Chapter 9: Problem 36

The oldest rocks found on the continents are about 4 billion years old. By contrast, the oldest rocks found on the ocean floor are only about 200 million years old. Explain why there is such a large difference in ages.

Short Answer

Expert verified

The large difference in ages between rocks found on continents and the ocean floor is primarily due to the cycle of seafloor spreading and subduction. New oceanic crust is continually formed and subsides back into the mantle over time, keeping the average age of the ocean floor relatively young compared to continental rocks, which go through the rock cycle and remain in place over much longer geological timescales.

Step by step solution

Understanding Plate Tectonics

Plate tectonics is the theory that Earth's outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core. These tectonic plates include both the land and sea floor.

Elaborating the Concept of Seafloor Spreading

Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. This constant production of new seafloor and gradual movement leads to the older rocks being pushed away from the ridge.

Describing the Rock Cycle

The rock cycle explains the processes through which one type of rock can become another type over time due to environmental changes. This process contributes to the older age of the rocks on the continents, as they get recycled and transformed but remain on the continent.

Linking the Concepts

In the context of these geological processes, the young age of the rocks on the ocean floor can be explained. With new seafloor constantly being formed through seafloor spreading, the old ocean floor is subsumed back into the mantle in a process called subduction, preventing it from achieving the same age as continental rocks.

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The oldest rocks found on the continents are about 4 billion years old. By contrast, the oldest rocks found on the ocean floor are only about 200 million years old. Explain why there is such a large difference in ages.

Short Answer

Step by step solution

Understanding Plate Tectonics

Elaborating the Concept of Seafloor Spreading

Describing the Rock Cycle

Linking the Concepts

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