 439–436 (2010).7. Keir, D. et al. Geology 37, 59–62 (2009).8. Hamling, I., Wright, T., Bennati, L., Calais, E. & Lewi, E.

Nature Geosci. 3, 713–717 (2010).9. Chang, S-J., Merino, M., van der Lee, S., Stein, S. & Stein, C.

GSA Abstr. Programs 42, 5 (2010).10. Hansen, S., Schwartz, S., Al-Amri, A. & Rogers, A. Geology

34, 869–872 (2006).

global sea-level rise resulting from climate change over the next century may affect a substantial percentage

of the world’s population living on or near the coast. An often overlooked effect of sea-level rise is the potential intrusion of salt water into the coastal groundwater aquifer systems that supply fresh water to coastal dwellers. There is a precedent for dramatic sea-level rise: by the time the ice sheets that built up over the last glacial period fully melted some 7,000 years ago, sea levels had risen by more than 100 m, compared with the lowstand of the last glacial maximum1. The effects of these changes in sea level on coastal groundwater systems can still be observed today: many areas of the continental shelf that are now covered by oceans were exposed to groundwater recharge from rainfall during this time and accumulated large volumes of fresh water. Traces of this fresh groundwater can still be detected today along the Atlantic coasts of North America2 and Europe3. Lower sea levels during the last glacial period would also have affected the groundwater flow of inland systems that are now coastal environments, and most profoundly so in aquifers such as the Floridan aquifer system, located in thick, permeable carbonate platforms, because these aquifers more readily transmit the sea-level changes into and through the groundwater system.

Writing in Nature Geoscience, Morrissey and colleagues4 use geochemical evidence from a network of wells to document substantive changes in the direction of groundwater flow beneath Florida as sea level rose at the end of the last glacial period.

One of the world’s larger aquifers, the Floridan aquifer system supplies water to all of the state of Florida, as well as to several large cities in neighbouring states. Unlike most aquifer systems, regional flow in the Floridan aquifer system is controlled to a large extent by the presence of sea water on three of its sides. Because of this unusual configuration, density differences contribute to groundwater flow in the shallow aquifers, and are suspected to drive deep regional flow as well5,6. On the surface of the aquifer lies a variably thick lens of fresh water that is separate from the underlying salt water but occasionally mixes with it (Fig. 1). At the present time, rainwater enters the aquifer at the surface, moves downwards into this freshwater lens and then laterally towards the coastline over centuries to millennia. But at depth, excess pressure from the cold, dense sea water in the Straits of Florida drives salty water westwards into and eventually through the lower aquifer7. Ambient heat rising through the Earth’s crust warms this saline, low-level groundwater slightly as it flows westwards. It ultimately exits through thermal, saline

springs along the west coast of Florida. This modern-day groundwater-flow system has been characterized by hydraulic analysis and computer simulations7,8. But hydrogeologists have mostly had to speculate on the state of the system during the last glacial period9.

Morrissey and colleagues4 measured environmental tracers in 60 wells across southern Florida at various depths, to obtain an overview of the age and sources of water that are found today in different parts of this complex system. They analysed the water for salinity, carbon and oxygen isotopes, and helium and other noble gases. The helium data were used to estimate the age of the various water samples relative to each other, and water temperature at the site of recharge was obtained from the concentrations of other noble gases. Together, these two data sets clearly differentiated between the presence of three water masses: fresh water recharged at 25 °C during the Holocene, fresh water recharged at 20 °C during the last glacial period, and salt water that has consistently been recharged at 9 °C from the Straits of Florida. On average, fresh water during the last glacial period was found to be slightly enriched with oxygen-18 relative to the Holocene fresh water, consistent with expectations given that the lighter isotopes were preferentially locked in ice sheets at the time. Overall, Morrissey and

gRounDWaTeR HyDRology

coastal flowHow groundwater flow varies when long-term external conditions change is little documented. Geochemical evidence shows that sea-level rise at the end of the last glacial period led to a shift in the flow patterns of coastal groundwater beneath Florida.

Ward e. Sanford

ngeo_N&V_OCT10.indd 671 21/9/10 10:39:46

© 20 Macmillan Publishers Limited. All rights reserved10