Monowai (seamount) facts for kids
Monowai is an active underwater volcano, also known as a seamount, located north of New Zealand. It's made up of a large bowl-shaped hollow called a caldera and a volcanic cone. The top of this cone is usually about 100 metres (330 ft) below the ocean surface. However, its depth changes often because of ongoing volcanic activity. This includes parts of the volcano collapsing and new lava domes growing. Scientists first noticed volcanic activity here between 1877 and 1924. The volcano was officially named "Monowai" in 1980, after a research ship.
Monowai is part of the Tonga-Kermadec Ridge, where the Pacific Plate is slowly sliding under the Australia Plate. This movement causes lots of volcanic and hot water (hydrothermal) activity. Monowai is one of the most active volcanoes in the Kermadec volcanic arc. It might even be the most active underwater volcano in the world! Since 1977, it has erupted many times. When it's active, you can see gas coming out and the water changing colour. There are also many earthquakes and the volcano grows quite quickly. Hot water vents on Monowai are home to many different kinds of sea creatures.
Contents
Discovering Monowai
Volcanic activity at Monowai was first noticed between 1877 and 1924. A shallow area was marked on maps in 1944, but this might have been a floating raft of pumice rock or just disturbed water. Scientists officially recognized Monowai as a volcano in 1977. It was named in 1980 after the research ship HMNZS Monowai (A06). The large caldera was only discovered much later, in 2004. Researchers have explored Monowai using special underwater vehicles like Pisces V and ROPOS, and during many research trips. It's also sometimes called "Orion seamount".
Where is Monowai?
Location in the Pacific
Monowai is found in the southwestern Pacific Ocean, north of New Zealand. It sits about halfway between Tonga and the Kermadec Islands. This area is part of the Tonga-Kermadec Ridge, which is a long chain of volcanoes. This chain is about 2,500 kilometres (1,600 mi) long and includes about 12 volcanic islands and at least 37 underwater volcanoes. These volcanoes are spaced out about every 50 kilometres (31 mi). Many of them have only recently been found and haven't been studied much. Hot water activity has been seen at many of them.
Monowai is one of the busiest volcanoes in the Kermadec arc. Other active volcanoes nearby include Raoul Island and Rumble III. Several others, like Clark and Brothers, also have hot water vents.
How Monowai is Shaped
Monowai is a large volcano. It's shaped like a cone, about 8 kilometres (5.0 mi) wide. It rises 1.2 kilometres (0.75 mi) from the seafloor, which is about 1,000–1,500 metres (3,300–4,900 ft) deep. The very top of the volcano is less than 100 metres (330 ft) below the ocean surface. The exact height of the summit changes often because of the ongoing volcanic activity. For example, its height changed by up to 176 metres (577 ft) between surveys done in 1998, 2004, and 2007. Recent eruptions have made the volcano's sides smoother and covered them with small volcanic rocks like lapilli and scoria.
North of the main Monowai volcano is a large, deep caldera. This caldera is the biggest in the northern Kermadec arc, covering an area of 84 square kilometres (32 sq mi). It looks like it might be two calderas nested inside each other. The edge of the caldera is about 500–1,000 metres (1,600–3,300 ft) deep.
Smaller cones can be found around the caldera and even inside it. One cone inside the caldera is about 250 metres (820 ft) tall. Another cone, about 500 metres (1,600 ft) high, sits between the main Monowai cone and the caldera. The entire volcanic area covers about 530 square kilometres (200 sq mi).
Hot water (hydrothermal) activity happens at a place called Mussel Ridge inside the caldera. Here, several vents release warm water, less than 60 °C (140 °F). More vents are found on the main Monowai cone and its sides. Scientists also think there are more vents deeper down because they've seen large plumes of hot water. Sometimes, plumes of water that aren't hot also come from Monowai. These might be caused by landslides or material being moved by eruptions.
What Monowai is Made Of
The volcanism in the Kermadec arc happens because the Pacific Plate is sliding under the Australian Plate. This movement happens at a speed of about 24 centimetres per year (9.4 in/year).
Monowai has erupted different types of rocks. Most of the main cone is made of basalt, which is a dark, heavy volcanic rock. Around the caldera, you can find andesite and basaltic andesite. These rocks contain small crystals of minerals like olivine and plagioclase. The molten rock (magma) that forms Monowai comes from deep within the Earth, from a part called the mantle wedge. This magma then changes in a magma chamber at very high temperatures, between 1,080–1,200 °C (1,980–2,190 °F), before it erupts.
Life Around Monowai
The hot water vents at Mussel Ridge are home to a rich variety of sea life. You can find anemones, crabs, crustaceans, fish, mussels, polychaetes, shrimps, sponges, and tube worms. There are so many mussels that they can completely cover the seafloor! Fish and mussels have also been seen on the main Monowai cone.
Tiny living things called microbes use chemicals from the Monowai volcano to create their own food. This process is called chemosynthesis. These microbes then become a food source for other animals. After eruptions, the volcanic ash and material can also make the ocean water richer in nutrients, which helps tiny plants called phytoplankton to grow.
Eruptions at Monowai
It's hard to know the exact history of Monowai's eruptions because there isn't a way to date them precisely. For example, we don't know if the two nested calderas formed during the same eruption. However, the smaller cones around the caldera seem to be older than the main Monowai cone. Most of the volcano probably formed within the last 780,000 years. The caldera likely formed without a huge explosion, and regional movements of Earth's plates might have played a role.
Today, activity happens at the top of the Monowai cone. This includes earthquakes, discoloured water, gas coming out, and floating pumice rafts. People have also heard rumbling sounds and seen water bubbling up. Underwater, this activity creates new cones, debris flows, lava flows, and fast-moving pyroclastic flows. Parts of the volcano also collapse, and new lava domes grow, which has caused the summit to shift.
Several groups of seismic swarms (many small earthquakes) have been observed at Monowai. A strong swarm in May 2002 might have been linked to a part of the volcano collapsing. Sounds from the volcano have even been recorded as far away as Ascension Island in the Atlantic Ocean!
Monowai is a very fast-growing volcano. It can grow between 0.004–0.02 cubic kilometres per year (0.00096–0.00480 cu mi/a). During some periods, the amount of magma it produces is even higher than many other ocean volcanoes, like those in Hawaii. This fast growth is balanced by frequent landslides and collapses that move material down its sides. While these landslides are smaller than those at other volcanoes, they happen very often. Underwater landslides can sometimes cause tsunamis, but there's no evidence that eruptions at Monowai have caused tsunamis.
The first observations of volcanic activity at Monowai were in 1977 and 1978. The most recent eruption sequences might have happened in October 2014 or May 2016. During these times, pumice rafts and discoloured water were seen. Both events were accompanied by earthquakes, suggesting that between April 2014 and January 2017, there were about two eruptions every month. Monowai might truly be the most active underwater volcano in the world.
