Two "faces" of submarine volcanoes: magnificent yet ruthless

Iceland has been very restless recently. Since the end of October 2023, about 24,000 small earthquakes have been recorded on the Reykjanes Peninsula in the southwest of Iceland. According to experts, the scale of underground magma intrusion and the speed of movement indicate that there is a "high possibility" of a volcanic eruption here. In fact, there have been three volcanic eruptions on the Reykjanes Peninsula since 2021. Undersea volcanic eruptions created the beautiful Iceland, but now they are threatening Iceland's safety. How different are submarine volcanoes from land volcanoes? How harmful are they?

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Why can't seawater extinguish volcanoes?

71% of the Earth's surface is covered by vast oceans. If all the water in the oceans were drained, we would see a landform as spectacular as that on land, with mountains and valleys. The largest number of these is the scattered submarine volcanoes.

The location of volcanoes is not random, nor is it evenly distributed. Instead, they are concentrated at the boundaries of plates. When one plate collides with another, the denser plate is forced to dive downward, causing volcanic eruptions. Some submarine volcanoes erupt from mid-ocean ridges, which are like the seams on a baseball. They are the discrete boundaries of plates and the birthplace of new oceanic crust. For example, Iceland is an outcrop of the Mid-Atlantic Ridge on the surface, and active volcanic activity there indicates that the Atlantic Ocean is still expanding.

Schematic diagram of submarine volcanic eruption: 1. Water vapor cloud 2. Seawater 3. Strata 4. Lava flow 5. Magma channel 6. Magma chamber 7. Dike 8. Pillow lava (Wikimedia Commons)

In addition, some submarine volcanoes overflow from "hot spots" within oceanic plates. These are local heat sources deep in the earth. When the plates move slowly over the "hot spots" like a conveyor belt, a series of volcanoes will form, such as the dense series of volcanoes in the Pacific Ocean from the island of Hawaii to the northwest.

It has been erupting since 85 million years ago and has continued to erupt until today, forming an island chain about 6,000 kilometers long. (Source: Wikipedia)

Most submarine volcanoes are extinct, with only about a hundred still active. The strange thing is, why can't seawater extinguish submarine volcanoes? The reason is that submarine volcanoes do not spew out blazing flames, but hot liquid lava. Cold seawater can only cool the lava, but cannot extinguish the powerful volcano.

Geologists have found that due to the effect of seawater, submarine volcanoes are very different from terrestrial volcanoes. The most obvious difference is that submarine volcanoes are in a high-pressure, low-temperature environment, while volcanoes in the deep sea generally do not produce volcanic ash due to the strong pressure of the overlying water body. In addition, the lava of submarine volcanoes cools much faster than that of terrestrial volcanoes. Once the lava comes into contact with seawater, its surface will quickly condense to form a solid shell, which wraps the lava inside, thus forming an irregular oval shape with a diameter ranging from a few centimeters to a few meters, which looks like a pillow, so it is called pillow lava.

Pillow lava produced by Hawaiian submarine volcanoes (Source: Wikipedia)

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God's work : Wonders created by underwater volcanoes

Undersea volcanic eruptions are an important force that changes the earth's appearance. In the ocean, many volcanic islands are formed by volcanic eruptions.

There is a strange island in the South Shetland Islands at the northeastern end of the Antarctic Peninsula, called "Deception Island", across the sea from South America. If we look down from above, we will find that the shape of Deception Island is indeed very strange, with a horseshoe-shaped outline. In fact, it is a ring-shaped island with a diameter of about 12 kilometers, filled with sea water in the middle, and connected to the sea through a gap in the southeast.

Geologists have discovered that Deception Island is actually a volcanic island. In ancient times, a strong volcanic eruption formed an island in the ocean. Later, the crater collapsed and seawater poured in, turning it into what it is today. To this day, the Deception Island volcano is still active. The coast of the island is often steaming. If you dig up the black rocks and sand, you may find hot springs gushing out, so it has become a famous tourist destination.

Satellite image of Deception Island taken by Sentinel-2 (Source: Wikipedia)

In the Atlantic Ocean in the northwest of the African continent, there is an island group consisting of 7 small islands, called the Canary Islands. The Canary Islands are lined up in the Atlantic Ocean from east to west, like a beautiful string of pearls. In terms of origin, they are all volcanic islands. El Hierro Island is the smallest and youngest one among them, formed about 1.2 million years ago. It is about 29 kilometers long from east to west and about 16 kilometers wide from north to south, with an area of ​​only 268.71 square kilometers. Looking down from above, it presents a triangle with three concave sides, which is somewhat similar to an irregular heart-shaped pattern.

To this day, the island still retains very typical volcanic relics. According to statistics, there are more than 500 volcanic cones, more than 70 lava caves and passages on the island, which can be regarded as a natural laboratory for studying volcanoes . In addition, the geological structure here is still very active today, and earthquakes and volcanoes occur from time to time.

The Galapagos Islands, located in the vast Pacific Ocean off the west coast of South America, are also famous volcanic islands, consisting of 21 small islands and hundreds of reefs, with a total area of ​​about 7,880 square kilometers. The Galapagos Islands are located in a volcanically active area because it is located at the junction of the smaller Nazca Plate and the larger South American Plate. The Nazca Plate drifts eastward and dives under the South American Plate at a rate of 6.4 cm per year. Therefore, the crustal activity in the area is very active and volcanoes erupt frequently.

There are many craters not only on the island, but also on the seabed . In 1977, oceanographers took the manned submersible "Alvin" to the seabed more than 2,500 meters deep near the Galapagos Islands for exploration and found some vents with black smoke. The temperature was very high and they were vividly called "undersea black chimneys". These hydrothermal fluids are rich in minerals and nourish some organisms. They can exist independently without sunlight, forming a unique underwater extreme environment ecosystem .

La Cumbre volcano on Fernandina Island in the Galapagos Islands (Source: Wikipedia)

Alvin (Source: Wikipedia)

Giant tube worms gather around vents in the Galapagos Rift Valley (Source: Wikipedia)

In addition, the Hawaiian Islands, the Aleutian Islands, and South Korea's Jeju Island are all formed by the eruption of submarine volcanoes. According to statistics, about 85% of the world's active volcanoes are located in the Pacific Ocean. Its east coast and west edge are the areas with the most intense volcanic activity in the world, so the islands here are generally volcanic islands.

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Ruthless : The huge dangers of submarine volcanoes

Although submarine volcanoes are far from human settlements, they are still very dangerous when they reach a certain size. Not only can they spew out a large amount of harmful gases, but they may also disturb the sea water to form a tsunami . From January 14 to 15, 2022, the submarine volcano of Hongaha Apai Island in the South Pacific island nation of Tonga erupted violently, attracting global attention and concern. The Tonga Hongaha Apai Island volcano was originally an underwater volcano, which has gradually emerged from the water due to multiple eruptions in recent years.

The eruption triggered a massive tsunami that affected almost the entire Pacific coast. According to statistics, 85% of the population of Tonga was affected, and about 600 buildings were destroyed by the tsunami. Even the northern coast of Chile, more than 10,000 kilometers away, was hit by huge waves more than two meters high, and some urban areas were flooded with seawater.

In addition, submarine volcanic eruptions can also damage the ecological environment . Between October 2011 and March 2013, submarine volcanoes near El Hierro Island in the northwest of the African continent erupted, releasing a large amount of heat and carbon dioxide, causing the disappearance of a large number of plants and animals in the surrounding seas. Volcanologists used unmanned underwater detectors to investigate and found that after many years, the marine ecosystem near El Hierro Island has gradually recovered, and many zooplankton, phytoplankton, fish, shrimp, crabs, and shellfish have reappeared in the lava caves.

One day in October 2021, someone found a large number of rocks floating on the sea in Okinawa, Japan, and many ships were trapped in the port. What happened? After investigation, it was found that these rocks came from the Ogasawara Islands more than a thousand kilometers away. Not long ago, an underwater volcano erupted, and the pumice floated on the sea, and later reached the coast of Okinawa Prefecture with the ocean current to the west.

The surface of pumice is very rough and full of pores. Its density is lower than that of seawater, so it can float on the water. When magma erupts from an undersea volcano, the pressure drops rapidly, and the solubility of the gas in the magma decreases accordingly, and the gas in it continues to escape. This process is like seeing a lot of bubbles coming out when we open a carbonated drink. However, as the pressure decreases, the temperature of the lava also drops sharply, and many bubbles are condensed in the rock before they can escape, so pumice with pores all over the body is formed.

Pumice stone covered with pores (Photo: Ma Zhifei)

Pumice is not a good thing for ocean shipping . Pumice of all sizes emerge from the seabed, like floating islands, called "pumice rafts", which can be more than tens of kilometers long. Once this happens, it is usually difficult to disperse in a short time, affecting the normal passage of ports and even causing ship engine failures and navigation accidents.

Marine biologists are also very worried about the appearance of pumice. They believe that pumice will affect the direct sunlight to the seabed , which is not conducive to the growth of plankton and has a destructive effect on the marine food chain . In addition, floating pumice may also become a "natural ferry" for some alien species, such as some shellfish, which float on pumice rafts to land or islands they have never been to before, and may become invasive species, threatening the survival of other organisms.

Author: Ma Zhifei, a popular science writer and member of the Beijing Science Writers Association, has written more than 10 popular science books, including "A Dream of Red Mansions" and "Glass Earth". He has won the Ministry of Natural Resources' Excellent Popular Science Book Award, the Wu Dayou Popular Science Book Award, and the China Natural Resources Writers Association's "China Gem Literature Award" Popular Science Works Award.

Reviewer : Dong Hanwen, Associate Researcher, Institute of Geology, Chinese Academy of Geological Sciences

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.