VOLCANISM

 

Of course, intimately connected to plate tectonics & subduction is volcanic activity. Volcanism is the process associated with the transfer of molten material from the interior of a body to its surface. It is a major process in sculpting the landscapes of both Earth and Mars, although it operates in slightly different ways on these planets.

 

It is surprising to most people (including me!) that the addition of heat to a mass of solid rock is not the main mechanism by which rock becomes molten. Rather it is usually a combination of decompressional melting and hydration-induced melting.

 

 

Mount St. Helens

 

	Most liquids, water being an very important exception, will solidify when compressed sufficiently. This is because in the liquid form, the molecules are spaced sufficiently apart to make them free to roam around. Compressing them enough to force the molecules into “touching” hinders their motion, often allowing them to be bonded to one another, which is what a solid is. Decompressing them allows them to more easily break free and roam  become liquid.   Because of this, rising regions of the mantle, which are undergoing solid-state convection, may melt as they approach the surface. This mechanism encourages the production of magma near oceanic spreading zones and continental rift zones.   Adding water to a mixture of minerals can have the effect of lowering their melting point, again making the minerals malt more easily.
		This latter process helps provide a mechanism to melt rock on an oceanic plate that is being subducted below a continental one. The water that is carried along with the rock (sometimes as part of its structure) will induce partial melting, and promote volcanic activity near these subduction zones.

 

Mineral Types with Depth

 

The major rock type of the Earth’s crust and mantle is perodotite. The major minerals that make up this rock are olivine, pyroxene, plagioclase feldspar, spinel and garnet (see Table 2.1 in your textbook). While olivine and pyroxene are stable to depths of hundreds of km, the others are not. Plagioclase feldspar in only stable at lower pressures (depths less than 30 km), and is gradually replaced by spinel and then garnet at greater depths.

 

Composition of the Melt

 

The basalt rock that actually makes it to the surface has had its chemistry altered from that of regular peridotite, being enriched in some elements and depleted in others. Further separation occurs during partial melting, as some elements “prefer” the melt over the solid. The exact composition depends of the degree of melting, and for example, plagioclase feldspar melts at a lower T than olivine or pyroxene.

 

The first atoms to enter the melt are the most volatile: Na and K (sodium & potassium), followed by less volatile ones.

 

Ancient v Modern

 

The exact composition of the magma produced depends on many factors. Those produced when the TPs were very young and hotter to great depths could incorporate material from deeper inside to produce what geologists call Komatiite. These are are on today’s Earth, but are expected to be more common on Mars.

 

Effusive Volcanism & Lava Flows

 

 

Explosive Volcanism

 

Much more dangerous than effusive volcanism  these can be killers. What happens depends on the viscosity and amount of dissolved gases in the magma.

 

Ascent due to buoyancy (less dense material float to the surface) and pressure from below. During ascent. The lowering of pressure allows gas bubble to form & expand, adding even more pressure.

 

In low-viscosity magmas, this can result in fire mountains. High viscosity magmas allow higher pressures to be built up, and a more explosive eruption onto the surface. These can produce:

Semi-molten blob of lava (lava bombs)   Pumice (solidified magma with gas bubble frozen in)   Ash (small particles)
	A lava bomb

 

These are ejected in an eruption column. The heat within the columns can carry this material further upward by inducing convection. If the lighter material reaches sufficient height, it can be carried by high-level winds over great distanced. The rest usually collapses (including air, ash, and other debris) in a pyroclastic flow, which is usually the BIG killer, since it can travel very fast and engulf everything in its way. The USGS has a little video at http://volcanoes.usgs.gov/Movs/anim_pf.mov and you can see a nice movie of one at http://faculty.gg.uwyo.edu/heller/SedMovs/Pyroclastic.htm There is also a host of information at the Volcanoes of the World web site: http://www.volcano.si.edu/world/

 

 

 

 

 

 

Effects of Environment

 

Local Surface Gravity

 

Buoyancy is caused by the difference in the force of gravity on one material compared to that of its surroundings. The greater the difference in density, and the greater the strength of gravity, the greater the buoyancy will be.

 

With less gravity, the depth at which the pressure allows dissolved gases to form bubble is deeper.

 

Lower gravity also requires thicker lavas to be formed before it will flow:

 

 

The surface gravity of Mars is less than that of the Earth, so all of these will make volcanism on Mars a little different than on Earth. For example, because  is smaller on Mars than the Earth, the flows on Mars will have to be thicker.

 

Surface & Atmospheric T

 

 This will obviously affect how rapidly molten rock will cool, and hence how far it will flow.

 

Atmospheric Pressure

 

Given the low atmospheric pressure on Mars, the effect of the convective region above the eruption column will affected.

 

 

 

Olympus Mons

3 Stages of Crust Development

 

Primary Crust - original solidification, subject to heavy impact during early solar system (“late heavy bombardment period”).    Secondary Crust - hot mantle material reaches surface through volcanic action and new crust exposed in seafloor spreading.   Tertiary Crust - further re-working of crust material by subduction, metamorphic processes, erosion, and sedimentation.   On Earth:   Today: no primary crust remaining, due to the effects of erosion, subduction, etc.   Secondary crust: production is still occurring at Mid-Atlantic Ridge, large shield volcanoes such as Hawaiian Islands   Tertiary crust: continents