The biggest island in the United States also is the fastest growing. Thanks to a steadily rising hot plume from the lower mantle, Hawaii Island’s land area grows by an average of a little more than 40 acres per year. Though geophysicists have known about hot mantle plumes for several decades, only in the past few weeks have they gained an understanding of the source and mechanisms that produce hot plumes.
Earth’s mantle, the silicate high-viscosity layer between Earth’s crust and core, makes up 82.5% of Earth’s volume and 67.1% of its mass. In spite of tens of thousands of seismic measurements, Earth’s mantle remains the least understood of Earth’s interior components. However, a new analysis of seismic readings is unveiling features that prove critical for sustaining our global civilization.
Strong Earthquakes to the Rescue
The most mysterious part of Earth’s mantle is the lowermost part, the deep mantle. The reason for this is that it takes strong earthquakes occurring 200 kilometers (125 miles) or deeper below Earth’s surface to produce the seismic shear wave signals capable of revealing information about the conditions of the deep mantle.
A team of five American and Israeli geophysicists, led by Doyeon Kim, used a self-learning sequencing algorithm to analyze over 7,000 seismic shear wave records generated by magnitude 6.5 or greater earthquakes that occurred more than 200 kilometers below Earth’s surface between 1990 and 2018 in the Pacific Ocean Basin.1 The earthquakes’ frequency and strength made the research possible.
Deep Mantle Research Findings
The team found that the “textbook picture” of Earth’s mantle needs revision. It is not a viscous layer thoroughly homogenized by widespread aggressive convection (heat transfer) akin to a jar of homogenized creamy peanut butter. Yes, the energy transport in the mantle occurs predominantly by convection. However, while the mantle is well mixed, it is not thoroughly mixed.
Kim’s team found two large blobs, each measuring about 1,000 kilometers (600 miles) across and about 25 kilometers (15 miles) thick, and several much smaller ones. These blobs had a composition and density distinct from both the molten iron-nickel alloy at the top of the inner core and the slushy rock at the bottom of the mantle.
The two large blobs were located under the Hawaiian Islands and the Marquesas Islands (about 2,000 miles south of Hawaii). Both blobs are associated with the hot vertical plumes rising up from the lower mantle through the upper mantle and crust to form the volcanoes that built the islands. This island-building has been progressing for the past 28 million years for the Hawaiian Islands and 5 million years for the Marquesas Islands, continues to this day, and will be sustained for several more million years.
Such rising hot plumes from the lower mantle have created geological features beyond the Pacific Ocean Basin. Present-day examples include Iceland and Yellowstone. A past example would be Greenland.
This blob-to-plume-to-surface activity over millions of years has generated at least five benefits for modern civilization.
1. Nutrients for life. The volcanoes that formed as a result of the plumes rising up from the lower mantle provide nutrient-rich soils. This enrichment occurs not just on the resultant volcanic islands. The Yellowstone supervolcano—when it last erupted 700,000 years ago—deposited nutrient-rich ash over most of the continental United States.
2. Ice age cycle. If it were not for Greenland’s tectonic movement northward and settling over a rising plume, Greenland would not have the high average elevation that it possesses. Thanks to its high elevation, nearly all of Greenland is covered with thick ice. This ice cover played a key role in producing the ice age cycle. As I explain in my latest book Weathering Climate Change,2 the ice age cycle is one of the most important components that have made global human civilization possible.
3. Unparalleled beauty. Anyone who has visited the Hawaiian or Marquesas Islands or Iceland, recognizes their spectacular scenic beauty. Credit for this beauty goes not only to the volcanoes but to the unique weather patterns they produce that engender the powerful erosion forces that create the deep canyons, knife-edge ridges, and waterfalls. The unique landscapes also produce ecosystems for species that are found nowhere else on Earth. Such scenic wonderlands provide much-needed relief for the technologically stressed urbanites that now comprise more than half the world’s human population.
4. Energy and minerals. The geothermal energy sources formed by the rising plume under Iceland today provide most of the heating and electricity for Iceland’s citizens and visitors. Rising plumes also yield valuable minerals for sustaining the world economy.
5. Just-right magnetic field. Plumes rising up from the lower mantle play a vital role in ensuring that neither too much nor too little mantle convection occurs. Mantle convection governs the rate at which heat flows out from the core to the crust, hydrosphere, and atmosphere. This rate plays a critical role in ensuring that the magnetic field generated in the liquid outer core remains both strong and enduring. Thanks to Earth’s strong magnetic field throughout the past 4.2 billion years, solar radiation has not eroded away Earth’s atmosphere and oceans. That same strong, enduring magnetic field3 has protected Earth from deadly cosmic and solar radiation.
In this way, scientific advance not only revises models for Earth’s interior but also provides provocative evidence. These curious blob structures just above core-mantle boundary yield another demonstration that the more we learn about the interior of our planet, the more evidence we gain for its design being for the specific benefit of human beings and all of Earth’s life.
Featured image: Pali of Kauai
These knife-edge ridges formed through the rapid erosion of deep mantle material pushed up to Earth’s surface via a rising mantle plume.
Image credit: Hugh Ross