Utrecht University geologist Suzanna van de Lagemaat has reconstructed a previously unknown giant tectonic plate that was once a quarter the size of the Pacific Ocean. Her colleagues in Utrecht predicted its existence more than a decade ago based on fragments of ancient tectonic plates found deep in the Earth's mantle.

A geologist has successfully reconstructed a previously unknown tectonic plate named Pontus that was once about a quarter the size of the Pacific Ocean. The discovery was made after fieldwork in various regions including Japan, Borneo and the Philippines. Image credit: Suzanna van de Lagemaat/Utrecht University

Van de Lagmathe reconstructed the lost plates through field research and detailed surveys of mountain belts in Japan, Borneo, the Philippines, New Guinea and New Zealand. To her surprise, she discovered that the remnants of the ocean north of Borneo must belong to a long-suspected plate that scientists named the Pontus Plate. Now, she has reconstructed the entire plate.

The Philippines lies at the complex intersection of different plate systems. The area consists almost entirely of oceanic crust, but some massifs rise above sea level and exhibit rocks of varying ages.

Tectonic plates make up the Earth's hard outer shell, and understanding their movement is crucial to understanding the Earth's geological history. The movement of these plates strongly affects the long-term changes in the earth's paleogeography and climate, and even affects the discovery of rare metals. However, oceanic plates throughout geological history have disappeared into the mantle through subduction. They left behind only fragments of rock hidden in mountain belts. VandeLagemaat studies the most complex plate tectonics region on Earth: the area around the Philippines.

Suzanna van de Lagemaat's reconstruction of the Pontus Oceanic Plate: its position in the ancient Pacific 120 million years ago and what remains today. An earlier study suggested that there must have been a large subduction zone in the western ancient Pacific that separated the known Pacific Plate to the east from the putative Pontus Plate to the west. VandeLagemaat's research has now independently proven this hypothesis. Image credit: SuzannavandeLagemaat, Utrecht University

VandeLagemaat first used geological data to reconstruct plate motion in the area between Japan and New Zealand. This reveals how large an area of ​​​​the plate is now lost in the western Pacific.

"We also conducted fieldwork in northern Borneo and found the most important piece of the puzzle there. We thought we were dealing with the remnants of a lost plate we already knew about. But our study of these rocks in the magnetism laboratory showed that our findings originally came from further north and must be the remnants of another previously unknown plate. But the important discoveries are yet to come."

Douwe van Hinsbergen, van de Lagmat's doctoral supervisor, explains: "Eleven years ago we thought that the remains of Pontus might be located in northern Japan, but we later dismissed this theory. The proposed Pontus plate did not become apparent until Suzanne systematically reconstructed half of the 'Ring of Fire' mountain belt from Japan through New Guinea to New Zealand, including the rocks we studied in Borneo."

Remains of the Pontus Plate are found not only in northern Borneo, but also in Palawan, in the western Philippines, and in the South China Sea. Van de Lagmathe's research also shows that stretching from southern Japan to New Zealand is a single coherent plate tectonic system that has existed for at least 150 million years. This is also a new discovery in this field.

The existence of Pontus was previously predicted because subducting plates leave traces as they "sink" into the mantle: areas in the mantle with unusual temperatures or compositions.

These anomalies can be observed when seismometers pick up seismic signals. Earthquakes send seismic waves through the Earth's interior, and when the waves pass through anomalies, such as fragments of old plates, the anomalies can interfere with the seismic signal.

Geologists can use these disturbances to trace phenomena in the mantle, such as fragments of tectonic plates. This allowed them to see 300 million years ago; fragments of older plates had "dissolved" at the boundary between the mantle and core.

Research 11 years ago suggested that a large subduction zone must have passed through the western ancient Pacific, separating the known Pacific Plate to the east from the putative Pontus Plate to the west. VandeLagemaat's research has now independently proven this hypothesis.