Class 18 - Volcanism + Tectonics

First, READ pp 237-244 - about petrology - rock types. You can skim this pretty cursorily - I am not going to test you on mineral types when my knowledge does not extend much beyond GRANITE vs. BASALT.

Main READING = Chpater 10.

Volcanism

Just like the cratering chart in the last class, this chart shows how the FORMATION PROPERTIES relate the FACTORS that CONTROL GEOLOGY to VOLCANISM. The most obvious and important arrows are those linking the size of the planet (and composition) to the internal temperature and to volcanism.

Figure 10-4 shows a range of volcano types. The most basic way to think of volcanism is just molten rock reaching the surface. But the nature of the volcanism depends on (a) the viscosity of the lava (which tends to depend exponentially on temperature) and (b) the eruption quantity. Here are a couple of diagrams similar to Figure 10-4

And for VERY viscous lava...

- appropriate for Venus - see Figs 10.54 and 10.55. - there are much better examples HERE.

Finally - think about the BIG volcanos of the solar system....

What allows Mars to have the biggest mountain in the solar system? (i) a volcanic plume persistently kept erupting lava onto the surface from one location (unlike on Earth where the crustal plates move around relative to the underlying convection; and (ii) lower gravity means that the mass of lava on top weighs less; and (iii) Because Mars cooled down, the crust is thicker and cooler - so more able to hold up a big mountain.

Tectonics

Although there are some notable exceptions (e.g. cracking due to shrinking of crust on cooling) tectonics tends to go along with volcanism - so, take a look at the tectonics chart and see if the cause-and-effect relationships make sense.

Note the similarities with volcanism - similar links. The controlling factors that lead to more volcanism also tend to result in more tectonics.

Plate tectonics is an extreme form of tectonics - the whole surface is turned over, driven by convection in the mantle, driving the plates into each other (-> mountain chains), apart at ocean ridges and sinking at subduction zones.

VENUS

So - why is Venus so very different from Earth?

"Is Venus dead, asleep or just confused?" - David Grinspoon, author of Venus Revealed.

Volcanism on Venus - there are both low-viscosity lavas (shield volcanoes, lava rivers, festoons) as well as viscous lava (domes, coronae). The uniform cratering over the surface and the crater density tells us that the whole surface was re-surfaced about 600 million years ago.

Tectonics on Venus - lots of cracks - on the plains and around volcanoes. But the evidence for plate tectonics is limited (no substantial mountain ranges, no subduction zones, no clear evidence of continents.

WHY no plate tectonics on Venus? There are 2 possible reasons:

(1) what drives the plate tectonics on earth is as much the sinking of plates in subduction zones as the convection - when basalt and water are under pressure - 60 km down in the subduction zone - the plate becomes eclogite, a denser rock - and the tip of the plate sinks, pulling the plate with it downwards. But you need WATER in the crustal rock.

(2) to drive mantle convection you need a temperature GRADIENT - a difference in temperature between the bottom and the top. But with Venus the greenhouse effect keeps the top warm - supressing convection. (Could this lead to an unstable, oscillating equilibrium leading to sporadic eruptions?)

And why not a magnetic field? Contrary to common myth, the problem is not a lack of rotation at Venus. Remember, the Earth's dynamo is driven by convection caused by the compoitional change at the inner core (where pure iron solidifies out of an iron/sulfur or iron/oxygen mix in the outer core). In Venus there probably is not a solid inner core - could this just be due to the lower central pressures due to the 81% mass of Venus (compared with Earth) preventing the inner core from solidifying? That's Dave Stevenson's story. Or, perhaps the warmer mantle (and warmer crust) prevents sufficient temperature gradients developing and inhibiting convection.

Thus, the major differences between Venus and the Earth could be due to the small initial differences in mass, composition, distance from the Sun - but the runaway greenhouse effect on Venus means Venus is DRY and HOT at the surface - the atmosphere perhaps having an important influence on the planet's geology.