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

~~~
vanderZwan
> 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.

~~~
cossatot
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...](http://onlinelibrary.wiley.com/doi/10.1002/tect.20053/abstract)

~~~
ta1234567890
Thank you for sharing your research and experience. Your comment was a really
nice read.

What does someone with a PhD like yours does afterwards?

What is currently the holy grail of geology?

~~~
mogadsheu
Fellow (former) geologist here.

The most common options are:

-Resources (oil/mining). It’s a surprisingly diverse field within this realm

-Academia/research. Also very diverse

-Disaster modeling/prediction/studies like earthquakes and flooding

-Some engineering (ie geological surveys for real estate/infrastructure development)

-Space-related like satellite imaging/remote sensing. Or becoming an astronaut

~~~
cossatot
Good list. A lot of folks go into environmental remediation as well.
Unfortunately in practice a lot of this is 'helping companies pollute to the
maximum extent allowed by law' but it's better than nothing.

------
ta1234567890
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 :)

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

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

