Aki Roberge
April 21, 1998
Second Year Seminar Paper
Johns Hopkins University
The Planets After Formation
Tectonics:
Plate tectonics on Earth involves the formation, lateral movement, interaction, and destruction of lithospheric plates.
This complicated process is driven by the transport of internal energy, as discussed in the previous section.
The brittle lithosphere floats on top of the fluid asthenosphere. Convective currents in the asthenosphere drag the lithosphere,
which gets broken into smaller plates by the strain, forming what we call continents.
A) Continental Drift
There are eight large continental plates on the Earth: the African, Antarctic, Eurasian, Indian-Australian, Nazca, North American,
Pacific, and South American plates. The plates are rigid and deformation occurs at plate boundaries only. The plates move about
5-10 cm per year and have moved all over the surface of the Earth ever since their formation. Below is a figure showing the
positions of the continents over the last 225 million years.
The reason for continental drift is found in the convective motions of the fluid portion of the Earth's interior. In a mantle
upwelling, hot material rises from the interior and reaches the crust. It then cools and spreads out laterally away from the
upwelling. This lateral motion of mantle material drags the continents around. Thus, the continental plates are sitting on or
moving toward cold spots, except for the continent of Africa, which was the heart of the supercontinent Pangea.
B) Types of Faults
As mentioned earlier, deformation of the plates occurs only at plate boundaries, because they are quite stiff and rigid.
So, we will now focus on the types of interactions that can occur at plate boundaries, where the action occurs.
1) Divergent boundaries
At these tyes of boundaries, the plates are moving away from each other. New material from the middle mantle is added to the
oceanic lithosphere (or crust). An example of this sort of boundary is a mid-ocean ridge, such as the one that runs down the
center of the Atlantic Ocean. The new lithosphere cools and contracts as it moves away from a ridge. Thus, the ocean depth
above it increases with distance from the ridge. (Distance from the ridge is proportional to the age of the lithosphere.)
The lithosphere becomes denser than the upper mantle as it cools; remember the comments about this in the previous section.
Therefore, cold lithospheric material sinks back into the mantle at convergent boundaries.
2) Convergent boundaries
Here, the plates are approaching each other; an example of this type of boundary is an oceanic trench or subduction zone.
The subducting plate sinks under the overriding plate and can often penetrate the mantle to depths of 700 km. Although plates
are made up of oceanic and continental material, only the oceanic part of any plate is destroyed. This is due to the fact that
the continental crustal density is lower than the oceanic crustal density. If thick, low-density continental material reaches a
subduction zone, it could descend a short way, but the downward motion cannot continue because the material is always more buoyant
than the mantle material. The subduction zone therefore ceases to operate in that place and moves to a more favorable location.
Since the continental material cannot go down, the only place to go is up. Mountains can be built over convergent boundaries as a
result of continental collisions. An example of this type of mountain is the Himalaya mountain range, the tallest in the world.
To sum up, continents are like rafts of light material floating around on the mantle; they are permanently buoyant and remain
on the surface of the Earth. The dense oceanic lithosphere can be subducted under oceanic or continental lithosphere.
Thus the ocean floor is continuously being recycled; it's less than 90 million years old, which is very young in geologic time scales. The cool oceanic lithosphere is get inserted into the warm mantle; this is the main mechanism for cooling the mantle below 100 km.
Conservative boundaries
At these boundaries, lithosphere is neither created or destroyed; the plates slide relative to one another. An example of this is a transform fault, like the San Andreas in California. Below is a figure illustrating the various interactions at plate boundaries.
C) Other Planets
The Earth is apparently unique in its use of deep subduction of lithosphere as a cooling mechanism. This process can only occur if the lithosphere gets cool enough to cause it to become unstably dense and sink into the mantle. On a planet with a thicker crust, a hotter surface, and/or a colder interior, the whole lithosphere may be permanently buoyuant.
1) No tectonics
Moon:
The Moon's lithosphere is too thick to break up; thus, it has a single lithospheric plate. There are no true tectonic features on the Moon, only a few expansion/compression faults formed as the body cooled early in its history. Thus, plate tectonics cannot occur. It is a geologically dead world.
Mercury:
This world shows a few expansion/conpression faults. It has a thick lithosphere and probable no asthenosphere. Again, plate tectonics cannot occur.
Mars:
Mars is more geologically diverse, although it is also a single plate planet. Expansion and/or contraction during initial cooling produced many fractured areas and an enormous rift canyon, called the Valles Marineris. Mars also shows more volcanic features, which will be discussed later.
The above planets differ from the Earth mainly due to their smaller size, which led to more rapid cooling and thick lithospheres. Any faults or fractures are due to radial (vertical) motions, rather than the lateral movements of plates on their surfaces. These worlds transport heat out of their interiors mainly by conduction through thick, static outer shells, and therefore have very old surfaces.
Venus:
This world should be similar to the Earth, due to its similar size. Its surface is younger than the Moon's but older than the Earth's. There is some evidence for tectonic features. A region called Aphrodite Terra seems to show crustal spreading characteristic of a divergent boundary. Another region called Ishtar Terra shows compression-generated folded mountain belts, like the Himalayas, characteristic of a convergent boundary. However, we don't see true subduction zones, where lithospheric material is destroyed. We can only conclude that Venus has a significantly different tectonic style than the Earth.
Io:
This world has a very young surface and extensive volcanism, both of which usually are consequences of tectonics.
However, there is no evidence for plate tectonics, like faults or folded mountains.
2) Maybe tectonics
Ganymede:
There are many ridge and trough systems visible on parts of the surface of this moon of Jupiter, but the ridged surface is relatively old.
This suggests that perhaps Ganymede had some sort of tectonics early in its history. But again, like Venus, there are no identifiable subduction zones.
Europa:
This moon has a younger surface than Ganymede and it's covered with surface stripes and ridges.
It has been suggested that perhaps the ice-covered surface of Europa may be experiencing "pack-ice" motions on an ocean of liquid water.
This could perhaps be viewed as a sort of continental drift.
Enceladus:
This moon of Saturn has a young, uncratered surface, indicating major re-surfacing in the recent past. It is suspected that the interior is liquid
and continues to churn. The reason that Enceladus is thought to still be liquid somewhere inside is due to the fact that Enceladus and the Saturnian
satellite Dione happen to be in a tidal resonsnce with each other. This is thought to increase the amount of tidal heating experienced by them, keeping
Enceladus fluid while the other Saturnian satellites are expected to be totally frozen.
The nature of the geologic activity on Enceladus is unknown, but since a debris torus is observed in orbit near the moon,
it may spew material off the surface in geyser-like bursts.