The waters are obviously not very deep, and a volcano is always as "deep" as the distance from the cone (if this has one) to the Earth's mantle.

Volcanic sea mounts are like icebergs: El Hierro rises just over 1 km above sea level - and appears to run at least 8 or 9 km down to the Atlantic Ocean floor below.

Honestly, it's not likely to turn into a full-scale eruption event: So far, seismic energy released since July 17th is estimated to be a smallish amount almost equal to 650 tonnes of TNT. It is troubling that the last 300 tonnes of that energy was released mainly in October, which indicates a faster rate of energy release once the volcanic vents opened to the south of La Restinga.

There are two magma chambers involved: There's one at the Atlantic plate and African plate boundary - about 20 km below sea level on the El Golfo Bay side; and there's another one about 15 km below sea level - somewhere just below the granitic root of the island - lying directly below the north-south strike fault line that cuts the island in half.

With two chambers, it's kind of like having a catalytic convertor and a muffler: The convertor is very hot and pressurized, and the other makes an honest, gassy, car noise that won't disturb the neighbors.

At El Hierro, the honest car noise is the harmonic tremors - likely to be caused by magma moving through the pipes below. It has been reported to be something like a constant low level rumble coming from all over the island.

If you plug the muffler - you can stall a car engine - which can then backfire: The infrequent significant quakes (above 2.0 magnitude) indicate potential magma flow blockages "backfiring" as rock cleaved from the pipe surrounding the flow - creates a "temporary" pressure restriction.

Significant - aka felt - quakes can also indicate magmatic hydraulic fracturing of basaltic bedrock - as pressure builds up - or releases too quickly - for the rock to flex around the pipes and around either magma chamber.

The 20 km deep magma chamber is likely to consist of a mix of mantle magma and melt from the hydrated rock layers of the Atlantic plate slipping under the lighter African plate. Because the rock melt is a mix of hydrated rocks - there's an increased potential for gases to be dissolved into the melt, which can emerge explosively when the pressure comes off the magma. This is typical for strato-volcanoes - like Mount St. Helens - which emerge along fault lines above overlapping tectonic plates.

We have not seen an explosive eruption to date at El Hierro because the water column pressure above the volcanic vent located south of La Restinga in the Las Calmas Sea acts as a "slow-release" buffer. As the vent rises towards sea-level, the flow will become more agitated as the pressure difference between magma and water, and then open air, increases.

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As of today, Thursday, 11/10, following a 3.1 magnitude quake around 18:41 GMT 11/09 on the El Golfo side of the island to the northwest, there is NO out-gassing from the volcanic vents south of El Hierro.

Quakes have become localized but less frequent under the El Golfo Bay side of the island, which suggests that the 3.1 quake reflects a blockage of the magma pipeline from the 20 km deep magma chamber to the 15 km deep chamber under the island. CO2 levels are reported to be dropping along beaches near La Restinga, whereas yesterday, two beaches had to be closed because the CO2 concentrations were high enough to be intoxicating.

Even the amplitude of harmonic tremors has been reduced this morning. That does mean that there is still some magma movement going on below the island's sea level - but the upward pressure from 15 km to the underwater volcanic vents has probably equalized with the water column above the vents - enough to shutdown the visible eruption at the water surface.

With a little luck, the plugging of the deeper 20 km magma chamber will hold for awhile -- but we'll just have to see how things proceed day-to-day over the short term.

As I mentioned earlier somewhere else today, the worry is not an explosive eruption: The worry is the magnitude of the constant shaking under the north shore's El Golfo Bay.

The geology of the island is conducive to water intrusion under pressure. Water could lubricate several key fault planes in the structure of the island that would allow the northwest side of the island to "fail" and then cascade into the ocean, in the worst case. One key is the geometry of the north shore's escarpment: It exceeds a 34-degree angle from horizontal - and is considered unstable.

Early indication of this process could be a series of slow motion underwater rock slides over several weeks and or months, but if the shaking in El Golfo Bay were strong enough and long enough, the island could separate along the north-south strike fault as the pile of rocks eased itself into the Atlantic Ocean in a north-westerly direction.

The pressure release from the upper magma chamber would probably be slow enough that there wouldn't necessarily be an explosive event at the water surface: The escarpment would fail, and there might be a lot of gas and steam released from below into the falling rocks - increasing the turbulence of the landslide - which explains the long landslide runouts from prior landslides over 13,000 years ago - one of which is well over 60 km in length across the ocean floor and represents over 180 cubic kilometers of rock and debris.

As I understand it, the main north-south highway tunnel on El Hierro is currently closed due to a continuing threat to car traffic from very small rock slides.