This is a map of sea level, exaggerated 10,000 times.

Why is it so weird and lumpy? Sea level is not defined by distance from Earth's center - it's an equal-gravity surface.

Understanding the #geoid is key to understanding #climatechange impacts. 1/7

en.wikipedia.org/wiki/Geoid Source: http://doi.org/10.5194/essd-11-647-2019.
The "lumpiness" comes from variations in density and topography. Mountains have gravity, so the #geoid is generally higher in mountainous regions. But inside the Earth there are variations, too - from the different kinds of rocks and the thickness of the crust. 2/7
Elevations on Earth are defined relative to the geoid. So every time you look at a topographic map, there's a secret geoid hidden behind that data! 3/7
The #geoid is not fixed - it changes as the balance of mass on Earth shifts. Mostly this is slow: tectonic movements, plate collision, erosion. Some of it is fast, but minor: volcanic eruptions, earthquakes.

Melting ice caps change the geoid too! 4/7

climate.nasa.gov/blog/2861/arct…
As the ice caps melt, that mass gets distributed into the oceans. This lowers the geoid near the ice caps, and makes it appear that the land is locally rising.

This doesn't work for sea ice - that ice is already floating in water, so balanced. Only ice on land matters. 5/7
This is why we can't talk about #sealevelrise with a single number: local impact depends on local geoid changes. So the weird lumpy Earth geoid - and its changes over time - are central to how coastal regions will be affected as the climate changes. 6/7

sealevel.nasa.gov/news/43/a-nasa…
If you have a 3D printer, you can print your own lumpy #geoids at home! A great stocking-stuffer if you want to confuse and offend people. 7/7

oceanservice.noaa.gov/facts/geoid.ht…

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More from @JudithGeology

13 Nov
The Earth's most common elevation is sea level - and yet, most of Earth's surface lies below sea level - FAR below sea level, at 4-6 km depth.

Why?

The answer is a combination of #platetectonics and Earth's oceans. 1/9

#tectoplot ImageImage
You might think that the oceans are just parts of the land that are covered with water. Actually, that's really not the point - the oceans are there because the rocks *below* the oceans are fundamentally different from those below continents - and it's all because of magma! 2/9
Below the crust, the mantle is convecting. This is driven by heat given off by radioactive delay deep inside the Earth.

The mantle is solid rock - but every now and then a pocket melts: due to the addition of water, release of pressure, or extra added heat. Magma! 3/9 Source: https://concord.org...
Read 9 tweets
11 Oct
An #ophiolite is a rock with a secret: it tells the story of an ocean that lived and died.

Ophiolites are pieces of crust and mantle that formed at #spreadingcenters below an ocean. Why do we find these rocks (black dots) in mountain belts (red)? 🧵

doi.org/10.1016/j.gsf.…
The #WilsonCycle describes how tectonic plates break apart, forming an ocean basin that grows around a spreading center. But the oceanic lithosphere is dense, and it eventually breaks and sink into the mantle. #Subduction closes the basin and the plates on either side collide.
Rocks that form at a #spreadingcenter have a distinctive sequence: sediments on top, then basalts that erupted underwater, then denser rocks crystallized from melted mantle, grading into mantle. You might find this sequence on land (an #ophiolite), but it formed under the ocean.
Read 5 tweets
14 Aug
A catastrophic #earthquake in 2010 on this fault system in #Haiti killed ~250k people. It just ruptured again, this time to the west. Hopefully the lower population density in this region, further from Port-au-Prince, will mitigate the impact. 😧

#tectoplot #haitiearthquake
The updated focal mechanism for the earthquake from GFZ indicates the rupture was on land, and oblique thrust - similar to the overall 2010 event, which combines a mostly strike-slip mainshock with a cluster of smaller thrust earthquakes.

#tectoplot #haitiearthquake
Scattered aftershocks from the 2010 earthquake were still occurring up through this year, although none were close in magnitude to the mainshock.

#haitiearthquake
Read 6 tweets
13 Aug
The #SouthSandwichIslands are a wonderful example of #platetectonics in miniature. The #SouthAmericanPlate is subducting west at ~7 cm/yr below the #SandwichPlate; this #subductionzone hosted a Mw7.5 #earthquake yesterday (Aug 12). 1/4

#tectoplot Image
The depth of the #earthquake is still poorly constrained. GFZ puts it shallow, above the plate interface, dip 11°. USGS puts it deeper, within the slab, dip 26° and non-double-couple. Historical events of this scale in the region are old so not much help - 1929, 1933, 1964. 2/4 ImageImage
Given the curvature of the #subductionzone, it would certainly be reasonable to have some intra-slab deformation, and fracturing could be complex, leading to non-double-couple. The closest large event (1964) was apparently quite deep (125 km). 3/4
Read 4 tweets
12 Aug
At this point I just assume no one knows anything. (Including myself...) This is especially important when you're working between fields - the same word can mean different things to different people.

Example from #earthquakescience below...
...Fault slip rate = (1) average slip rate recorded by geology, (2) modeled average slip rate from GPS, (3) how fast the fault slips in an earthquake. But somehow, NOT (4) the rate the fault is slipping right now (probably zero)...
...Aseismic = (1) has not generated recorded seismicity, or (2) cannot generate earthquakes. These are very different things!...
Read 4 tweets

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