9.40 Underwater Volcano Facts

Underwater volcanoes (or submarine volcanoes) form along underwater mountain ranges or over hot spots. Explore the volcanic ocean floor and learn about underwater eruptions.

The Volcanic Ocean Floor

Did you ever see a round fish bowl made of smooth glass? The ocean is like an enormous fishbowl, but the ocean floor is not smooth. If you were to drain out the water, you’d find mountains, and many of those mountains would be volcanoes!

We find many underwater volcanoes grouped together along the mid-ocean ridges, which are like mountain ranges sprouting up from the ocean floor. These ridges are formed at the places where tectonic plates come together.

When you stand on the ground, you’re actually standing on a movable section of the earth’s crust, called a tectonic plate. Tectonic plates are large sections of the earth’s crust that float on a layer of very hot melted or molten rock. As plates on the ocean floor shift, the molten rock below pushes up through the crack, cools, and solidifies into new rock; this helps to build up the mid-ocean ridges.

Volcanoes can also happen in the middle of the ocean’s tectonic plates if the plates move over a hot spot, which is a very hot pocket of molten rock. If conditions are right, the hot spot spews molten rock up through the plate.

The products that erupt from submarine volcanoes settle on the bottom of the ocean and help to shape the ocean floor. These products can build up to become mountains that reach above the surface of the water, which creates an island. In fact, the Hawaiian Islands were formed by underwater volcanoes. If you’ve ever been to Hawaii, you’ve stood on a mountain that was once a volcano!

1. Main points:

The ocean floor is home to mountains and volcanoes, formed along mid-ocean ridges and hot spots due to shifting tectonic plates. These underwater volcanic activities contribute to the formation of islands, such as the Hawaiian Islands.

2. Questions:

• Content Analysis: What are mid-ocean ridges and how are they formed?
• Thematic Analysis: How do tectonic plates contribute to the formation of underwater volcanoes?
• Socio-cultural Analysis: What is the significance of islands like Hawaii being formed from underwater volcanoes?

3. Further Discussion:

• How do you think the discovery of underwater volcanoes has changed our understanding of the Earth?

4. Answers:

• Content Analysis: "We find many underwater volcanoes grouped together along the mid-ocean ridges… These ridges are formed at the places where tectonic plates come together."
• Thematic Analysis: "As plates on the ocean floor shift, the molten rock below pushes up through the crack, cools, and solidifies into new rock; this helps to build up the mid-ocean ridges."
• Socio-cultural Analysis: "The Hawaiian Islands were formed by underwater volcanoes. If you’ve ever been to Hawaii, you’ve stood on a mountain that was once a volcano!"

Underwater Eruptions

Submarine volcanoes are surrounded by water, which has weight. If the volcano erupts just below the surface of the water, it can overcome this weight and burst water and volcanic debris into the air. However, if the volcano is under too much water, the water’s weight presses down on the volcano and suppresses the eruption. The water quickly extinguishes the molten rock from the eruption, so the lava that flows out of an underwater volcano only travels a short distance before it cools and hardens.

1. Main points:

Submarine volcanoes are influenced by the weight of the surrounding water. If close to the surface, they can erupt forcefully, but if deeper, the water’s pressure suppresses the eruption, causing the lava to cool and harden quickly.

2. Questions:

• Content Analysis: How does the depth of water affect submarine volcanic eruptions?
• Thematic Analysis: What happens to lava from an underwater volcano eruption?
• Socio-cultural Analysis: Why is it important to understand the behavior of underwater volcanoes?

3. Further Discussion:

• What do you think would be the challenges in studying underwater volcanoes?

4. Answers:

• Content Analysis: "If the volcano erupts just below the surface of the water, it can overcome this weight and burst water and volcanic debris into the air. However, if the volcano is under too much water, the water’s weight presses down on the volcano and suppresses the eruption."
• Thematic Analysis: "The water quickly extinguishes the molten rock from the eruption, so the lava that flows out of an underwater volcano only travels a short distance before it cools and hardens."
• Socio-cultural Analysis: Understanding the behavior of underwater volcanoes is crucial for comprehending the geological processes that shape the ocean floor and potentially impact marine life.

Lesson Summary

Underwater volcanoes, or submarine volcanoes, are volcanoes that form below water. Underwater volcanoes can be found grouped together along the mid-ocean ridges, which are underwater mountain ranges that form where tectonic plates come together. Volcanoes can also happen in the middle of tectonic plates if the plate moves over a hot spot, which is a very hot pocket of molten rock. This is how the Hawaiian Islands were formed.

Additional Activities

Underwater Volcano: Discussion Questions

In this activity, you’ll check your knowledge regarding the features and significance of underwater volcanoes.

Directions

For this activity, print or copy this page on a blank piece of paper. Then, carefully read the given narrative and provide a written response to the questions that follow. You may use the web to search for ideas in answering the questions.

The Marianna Arc

The Marianna Arc lies in the Western Pacific Ocean, somewhere between Guam and the Philippines. It is a chain of volcanic mountains, both above and below the ocean surface, roughly the same area or dimensions as Japan. The region includes more than 60 active underwater volcanoes, known as seamounts. The eruption of seamounts does not cause the ocean to boil, thanks to the immense pressure. The volcanic activity is caused by the motions of two of the plates that make up the Earth’s crust. One plate is plunging below the other. The intersection between them has created the deepest spot in the oceans, a canyon known as the Mariana Trench. Sunlight barely reaches the deepwater volcanoes of the Marianna Arc. Despite this, all the expeditions in the Marianna Arc found abundant life in this seemingly hostile environment. Bacteria gather around hydrothermal vents near active volcanoes and use the released chemicals to grow. Various marine organisms, including crabs and tubeworms, survive by eating these bacteria.

Questions

1. What are seamounts?
2. Is the Marianna Arc an example of a midocean ridge? Why or why not?
3. What caused the formation of the Marianna Trench?
4. How do organisms on the deep ocean floor survive without sunlight? Explain your answer.
5. Does water in the Mariana Arc boil as the active volcanoes erupt? Why or why not?

Sample Answers

1. A seamount is an underwater mountain or volcano with steep sides rising from the seafloor.
2. Yes, the Marianna Arc is a mid-ocean ridge and consists of a long chain of volcanoes.
3. The Marianna Trench was formed from the convergence of two tectonic plates.
4. Bacteria gather around hydrothermal vents where they use the released chemicals, such as hydrogen sulfide, in a process known as chemosynthesis. These bacteria initiate the food chain, serving as a food source for other deepwater creatures.
5. No. The pressure in the Mariana Arc is high enough to prevent water from boiling during an eruption.