Plate tectonics to earthquakes, and everything in between • Asst Prof @NTU_ASE @EOS_SG • Prev @ Caltech & Harvard • Author • Mom of 3 • She/her
Mar 28 • 10 tweets • 5 min read
I saw these weird circular mounds in the ocean off the west coast of Scotland, and wondered what they were.
Turns out: they're extinct volcanoes, beveled flat by wind and waves, sunk deep beneath the sea. 🧵1/
Where did they come from? Look 1000 km to the NNW & you'll see Iceland - a region with not one, but two sources of volcanoes:
-> Atlantic Ocean spreading & associated decompression melting, and
-> A plume of hot material rising through the mantle.
I'm here to remind you that 20,000 years ago, sea level was ~130 m lower than today.
For Southeast Asia, that means that instead of isolated islands, the Sunda shelf was a vast landmass more than 1/2 the size of the continental USA. 🧵1/
Map source: deviantart.com/atlas-v7x
This history of lower sea level defines the ecology of the region. Plants, animals, and people could move across regions now covered by open ocean.
Australia, however, was still separated by a wide archipelago called Wallacea - allowing it to maintain its own unique ecology. 2/
Mar 10 • 16 tweets • 7 min read
The Galápagos Islands aren't just cool because of Darwin and his finches - they're also a fascinating region geologically.
And the story that the geology and ecology tell together may be the most intriguing one of all.
Like Hawaii, the Galápagos Islands are volcanic, formed above a plume of hot material rising in the mantle. That heat comes from radioactive material near the core-mantle boundary; hot material is low density, so it rises.
People sometimes talk about how the seafloor is one of the great unexplored mysteries - and it's true - but there's a vast region of land that also qualifies: the Tibetan Plateau. 🧵 1/
Standing at ~4.8 km above sea level (15,700 ft), the Plateau is extremely inhospitable to humans: oxygen is almost halved compared to sea level. Operating at this altitude is difficult: movement is exhausting, thinking is hard. 2/
Suddenly, iron dissolved in the oceans bonded with oxygen and precipitated, forming layers of rust on the ocean floor. The deposits got thicker and thicker, reaching 100s of meters. 60% of iron is mined from these layers! 2/n
When rocks *do* break (#earthquake!), we can use that to estimate stress. If you know the direction of slip, you can do even better. This even works for earthquakes that occurred long ago, if they left scratches on the fault!
A Mw6.6 #earthquake just occurred below the W tip of #Java, #Indonesia. Here, the Indo-Australian Plate is sinking below the Sunda Plate. To the north, this #subductionzone produced the devastating Mw9.1 2004 Indian Ocean earthquake and tsunami. 🧵1/5
Ever look at global #earthquakes from the top down? The #NorthAmericanPlate and #EurasianPlate seem simple around the Atlantic - they're pulling apart - but if you follow that boundary across the pole to Russia, it gets weird and diffuse. 🧵1/4
#tectoplot#Iceland provides a remarkable view of the plate boundary. Here, the plates are pulling apart over a #hotspot, so the spreading center is on land instead of at the bottom of the sea.
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
Nov 13, 2021 • 9 tweets • 4 min read
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.
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
Oct 11, 2021 • 5 tweets • 5 min read
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)? 🧵
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.
Aug 14, 2021 • 6 tweets • 3 min read
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. 😧
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.
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
Aug 12, 2021 • 4 tweets • 1 min read
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.
...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)...