The giant plateau
If you look at this bathymetric image showing the depth of the seafloor in the Western Pacific Ocean, one spot stands out more than any other. On this map I’ve marked it with the initials OJP – the Ontong-Java Plateau.
The Ontong-Java Plateau is a monster. It covers an area of about 1,900,000 square kilometers – larger than the state of Alaska. It sits entirely beneath the ocean, typically about 1.5 to 3 kilometers beneath the ocean surface and about 2-3 kilometers above the surrounding ocean floor.
It is so big and massive that as it rode along the Pacific Plate it ran into a subduction zone and would not go down. There is now an extinct subduction zone to the southwest of the plateau and subduction zones are developing on the other side of the Solomon Islands to take up the stress.
The Ontong-Java Plateau is a gigantic volcanic feature. Scientists drilled samples and found that it is a gigantic pile of basaltic lava that erupted rapidly around 120 million years ago. Most of the volume of the plateau was erupted within a short space of only a couple million years, maybe even less (it’s hard to tell the difference between 121 and 122 million year old rocks using the techniques that date basaltic igneous rocks).
The size of the plateau as seen today though, is only part of the story of this monster. A plateau this size was expected to sit much higher up and have even have volcanoes above the ocean waters, but drilling efforts at the highest part of the plateau in its western side only found rocks that erupted underwater. However, about a decade ago geologists realized something else – there were 2 other plateaus in the Pacific with the exact same age, the Manihiki Plateau in the open Pacific and the Hikurangi Plateau currently running into New Zealand (both marked on the map).
These 2 plateaus have also been drilled and have not only the same age as the Ontong-Java Plateau, they match in other chemical details as well. In 2005 a plate tectonic reconstruction was created showing that these 3 large provinces could have originally formed together and rifted apart shortly after their creation. Their shapes even fit together – the Manihiki plateau has a western ridge that fits well into a gap in the eastern Ontong-Java plateau. In that case, the original center of the Ontong-Java would be in the now-disrupted eastern portion of the plateau.
When scientists drilled in that area, they found volcanic rocks erupted above the Pacific Ocean and even hit pieces of buried trees. The hypothesis accurately predicted the geology of the plateau and now most scientists agree that these 3 plateaus once hooked together as a true monster.
Within the space of a couple million years a huge amount of lava poured out onto the Pacific Ocean floor. It piled up so high that large islands formed in the middle. After the eruptions finished, its mass was so great that gravity pulled it apart, creating faults and rift zones in the part that was formerly the center. This outpouring of lava at its largest covered about 1% of the surface area of the entire planet – almost the size of Australia today.
These gigantic eruptions have happened at various points in geologic history and produce areas we call “large igneous provinces” or LIPs. Although none that we know of are the size of the Ontong-Java plateau, many have similar properties – huge outpourings of basaltic lava forming within the space of 1-2 million years.
The most likely explanation for the formation of LIPs is a plume of extra-hot material rising through the mantle. When it hits the surface, it would melt like crazy, rapidly producing a huge outpouring of lava as the heat is removed.
That outpouring of lava would bring all sorts of new elements to the Earth’s surface, including metals, nutrients that life can use, and gases that can be toxic to life. Some LIPs, like those in Siberia, have been proposed as explanations for large mass extinctions on Earth. However, the Ontong-Java Plateau is by far the largest LIP on Earth and it is not associated with a mass extinction, maybe suggesting that the link between mass extinctions and volcanic provinces is more complicated. For example, the LIP in Siberia might have been especially deadly due to interaction with a coal layer, while the Ontong-Java plateau’s effects might have been minimized due to erupting mostly under water rather than at the surface.
Regardless of the lack of an Ontong-Java mass extinction, this feature is still an amazing monster. No other LIP compares to it in size, and it’s big enough to reshape the plate tectonic patterns in the Pacific Ocean.
-JBB
Image credit:
http://bit.ly/1Kr4PLP
References:
http://bit.ly/1U59oOU
http://bit.ly/1JuX0Vc
http://bit.ly/1HsSJzm
http://bit.ly/1BUWXza
