Saturday, November 23, 1991

Tropical scenery

Najwa A. Yassar

In 1931, the Waimata Valley mud volcano in New Zealand erupted, emitting 150,000 metric tons of mud. In 1936, the Sayncha mud volcano in the Apsheron Peninsula, U.S.S.R., emitted 100,000 cubic metres of mud; this covered an area of 112 ha, reaching a maximum thickness of 60 metres. In 1964, an island appeared off the southern coast of Trinidad, reaching a height of 2 metres above sea level in two days. The Chatham Island mud volcano, as it was called, was estimated at 255,000 cubic metres of mud and rock.

Although not always this spectacular, mud volcanoes are always fun to look at. They can be heard from a fair distance, constantly bubbling mud and periodically making small explosions. The water in them is so salty that they always form clearings in vegetation where no plants can grow. Old records suggest that some examples of South America occasionally erupt small fish, presumably where they encounter nearby lakes!

Map of the globeMud volcanoes are widespread, occurring in areas like Taiwan, Indonesia, Malaysia, Iran, Italy, Mexico, Colombia … the list goes on (see map). They can be anything from a few centimetres to a few kilometres wide, or small pools to hills up to a few hundred metres in height. Early researchers in the field thought that they were associated with real volcanoes and heat. In fact, a couple of hundred years ago, the inhabitants of a small Indian village in South America called Turbaco thought that they were formerly real volcanoes, until a monk sprinkles holy water on them. A few mud volcanoes are associated with heat, but these tend to be shallow hot mud erupting to the surface. Mud volcanoes are usually cold and can originate from depths of up to 3 kilometres. So what makes them erupt?

To answer that question, we must first do a bit of detective work and look at the main features common to as many examples of cold mud volcanoes as we can find. The following trends were found:

  1. They usually occur in areas of earthquake activity (see map), associated with Cenozoic fold belts.
  2. They originate from thick clay beds at depths of up to 3 kilometres.
  3. They are geologically young.
  4. They usually erupt along geological faults.
  5. They bubble salt water, gas (usually methane) and sometimes oil.
  6. They often produce blocks of rock from great depths (like real volcanoes).

Digity mud volcano, Trinidad.

Digity mud volcano, Trinidad.

Volcanologists can only imagine the source of the magma that comes out at the earth’s furface, but we are luckier with mud volcanoes. As mentioned above, mud volcanoes emit oil and gas very often; they are, in fact, usually in petroleum producing areas. Oil companies commonly drill down to several kilometres, taking core samples of the rocks encountered, so we can actually study drilled core and compare it with the mud that comes out at the surface. This includes analysis of the water chemistry, minerals, particle sizes and fossils in the core and the mud to compare their composition and age. Interestingly, the source usually coincides with clays of abnormally low density for their depth of burial, and they are always highly pressured. In fact, these overpressures cause many problems to the drillers. In Trinidad, an onshore well set up next to Digity mud volcano (opposite) disappeared overnight when it hit the source bed of the volcano. Luckily, nobody was nearby when this happened. When I visited this volcano in 1986, there were bits of the well still coming up with the mud and scattered around the cone!!

Gas bubble, Wushanting mud volcano, Taiwan.

Gas bubble, Wushanting mud volcano, Taiwan.

So the overpressures must be an important clue as to the origin of mud volcanoes. There are many ideas on what causes these overpressures. The clay beds are usually deposited very rapidly at the edge of continents, by large river deltas, or by gravity sliding down the continental slopes. Some scientists think that the clay sequences are buried so quickly that they do not have the time to get rid of all the excess water in them, so they become overpressured. Others, like me, think that seismic activity is important, in that it pumps the clays, or squeezes them, until they become overpressured. Others think that the generation of oil and gas at depth causes the pressures to go up. If we look at the features common to mud volcanoes again, we will see potential evidence for all these mechanisms. It is very likely that a combination of the mechanisms affects the clays, and their importance can certainly vary from area to area.

Gas bubble exploding

Gas bubble exploding (every 2 minutes)

But what makes the clays come up to the surface? Well, as seen above, mud volcanoes are usually associated with earthquakes and also occur along geological faults. Piparo mud volcano in Trinidad erupted violently a few years ago, coinciding with a tremor that shook and deformed a nearby house. The faults must act as paths for the deep highly pressured mud to squeeze up to the surface. It probably becomes more runny near the surface where it encounters groundwater. Seismic sections show that some mud volcanoes originate from very large lumps of mud, up to several kilometres wide, which are slowly rising up through the overlying sediments.

The study of this phenomenon is not just interesting for "scientific curiosity", it actually helps geologists understand complex processes that occur during the deposition and deformation of sediments. It also helps them understand the generation and migration of oil and gas in the sediments, as well as associated potential drilling problems.

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