1994 Bolivian earthquake reveals mountains 660 kilometers below our feet

https://www.princeton.edu/news/2019/02/14/massive-1994-bolivian-earthquake-reveals-mountains-660-kilometers-below-our-feet

by zakki • 7 years ago

71 17 7 years ago

princeton.edu

mogadsheu3• 7 years ago

I believe this is a great use of public funding for basic research.

Plate tectonic theory is strangely both very recent and very old—it was conceived in the 1950s and has led to major discoveries in the private sector.

While the immediate implications of a breakthrough like this (extreme topography separating mantle transition zones) don’t likely have an impact on society at large, a series of them likely will.

It would be great to unlock additional resources, perhaps in less volatile areas, or more cleanly, by using a new model for earth processes.

vanderZwan2• 7 years ago

Plate tectonic theory is strangely both very recent and very old

In general geology is one of the forgotten natural sciences, even though it has had some pretty important fundamental contributions to the way we see the universe (the concept of deep time is fairly recent). Maybe because it is inherently dealing with time-frames and scales beyond human imagining.

cossatot6• 7 years ago

In my experience teaching geology, many people are fundamentally uninterested in rocks; they're mundane, pretty static, boring, etc. Many people are also not particularly spatially-inclined so maps aren't interesting. I think the spatial and temporal scale issues you bring up are also factors.

People do seem to be interested in landscape development, minerals (pretty crystals are cooler than pieces of shale), earthquakes, etc. Paleontology is typically housed in geoscience departments too.

From a research perspective, geoscience is extremely fun. Even to those infatuated with the Earth such as myself, the individual topics of study can get a little dry. However, geologic problems can be approached from a wide variety of methods, and to solve those problems that are still standing, a research group will come at it from many angles. This includes, but is not limited to, physics-based computational modeling, hiking, isotope geochemistry, fossil collection, statistics, satellite imagery analysis, acoustic physics, technical drawing, computer simulation, spherical geometry, and smashing boulders with sledge hammers.

For example, one chapter[0] of my PhD thesis (studying the development of fault-bounded mountain ranges in Tibet) I spent several months camping in beautiful Tibetan mountains doing geologic mapping and sampling (which is hiking, making qualitative and quantitative field observations and measurements, as well as digging holes and picking up rocks), then I came home and did a lot of radioactive isotope geochemistry of the minerals apatite and zircon within the rock samples: These minerals contain uranium and thorium; as the U and Th decay, they produce 4He (an alpha particle). At high temperatures, those particles diffuse out of the apatite and zircon crystals, but at low temperatures the 4He is retained in the crystals. So you measure the ratio of 4He to U and Th, and get an idea of when the crystals cooled below the diffusion 'closure temperature' (which is dependent on cooling rate). By taking these measurements over almost a vertical kilometer along the mountain range, I could estimate how fast the mountain range was rising, because it cools as it rises out of the surrounding crust. But it's a bit complicated to do the calculations right because there are lots of small issues (how fast the rock in the mountain range cools is dependent on the ambient temperature of the crust, the thermal properties of the rock, how much radiogenic heat production there is from K, Th and U decay, the temperature of the surface of the earth, the irregularities of the topography, ...) so to do it right you have to make a finite element model of the crust and simulate the thermal evolution as the crust deforms due to faulting and mountain range development. This is only a forward model, so to figure out the whole history of deformation including faulting and mountain range uplift rates through time, and deal with uncertainty in the thermal parameters of the crust, I had to run thousands of forward models. Lacking a cluster, I had to teach myself Python and how to use AWS EC2 instances to do embarrassingly parallel finite element modeling (I used the PiCloud library which was incredible but died in an aquihire event).

[0] http://onlinelibrary.wiley.com/doi/10.1002/tect.20053/abstra...

lkrubner• 7 years ago

"In general geology is one of the forgotten natural sciences"

Forgotten or hidden? Almost all the good jobs involve working for the fossil fuel industry, which then treats discoveries as industrial secrets. If you want to be a well paid geologist, you will end up working for an oil company, but for the most part you won't be allowed to talk about your research, except when the research is discovering a negative. For instance, in the 1970s, the oil companies decided they would no longer look for deposits in layers dating to the 50 million years after the Permian Triassic extinction -- there is nothing for them to find at that level, because there wasn't enough life to create fossil fuels. That kind of research is released to the public, but positive finds are not.

ta12345678903• 7 years ago

This is both really amazing and sad at the same time. Amazing because they are advancing a fascinating field and discovering what Earth's interior is like. Sad because it seems research like this barely gets any funding or attention.

Yesterday I read that NASA estimates that it would take $104B to go back to the Moon. I'd much prefer we would spend that kind of money on trying to get to the center of the Earth :)

cossatot6• 7 years ago

Some possibly interesting stuff about topography on the 660 km 'boundary':

The 660 km boundary (aka the 660 km discontinuity) is primarily* a mineralogical phase transition in the mid-Mantle. What that means is that the bulk geochemistry above and below this boundary are the same, but the crystalline structures are different (different mineral assemblages are stable at different temperatures and pressures given the same bulk rock chemistry, and the transitions are sharp-ish rather than gradual). The rock below the 660 boundary is maybe 10% more dense than the rock above. Because rock in the mantle is viscous (say, 10^20 Pa s) in the absence of persistent stress or pressure gradients, any topography on this boundary should eventually level out. So seeing kilometers of topography on the boundary means that there are some sort of persistent differences in force in the mantle of magnitude 10-100 MPa that can sustain the gravitational potential energy gradients produced by irregular topography and therefore irregular density distributions. Over broad scales (hundreds to thousands of km laterally) this could be from the big mantle convection cells that drive tectonic plate motion on the Earth's surface. At smaller scales I have no idea what could be driving it.

Another interesting thing about the 660 km (and 410 km and shallower) phase transitions is that I think that the density and viscosity contrasts due to the phase transitions causes a lot of the deep earthquakes such as the Bolivian earthquake from the paper. These earthquakes are in a subducted (sinking, falling if you will) old slab of oceanic crust. As these sink due to density contrasts (they are cold from being at the Earth's surface), they will slow down when they pass through the 410, 660, etc. phase transitions and should deform, kind of like a big icicle that falls of a cliff into water fracturing when it hits the water's surface (except it may continue to sink).

* Some say it's a chemical boundary as well, i.e. that the material above and below doesn't mix. I think most high-resolution seismic imaging of the mantle shows that many convective features such as mantle plumes and subducting oceanic plates penetrate the 660 discontinuity so I don't know if I believe this but it's hard to say and this isn't my area of expertise. It could be either or both although we have solid experimental and theoretical thermodynamic evidence for saying that these are phase transitions.

stretchwithme• 7 years ago

This complicates my planned journey to the center of the Earth.

mtnGoat• 7 years ago

make sure to take your hiking boots. crossing mountains is tough business.

1994 Bolivian earthquake reveals mountains 660 kilometers below our feet

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