What is a Deposition? - Definition & Explanation

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  • 0:00 Island Formation & Deposition
  • 1:27 Erosion & Deposition
  • 2:00 Volcanic Eruption & Deposition
  • 3:00 Water & Ice Deposition
  • 4:13 Wind & Gravity Deposition
  • 5:00 Lesson Summary
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Lesson Transcript
Instructor: Maria Howard

Maria is a teacher and a learning specialist and has master's degrees in literature and education.

Discover the many ways deposits of rock and soil can alter landforms as big as islands, mountains, and river deltas in this lesson. You'll explore both quick and slow changes in the land around us, and learn some fascinating geologic terms along the way!

Island Formation & Deposition

On November 14, 1963, fishermen off the southern coast of Iceland noticed a new island had suddenly appeared where they fished. The small island was soon named Surtsey, which is Icelandic for 'Surtur's island'. Surtur, the Norse ruler of fire, is a fitting namesake: Surtsey was formed from a volcano on the sea floor, called a seamount, which had grown so big it broke the surface of the ocean and became an island. Surtsey continued to erupt for over three years and now provides a young island for scientists to discover all they can about new land.

Surtsey is not the only new island. A 2006 eruption created a small island, Home Reef, near Tonga in the southern Pacific Ocean. Additionally, Mauna Loa and Kilauea on Hawaii's 'Big Island' both continue to erupt, adding acres of land each year to the island. Nearby, the underwater volcano Loihi is just short of becoming another Hawaiian island. This process of island formation is a type of deposition, or the geological process whereby rocks, soil, and silt are naturally deposited in such a way that new land masses are created or old landforms are added to or changed.

Erosion & Deposition

During erosion, which is part of the geologic cycle, land is worn away and carried elsewhere by elements like wind and rain. Rock and tiny pieces of rock called silt are carried away to make up sediment. Of course, all that rock and silt, or sediment, has to go somewhere. Sometimes deposition is as dramatic as the creation of a new island, but it often affects much smaller, incremental changes in land that, over time, can become new landforms like mountains and river deltas.

Volcanic Eruption & Deposition

While a few undersea volcanoes grow big enough to become islands, most become mountain ranges on the seafloor where the tectonic plates that make up the earth's crust spread apart and/or crash together. Magma from beneath the ocean floor oozes up to form new land in the spaces where tectonic plates pull apart through a process called seafloor spreading.

In addition to lava, volcanoes above ground spew ash and dust which settle near the volcano itself but also get picked up by the wind or are pushed far out by the force of the blast. The ash, dust, and pieces of volcanic rock then settle and become a part of the existing landscape. The bigger the eruption, the more deposits that are added. For example, when Mount Pinatubo erupted in 1991, so much dust and ash was released that roofs of houses collapsed under the weight, fields of crops were destroyed, and the land was left unusable for many years.

Water & Ice Deposition

Water in the form of rivers, waves, and ice has the power to move sediment, sometimes hundreds of miles. Rivers move sediment great distances, emptying it into oceans, lakes, and even other rivers. Sometimes the deposits get washed and sometimes, as is the case with the slow-moving Mississippi River, the sediment steadily builds up and creates deltas. Luckily for people, deltas provide fertile soil for agriculture and ideal areas for fishing and shipping.

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