Why will a rock sink if we put it in sand? This is because the Earth’s gravity pulls everything down as far as it will go, and the rock is denser than the sand. Gravitational sinking also happens with bigger objects, like tectonic plates under our oceans! This happens in a process called subduction.
The Whats and Where of Subduction
This process takes place at convergent plate boundaries. This is when two tectonic plates meet and push against each other. On land, where the lithosphere doesn’t give away easily, convergent plates push each other upwards and sideways. This creates mountain ranges, like the mighty Himalayas! Underwater, however, convergent plate boundaries do something different.
This type of plate movement happens between two oceanic plates, or an oceanic and a continental plate. When these two plates collide head-on, they don’t push each other and rise. Instead, the one that is denser sinks below the other one! The sinking plate slides down into the asthenosphere, and gets recycled back into the Earth’s mantle. Wherever this sinking process occurs is a subduction zone.
Subduction zones create a variety of different landforms. Oceanic trenches, for instance, form in the boundary between a sinking plate and the plate above it. But subduction zones can be very dangerous as well! Arcs of composite volcanoes can form on the plate that lifts up. While these don’t tend to be as active as shield volcanoes, they can still erupt and cause a lot of damage!
A subduction zone notorious for both earthquakes and volcanic activity is the Ring of Fire in the Pacific Ocean. That’s a very fitting name, given the horrible disasters that take place there.
Other Great Resources
USGS on Plate Boundaries: https://pubs.usgs.gov/gip/dynamic/understanding.html
Universe Today on Subduction Zones: https://www.universetoday.com/43822/subduction-zone/
Live Science on the Same Subject: https://www.livescience.com/43220-subduction-zone-definition.html
Study Lesson on Subduction: https://study.com/academy/lesson/subduction-definition-process.html
When Earth Erupts: https://www.youtube.com/watch?v=NbDqJy28hBw
Subduction Zones: https://www.youtube.com/watch?v=W0FvwWz0ulI