Dr Long began her lecture by noting humanity's fascination with the world beneath our feet, a fascination which has cropped up in popular culture for a long time. She displayed a simple, elegant image of the Earth's layers as characterized by current geology. Beneath the cool crust of the Earth, there is a rocky mantle which surrounds a liquid outer core primarily composed of iron (about 80%) and nickel (about 20%) surrounding a solid iron-nickel inner core. The interior of the Earth is characterized by dynamic processes.
Approximately 4.6 billion years ago, the Earth accreted from a planetary disc- collisions between accreting dust particles created kinetic energy which was stored as heat, the present day source of much of the interior's heat. The Earth has been cooling slowly to the temperature of the surrounding space, which will eventually result in a slow heat death billions of years from now. The Earth radiates 46 terawatts of heat- it is thought that approximately one half of this heat is residual primordial heat, and one half results from radioactive decay. Heat loss fuels plate tectonics and localized disasters such as earthquakes. While the Earth radiates 46 terawatts of heat, it receives 170,000 terawatts of heat from the sun, which can cause atmospheric disasters.
Dr Long indicated that the heat lost by the Earth is lost by convection- although the mantle is solid, there is a slow convection as rocks near the surface cool, become denser, and sink and rocks near the core heat up, become less dense, and rise. The process is slow, rocks move one to ten centimeters a year... to help the audience visualize the process, Dr Long likened it to the speed at which one's fingernails grow. Dr Long compared the process to the motion of the blobs in a lava lamp and joked that, as a geophysicist 'of course' she has a lava lamp. Plate tectonics is a surface expression of the convection in the mantle. Subduction zones are the regions in which the plates of the Earth collide and one plate slides under another plate. Oceanic spreading zones are regions in which plates are moving away from each other. Plate tectonics in boundary zones is the cause of earthquakes and vulcanism. Dr Long stressed the need to understand the physical properties of the Earth to mitigate disasters.
Dr Long then shifted the topic to methodology- how do we study the Earth's interior? Much of what we know about the interior is the result of studying seismology. Earthquakes are recorded at monitoring stations all over the planet- the 'wiggles' of the seismographs give researchers insights into the structure of the Earth. There are different sorts of seismic waves, such as body waves which move through the interior and surface waves. Body waves are further divided into primary P-waves (or compressional waves) and S-waves (shear waves). Seismic waves continually pass through the mantle of the Earth and the data can be compiled to create seismic tomography, in a process analogous to medical tomography. Images can be constructed from seismic waves. Dr Long showed us a gorgeous tomographic image of the mantle underneath the United States:
The blue regions are characterized by fast seismic waves traveling through older, colder, stiffer rock, the red regions are characterized by slow seismic waves moving through hotter rock with more vigorous seismic activity.
There are seismic stations all around the globe- a lot of earthquakes occur, providing a lot of data. While earthquakes are, as Dr Long put it, super-common, most of them occur in remote places, deep in the earth. A Global Seismographic Network with about two-hundred monitoring stations uploads data in real time. Earthquake occurrence is not evenly distributed, and oceanic monitors are difficult and expensive to place. With a global network, a seismic tomography of the deep earth is being compiled. One recent discovery is that subducting plates can form slabs which sink towards the core. Dr Long noted that many of the features observed in the deep Earth look like the predicated models, but that there are surprises- not all subduction slabs act alike, some slabs are slowly sinking all the way to the core-mantle boundary. Beneath Japan, one subduction slab sinks to a depth of 900 kilometers then stops sinking. Dr Long posed the question, why do some slabs sink to the mid-mantle level while others sink to the core-mantle boundary? All of these slabs are made of the same stuff.
There are also rising rock 'plumes'- solid rock rises up through the mantle. Dr Long cited the work of UC Berkeley geologists Scott French and Barbara Romanowicz who found slow velocity hot rock plumes under such volcanic hotspots as Hawaii and Iceland. Some of these plumes are one-thousand kilometers in width. There are also 'superplumes', more properly known as large low-shear-velocity provinces located at the base of the mantle and having low shear-wave velocities. There is a large low-shear-velocity province underneath the Pacific Ocean, and another under Africa. Dr Long characterized these LLSVP's as 'superweird'. Nobody knows what they are- they are at the base of the mantle, and they are hot, but they don't seem to be rising. Dr Long wondered if these structures had a different minerology/chemistry from other mantle sections and if they were formed shortly after the birth of the planet. LLSVP's remain a mystery. She recommended a TED talk by Dr Ed Garnero of Arizona State University on the subject:
Dr Long then brought up the topic of Earthscope, the largest earth science project funded by the National Science Foundation. Earthscope was designed to make transformative discoveries about the structure of the North American continent, earthquake physics, and the Deep Earth. The data derived from the project is free and open. The USArray placed approximately twenty-five hundred seismic systems throughout North America- before Earthscope, there were about one-hundred seismometers in the US. Dr Long contrasted the pre-Earthscope era to the present day using an analogy- it's like studying astronomy with a pair of binoculars versus studying astronomy with the Hubble Space Telescope. I seemed to detect a bit of 'football spiking' when Dr Long told us that PopSci, in 2011, named Earthscope the most awe-inspiring project of the year, edging out the LHC.
The Earthscope observatory nearest to the beautiful Bell House is N61A in Milburn, New Jersey. The USArray is a flexible array- detectors can be earmarked for specific seismic experiments. Dr Long's specific experiment is the poetically named MAGIC: Mid-Atlantic Geophysical Integrative Collaboration. The goal of the MAGIC project is to determine the seismic structure of the eastern United States, and to reconstruct the plate tectonics processes which formed the region. The geology of the eastern United States is largely covered by vegetation and I-95. It's a complicated geology- about 350 million years ago, the Appalachian Mountains were young, tall mountains like the Himalayas. At about 200 million years ago, the supercontinent of Pangea was splitting up, with present-day Africa separating from present-day North America- meaning that the current Eastern Seaboard would have looked much like Africa's Rift Valley. There are dramatic remnants of the Triassic Rift in the Hudson Palisades, part of the Newark Basin and New Haven's East Rock (visible from Dr Long's office), part of the Hartford Basin- both the Palisades and East Rock are lava formations, dramatic evidence of ancient tectonic processes.
Dr Long's MAGIC project is set up to determine the effects of tectonic processes on the deep structure of the crust and mantle underlying eastern North America- what lies beneath? Eastern North America has not been a plate boundary for 200 million years, but the seismic tomography indicates a couple of unusual features- red 'blobs' underneath New England and Central Appalachia, indicating slow velocity seismic waves. Dr Long chose to study the Central Appalachian 'blob'- what does the crust/mantle region look like in Appalachia? Central Virginia is known to experience earthquakes. Dr Long characterized the geology of Appalachia as 'superbizarre'- there are 500 million year-old rocks in the region, which should not be found in a passive-margin region with no plume or hotspot. Put succinctly, the structure is anomalous. MAGIC deployed 28 seismometers in Appalachia between 2013 and 2016. Dr Long announced her 'hot off the presses' findings... in the Central Appalachian region, the lithosphere, thought to be 100 kilometers thick, turned out to be a mere 70 kilometers in thickness, way thinner than was expected. At some time, perhaps 50 million years ago, some part of the lithosphere dropped off into the mantle. The dropped chunks of lithosphere may explain the earthquakes in this region. This unexpected finding raised other questions- is this region of thinness unique to the region, or is it a characteristic of old mountain ranges? What's special about Appalachia?
Dr Long ended her lecture with a plug for the Earthscope project and told us to stay tuned for new amazing discoveries, transformative discoveries, to come. Deep Earth research is important to understanding life, and to understanding hazards.
In the Q&A session following the lecture, some bastard in the audience asked Dr Long about the feasibility of earthquake prediction (impossible by today's standards). She noted that earthquakes cannot be predicted, but that broad forecasts can be made about which regions are earthquake-prone. While nobody can indicate if an earthquake is imminent, knowledge of risk factors can lead to better building codes in quake-prone areas. She indicated that there's not a lot of progress, and we may never get there, but increased knowledge can lead to better policy.
Another attendee asked if convection is random or if the LLSVP's play a role in the process- Dr Long indicated that nobody knows what factors control convection patterns. A question about the Chicxulub impact's effect on the planet had Dr Long stating that, while the impact was a mass extinction level event in the biosphere, it had little effect on Earth's deep structure... that being said, extinction events are often associated with volcanic flood basalts. Another question about our knowledge of extraterrestrial seismic studies had Dr Long talking about seismometers on the Moon and the Insight mission to place seismometers on Mars. Another question regarded the anomalous rise of upstate New York's Adirondack Mountains which is occurring 'faster than it should be'. The last question regarding the heat death of the Earth, and Dr Long noted that there are differing calculations depending on how much one attribute's Earth's heat to primordial kinetic energy or to radioactive decay- at any rate, we have tens of billions of years until it happens, and as Dr Long wisely put it- 'we have bigger problems'.
After the formal Q&A, Dr Long hung out at the beautiful Bell House for an informal chat session. One topic which came up was the discovery of a fault line not too far from the Indian Point nuclear power plant. Dr Long also broke the news to me that Dr Leo J. Hickey, gentleman and Renaissance man, had passed away four years ago.
Dr Long's lecture hit what I call the 'Secret Science sweet spot'- it was an entertaining and informative blend of hard science fact, methodology, and travelogue. In other words, the good doctor hit it out of the park. Kudos to Dr Long, Dorian and Margaret, and the staff of the beautiful Bell House. If you want a taste of that Secret Science effect, here's a video of the good doctor lecturing on natural disasters:
Crack open a beer and soak in that SCIENCE!
2 comments:
Good stuff. Thank you.
We're just waiting for the Hayward fault to tear loose and kill a million people in the bay area. It happens every 140 years, pretty much clockwork, and we're now about four years overdue.
I have updated my one week survival kit, but in reality we're pretty far to the west. We might have problems with water, electricity and communications, and the roads and bridges might be out of commission for a couple months, but our houses are not likely to collapse like the ones in the east bay...
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