I understand the process of volcanism and the reason for magma deposition on the surface of the earth.

What is not clear to me, or what seems to be less well explained is why eruptions occur and then stop. What is the operative cause or causes of a flow to start or stop?

As far as my understanding goes there would seem to be two possible obvious explanations:

(1) The magma rises in bubbles, like a lava lamp, through the earth, so the flow begins when the bubble reaches the surface and stops when the bubble is exhausted.

(2) The magma rises as a constant stream, but causes earth to build up around the stream, eventually snuffing it out by mass accumulation. In this explanation, the lava constantly searches for thin spots where it erupts until it blocks itself off, then the pressure forces its way through some new location.

Are either or both of these explanations correct or incorrect? Is there a mechanism I have not considered?

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Lava eruption in Iceland the size of Manhatten that began in 2014.

  • $\begingroup$ Your explanation 2 is an extreme simplification of things are, but it's much more correct than your explanation 1. However, not all magmas are alike. Basalts behave almost entirely like your (2), but granitic magmas may have a small (1) component to their motion. $\endgroup$ – Gimelist Feb 1 '17 at 0:02

Basaltic magma (pictured above) rises for several basic reasons: 1) High temperature, low density liquid mass that is buoyant relative to surrounding rock and 2) Low crustal overburden stresses that allow upward movement of the underlying buoyant magma.

Many magmas fail to break the surface if overburden keeps them "capped", but they can flow horizontally (ie between sedimentary bedding planes) if that's where the zone of weakness lies. Many may also simply lose their heat to surrounding rock and solidify on their upward migration. In many cases, buoyancy of rising magma may meet a layer of similarly dense material, effectively stopping it in its upward track.

Heat loss is important as magmas can stop flowing once relieved reservoir pressures slow upward flow to the point that rising magma cools faster than it can be replenished from below; as the flow trickles down, what's left just cools down, solidifies in place and flow stops.

On the other hand, a large reservoir contains both enormous buoyancy and, importantly, heat. That heat, constantly replenished by a large supply below, will prevent excessive conductive heat loss to surrounding rock and solidification of magma pathways, ensuring the path to the surface stays open. It's also worth noting that the local stress field changes as a magma body is emptied and can also work, along with heat loss, to cap flow.

  • $\begingroup$ Ok, so you are saying that explanation (2) in my question is relevant? $\endgroup$ – Tyler Durden Jan 31 '17 at 20:24
  • $\begingroup$ Not really.....earth doesn't "build up" around the flow snuffing itself out nor does it "look for thin spots". It will simply flow to areas of reduced isostatic stress, be they overhead, to the left, the right, magma doesn't care; it just follows the path of least resistance. Despite having risen 30,000 feet, the island of Hawaii is an example of 'continuous' flow that STILL hasn't snuffed itself out. $\endgroup$ – Knob Scratcher Jan 31 '17 at 22:13

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