Mariana Trench : How was the deepest point on earth formed?

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The journey over the Mariana Trench is wild. Imagine your are in Mariana trench region and the suddenly ocean evaporated, your ship would fall a whopping 10.9 kilometers! The geology here is even more striking—how did the Earth create such steep and deep formations?

The surprising fact is that about 200 km east of this deepest point, there is a chain of 15 islands perfectly parallel to the trench known as the Mariana Islands. On one side, a deep trench; on the other side, a collection of islands. We will explore how did Mariana islands were formed.

How did the Mariana Trench form?

Can you guess how the Mariana Trench formed? The only possibility is the collision of two tectonic plates. It was created when a highly dense tectonic plate collided with and was forced beneath a less dense plate. Let’s study this crucial collision process in detail.

Millions of years ago, the massive Pacific Plate, which was moving westward, crashed into the smaller Mariana Plate. When they collided, the Pacific Plate was forced to bend and dive beneath the lighter, more buoyant Mariana Plate. During this motion the Mariana Plate bends inward and travels downward, causing it to sink at a very steep angle. The Mariana Trench is the exact physical location of this collision. Subduction is the process where one tectonic plate sinks beneath another.

The deep, V-shaped scar on the ocean floor created the Pacific Plate bends and plunges down into the Earth's mantle. This is how the deepest trench on the planet was created.

How were the Mariana Islands formed?

Let’s explore how the Mariana Islands were formed. As the Pacific Plate sank, it dragged down seawater and water-logged sediment. The plate plunged hundreds of kilometers into the hot mantle.

The surprising thing is that while traveling into the mantle, the rock of the subducting plate melted much earlier than expected. The reason is the presence of trapped water and sediment in the rock. This reduced the melting temperature of the rock. This phenomenon is known as flux melting. This is simmilar to how salt decreases the melting point of the ice.

This newly melted rock is now hotter and less dense than the surrounding material, so it rises up and erupts onto the seafloor on the Mariana Plate. Over millions of years, repeated eruptions of lava and ash built up a massive underwater mountain. When these eruptions are huge, the top of one of these underwater volcanoes finally breaks the ocean's surface. This is how the Mariana Islands were formed, perfectly curved and parallel to the Mariana Trench.

The Challenger Deep

At the southern end of the Mariana Trench, there is an unusually deep point called Challenger Deep.

Why this region is suddenly so deep compared to the other parts of the trench? Scientists believe that at the southern end of the trench, the subducting plate actually tore. Because of this, this portion lost support from the remaining plate area. Without the side support, the torn portion is effectively in free-fall through the mantle. It is sinking vertically at a much steeper angle than normal subduction zones. This steep, vertical drop creates a much deeper V-shape in the trench floor.

Mariana trench - the water thief

Scientists have solved a lot of mysteries around the Mariana trench. But, one mystery still remains. Is Mariana Trench acting as a ‘water thief’? Here is the mystery. Researchers from University of Washington and Stony Brook University studied the movement of the tectonic plates in detail. They mainly analysed the hydrated rock which is sinking down to the mantle. Their study found that hydrated rock extends 32 km below the ocean floor. Their calculations proved that three times more water is penetrated into the mantle than previously thought.

This water should somehow reach the ocean back. Sea levels have remained relatively stable for a significant amount of time. The only option for this is volcanic eruptions. Here is the issue. Studies show the water coming out of volcanic eruptions is much lower than the water swallowed by the mantle. Where does the remaining water go?