Volcanism On Mars - Tectonic Boundaries

Tectonic Boundaries

Tectonic boundaries have been discovered on Mars. Valles Marineris is a horizontally sliding tectonic boundary that divides two major partial or complete plates of Mars. The recent finding suggests that Mars is geologically active with occurrences in the millions of years, and there is additional speculation. There has been previous evidence of Mars' geologic activity. The Mars Global Surveyor (MGS) discovered magnetic stripes in the crust of Mars, especially in the Phaethontis and Eridania quadrangles. The magnetometer on MGS discovered 100 km wide stripes of magnetized crust running roughly parallel for up to 2000 km. These stripes alternate in polarity with the north magnetic pole of one pointing up from the surface and the north magnetic pole of the next pointing down. When similar stripes were discovered on Earth in the 1960s, they were taken as evidence of plate tectonics. However, there are some differences, between the magnetic stripes on Earth and those on Mars. The Martian stripes are wider, much more strongly magnetized, and do not appear to spread out from a middle crustal spreading zone. Because the area with the magnetic stripes is about 4 billion years old, it is believed that the global magnetic field probably lasted for only the first few hundred million years of Mars' life. At that time the temperature of the molten iron in the planet's core might have been high enough to mix it into a magnetic dynamo. Younger rock does not show any stripes. When molten rock containing magnetic material, such as hematite (Fe2O3), cools and solidifies in the presence of a magnetic field, it becomes magnetized and takes on the polarity of the background field. This magnetism is lost only if the rock is subsequently heated above the Curie temperature, which is 770 degrees C for pure iron, but lower for oxides such as hematite (ca. 650) or magnetite (ca. 580). The magnetism left in rocks is a record of the magnetic field when the rock solidified.

Mars' volcanic features can be likened to Earth's geologic hotspots. Pavonis Mons is the middle of three volcanoes (collectively known as Tharsis Montes) on the Tharsis bulge near the equator of the planet Mars. The other Tharsis volcanoes are Ascraeus Mons and Arsia Mons. The three Tharsis Montes, together with some smaller volcanoes to the north, form a straight line. This arrangement suggests that they were formed by a crustal plate moving over a hot spot. Such an arrangement exists in the Earth's Pacific Ocean as the Hawaiian Islands. The Hawaiian Islands are in a straight line, with the youngest in the south and the oldest in the north. So geologists believe the plate is moving while a stationary plume of hot magma rises and punches through the crust to produce volcanic mountains. However, the largest volcano on the planet, Olympus Mons, is thought to have formed when the plates were not moving. Olympus Mons may have formed just after the plate motion stopped. The mare-like plains on Mars are roughly 3 to 3.5 billion years old. The giant shield volcanoes are younger, formed between 1 and 2 billion years ago. Olympus Mons may be "as young as 200 million years."

Norman H. Sleep, professor of geophysics at Stanford University, described how the three volcanoes that form a line along the Tharsis Ridge may be extinct island arc volcanoes like the Japanese Island chain.