6.1 - Volcanic Activity Lesson Objectives ● Explain how volcanoes form. ● Describe places where volcanoes occur. ● Describe what volcanic hot spots are and where they occur. Vocabulary ● Volcano ● Fissure ● Rift valley ● Intraplate activity ● Hot spot ● Mantle plume Introduction Volcanoes are fantastic displays of the power of the Earth. What is a volcano? How and where are they formed? Why do some places have lots of volcanoes? Where Volcanoes Are Found Volcanoes are a vent in the earth's crust through which lava, steam, ashes, etc., are expelled, either continuously or at irregular intervals. They are typically a mountain or hill, usually having a cuplike crater at the summit, formed around a vent from the ash and lava expelled through it. Volcanoes are found at plate boundaries and at hotspots. The map in Figure below shows where volcanoes are located. This map shows where many of the Earth’s volcanoes are located. Divergent Plate Boundaries There is a lot of volcanic activity at divergent plate boundaries in the oceans. As the plates pull away from each other, or diverge, they create deep fissures, an elongated fracture in the Earth’s surface. Molten lava erupts through these cracks. The East Pacific Rise is a divergent plate boundary in the Pacific Ocean (Figure below). The Mid-Atlantic Ridge is a divergent plate boundary in the Atlantic Ocean.Continents can also rift apart. When the mantle gets close enough to the surface, volcanoes form as does a rift valley. A rift valley is a long depression with very steep sides that forms as the opposite sides of a divergent boundary pull apart. Convergent Plate Boundaries Volcanoes also form along plate boundaries where the process of subduction occurs. The edges of the Pacific Plate are along subduction boundary. Lines of volcanoes can form at subduction zones on oceanic or continental crust. Japan is an example of a volcanic arc on oceanic crust. The Cascade Range and Andes Mountains are volcanic arcs on continental crust. The Pacific Ocean basin is a good place to look for volcanoes. The light blue wavy line that goes up the right-center of the diagram is the East Pacific Rise. Trenches due to subduction are on the west and east sides of the plate. Hawaii trends southeast-northwest near the center-top of the image. Volcanic Hot Spots - Video Most volcanic activity takes place at plate boundaries, but activity within a tectonic plate also occurs. This is called intraplate activity and much of it is found at hot spots. Hot spot volcanoes form as plumes of extra hot magma rises from deep in the mantle. Some volcanoes form over active hot spots. Scientists have counted about 50 hot spots on the Earth. Mantle plumes are found all over the world, especially in the ocean basins. The size of the eruptions is different at different plumes. As the tectonic plates move above a hot spot, they form a chain of volcanoes. The islands of Hawaii formed over a hot spot in the middle of the Pacific plate (video). The Hawaii hot spot has been active for tens of millions of years. The volcanoes of the Hawaiian Islands formed at this hot spot. Older volcanoes that formed at the hot spot have eroded below sea level. These are called the Emperor Seamounts. Loihi seamount is currently active beneath the water southeast of the Big Island of Hawaii. One day the volcano will rise above sea level and join the volcanoes of the island or create a new island (Figure below). A bathymetric map of Loihi seamount. Loihi will be the next shield volcano in the Hawaiian-Emperor chain. Hot spots can also be active at plate boundaries. This is especially common at mid-ocean ridges. Iceland is formed by a hot spot along the Mid-Atlantic Ridge. Hot spots are found within continents, but not as commonly as within oceans. The Yellowstone hot spot is a famous example of a continental hot spot. Lesson Summary ● Volcanoes form when magma reaches the Earth's surface. ● Volcanoes occur most often along plate boundaries. ● Convergent plate boundaries, where oceanic crust is forced down into the mantle, form many of the volcanoes found on Earth. ● Divergent plate boundaries produce huge mountain ranges under water in every ocean basin. ● Volcanoes like those that make up the islands of Hawaii form over areas called hot spots. Lesson Review Questions 1. What is a hot spot? Explain how a hot spot creates a volcano. 3. Why do hot spot volcanoes form in chains? 4. What plate tectonic setting produces the most volcanoes? Explain how these volcanoes form. 5. Volcanoes have been found on Venus, Mars, and even Jupiter’s moon Io. What do you think this indicates to planetary geologists?
