Question

Seafloor spreading occurs at divergent plate boundaries. As tectonic plates slowly move away from each other, heat from the mantle’s convection currents makes the crust more plastic and less dense. The less-dense material rises, often forming a mountain or elevated area of the seafloor. Eventually, the crust cracks and magma fueled by mantle convection, flows up to fill these cracks. This magma cools in the seawater to form igneous rock, a new part of Earth’s crust. This oceanic crust slowly moves away from mid-ocean ridge and the site of seafloor spreading. If the Earth's crust is continually growing, then why isn't Earth expanding? Hypothesize as to what is happening at the opposite tectonic plate boundary. Select a model that predicts the opposite bounda

Answer 1

At divergent plate boundaries, as you mentioned, seafloor spreading occurs where tectonic plates move away from each other, and new oceanic crust is formed from magma rising from the mantle. However, Earth's size does not increase significantly due to a balancing process that happens at convergent plate boundaries.

At convergent boundaries, two tectonic plates move toward each other. This interaction can occur between two oceanic plates, two continental plates, or one of each. When two oceanic plates collide, one typically subducts beneath the other due to differences in density, leading to the formation of deep ocean trenches and volcanic island arcs. In the case of a continental and oceanic plate collision, the denser oceanic plate is subducted beneath the lighter continental plate, creating mountain ranges and also leading to volcanic activity.

The process of subduction at convergent boundaries allows for the recycling of the oceanic crust back into the mantle. Here’s a hypothesis to understand this balance:

Hypothesis on Convergent Boundaries: At convergent boundaries, the oceanic crust that is generated at divergent boundaries is eventually destroyed through subduction. As tectonic plates converge, the oceanic crust is forced down into the mantle where it melts and is recycled. This recycling process counteracts the creation of new crust, stabilizing the overall size of the Earth’s crust.

To visualize the process further, consider the following model:

Model of Plate Tectonics:

• Mid-Ocean Ridge (Divergent Boundary): New oceanic crust is formed as magma rises and solidifies, continually pushing the older crust outward.
• Subduction Zone (Convergent Boundary): Older oceanic crust is pushed down at trenches, where it melts and becomes part of the mantle again.

The overall movements of tectonic plates create a dynamic equilibrium where the new crust formation at divergent boundaries balances the destruction and recycling of older crust at convergent boundaries, leading to a stable average size of the Earth's crust without significant expansion.

In summary, while seafloor spreading leads to the creation of new crust, subduction at convergent boundaries allows for the recycling of the old crust, maintaining the overall volume and size of the Earth.