Ocean trenches don't directly explain why the Earth doesn't get larger as new crust is added at mid-ocean ridges. Instead, they play a crucial role in balancing the process of crust creation. Here's how:

1. Seafloor Spreading and Crust Creation:

* At mid-ocean ridges, magma rises from the mantle, cools, and solidifies, creating new oceanic crust. This process, known as seafloor spreading, pushes the older crust away from the ridge.

* This continuous addition of new crust would make the Earth larger if there weren't a mechanism to remove it.

2. Subduction and Crust Destruction:

* Ocean trenches are deep depressions in the ocean floor where subduction occurs.

* Subduction is the process where one tectonic plate (typically an oceanic plate) slides beneath another (either oceanic or continental).

* As the denser oceanic plate descends into the mantle, it melts, and its material is recycled back into the mantle.

3. Balancing Act:

* Seafloor spreading creates new crust at mid-ocean ridges.

* Subduction destroys crust at ocean trenches.

* These two processes are in a dynamic balance, essentially a "give-and-take" relationship, that maintains the Earth's size.

In summary: Ocean trenches, through the process of subduction, act as the "sink" that balances the "source" of new crust created at mid-ocean ridges. This continuous cycle of creation and destruction helps keep the Earth's size relatively constant.