Pits and tectonic grabens in Phlegethon Catena
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I think that those pits are produced by subsidence of material into underlying voids opened by faults into the graben.
This is an evidence of different conditions into the martian crust. On Earth, such voids could form in exactly the same way into grabens, but they are immediatelly filled with lava from bottom or sediments from above. So I never heard about similar pits, even in the most active and recent graben, the rift valley in Somalia.
On Mars, such process don't take place (or much less) so we can expect that faults can open voids down to a great depth. This is still increased by the lower gravity.
Of course there can be other explanations, for instance those pits would be steam or volcanic explosions.
This is a bit mysterious anyway. On Earth a graben is basically a system of faults which cut all the crust through, from the surface down to a depth where rocks are softer from high temperature. And the loss of pressure automatically triggers the melting of those hot rocks, leading to the formation of basaltic eruptions up to the surface. This is visible as well in a recent graben such as the Rift Valley, or in an ancient one like in Connecticut. The same should go on on Mars, especially with all the system of rifts which goes all around the Tharsis dome. Basaltic lava, being slightly lighter that the cold basaltic crust, should rise up to the surface, and fill the bottom of grabens with a flat lava flow. Instead we see the bottom faults intact and pits from subsiding materials, indicating that the lava is well bellow or even inexistent. Mystery.
Either the whole thickness of the martian crust is lighter than the molten basalt, or those grabens are really different of Earth grabens formed by plate tectonics.
This is an evidence of different conditions into the martian crust. On Earth, such voids could form in exactly the same way into grabens, but they are immediatelly filled with lava from bottom or sediments from above. So I never heard about similar pits, even in the most active and recent graben, the rift valley in Somalia.
On Mars, such process don't take place (or much less) so we can expect that faults can open voids down to a great depth. This is still increased by the lower gravity.
Of course there can be other explanations, for instance those pits would be steam or volcanic explosions.
This is a bit mysterious anyway. On Earth a graben is basically a system of faults which cut all the crust through, from the surface down to a depth where rocks are softer from high temperature. And the loss of pressure automatically triggers the melting of those hot rocks, leading to the formation of basaltic eruptions up to the surface. This is visible as well in a recent graben such as the Rift Valley, or in an ancient one like in Connecticut. The same should go on on Mars, especially with all the system of rifts which goes all around the Tharsis dome. Basaltic lava, being slightly lighter that the cold basaltic crust, should rise up to the surface, and fill the bottom of grabens with a flat lava flow. Instead we see the bottom faults intact and pits from subsiding materials, indicating that the lava is well bellow or even inexistent. Mystery.
Either the whole thickness of the martian crust is lighter than the molten basalt, or those grabens are really different of Earth grabens formed by plate tectonics.
QUOTE (Richard Trigaux @ Feb 10 2006, 09:30 AM)
Either the whole thickness of the martian crust is lighter than the molten basalt, or those grabens are really different of Earth grabens formed by plate tectonics.
Richard:
Tectonics DON'T have to mean plate tectonics!
Bob Shaw
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