Chapter 4
Chapter 4: Up from the Inferno: Magma and Igneous Rocks
Feature Articles
The Rest of the Story: Does Plate Tectonics Occur on Other Planets?
by Stephen Marshak
Our solar system contains four terrestrial planets and numerous terrestrial-like moons. Do any of these planets or moons display the consequences of plate tectonics? Planetary geologists, who have studied images of these planets taken through telescopes or from exploratory satellites, say no. The Earth appears to be the only body in the solar system to experience plate tectonics. Why? Because Earth’s interior has remained warm enough for flow to take place in its mantle. For this reason, asthenosphere can rise at mid-ocean ridges, and can move out of the way of subducting plates. The mantles of Mercury, Mars, the Moon, and Venus apparently cannot flow like that of Earth, so these planets have no active volcanoes, no continental drift, and no sea-floor spreading. Earlier in solar system history, however, when they were warmer, Mars and Venus did have volcanoes, perhaps comparable to hot-spot volcanoes on Earth. Indeed, the pattern of faults on Venus suggests that plate-tectonics-like movement struggled to begin but never quite succeeded.
The Jovian planets consist mostly of gas, so they cannot possibly have a rigid lithosphere. Curiously, though, some of the moons of Jupiter and Saturn appear to exhibit tectonic features. In fact, the Galileo satellite in 1996 photographed a volcano in the act of erupting on Io, a moon of Jupiter. But these volcanoes reflect some other type of melting process, not plate tectonics.
The Human Angle: Volatile Forces Shape the History of a Region
by Elizabeth Lane Mason
The Eastern Mediterranean and Middle East is a historically turbulent region. As the world learned on the morning of September 11, 2001, much of the turmoil is the result of violent interactions between opposing human forces. However, there are other volatile forces at work shaping the history of this region. This chaotic terrain is set in an impressively active and complex plate tectonic framework.
The plate interactions of this region are dominated by convergent boundaries involving both subduction and collision. The Arabian and African plates are pushing northward into the Eurasian plate (map). According to recent plate tectonic models, the African plate is moving northward relative to the Eurasian plate at approximately 10 mm/yr and is being subducted along the Hellenic Arc. The Arabian plate is moving north-northwest relative to Eurasia at 18-25 mm/yr. The result is a continental collision along the Bitlis-Zagros fold and thrust belt. The smaller Anatolian plate is caught in the middle of all of this movement.
Sandwiched in between the Eurasian, Arabian and African plates, the Anatolian plate suffers the effects of the differential motion of these giants. Much like a ball bearing, the Anatolian plate is being slowly spun in a counter-clockwise direction to accommodate the more prodigious northward progress and slight westward penchant of the Arabian plate relative to the African plate. This motion is largely accommodated by left-lateral slip on the East Anatolian Fault and right-lateral slip along the North Anatolian Fault (McKenzie, 1970). The movement is slow, but persistent with occasionally spectacular results.
The North Anatolian fault is a strike-slip fault that bounds the Anatolian plate from the Aegean Sea in the east to the Karliova Triple Junction in the west (map). The fault has been the setting of twelve major (magnitude 6.7 or greater) earthquakes in the past 90 years. A trend of westward migration of several of these earthquakes helped geologists and geophysicists identify the Marmara segment of the North Anatolian fault which has a seismic gap. Consequently, many Global Positioning System (GPS) stations were established in the Marmara region to continuously record movement.
On August 17, 1999 a magnitude 7.4 earthquake struck the Marmara segment of the North Anatolian fault near the city of Izmit in the middle of an area that is home to one- quarter of Turkey’s population. Despite efforts that were underway to educate and prepare the population of this earthquake-prone region, the devastating tremor claimed more than 30,000 lives and caused billions of dollars in damages. Nonetheless, this tragic event afforded an excellent opportunity for the scientific community to learn more about plate tectonics and the resultant earthquake processes. Data recorded by the GPS network is providing information about strain and deformation that may help in future efforts to predict earthquakes on the North Anatolian fault and other similar strike-slip faults such as the San Andreas fault in California (Reilinger et al, 2000).
REFERENCES
- McKenzie, D.P. 1970. Plate tectonics of the Mediterranean region. Nature 226: 239–243.
- Reilinger, R. M.N. Toksoz , S. McCluskey, and Barka, A. 2000. GSA Today 10, no.1.