# Tag Info

31

I'm not quite sure if the question is asking about glacial, ice ages, or snowball Earth, and whether it's about the onset or end of a glacial period. I'll try to hit all three. Ice Ages and Milankovitch Cycles Ice ages are long spans of time that marked by periods of time during which ice reaches far from the poles, interspersed by periods during which the ...

23

Ocean lithosphere (geophysical definition of crust + upper mantle that acts as a 'plate') is primarily of basaltic composition - the upper levels are basalt and the lower levels are gabbro. The top levels have been proven with boreholes, whilst the lower levels have been inferred from transform fault sampling and comparisons with ophiolites. This sequence is ...

23

Sea-level only sort-of corresponds to the oceanic–continental crust boundary. In depth, they don't correspond at all: It's the same story at an active margin: the plate boundary at a subduction zone is buried several kilometres beneath a wedge of sediment: What about spatially? As you can see from this map, they don't correspond all that well spatially ...

22

Matan, the continents where we all live "float" on the Earth's mantle. The continents are made out of relatively brittle rock called the "Crust" and the mantle is made out of much more ductile material. The mantle, however, is NOT liquid. It is just much more ductile than the crust so, in geologic time, it can flow (like silly putty). Also, the mantle is ...

21

One of the key aspects of plate tectonics is continental drift. The person who came up up with the theory of continental drift was Alfred Wegener. He published his theory in 1912. One of the issues with the theory that geologists at the time had, was that Wegener was not a geologist, but a meteorologist. He was publishing a theory that wasn't associated ...

20

The best argument I've heard supporting strong skepticism of plumes, if not total dismissal, is that the theory is too flexible. To put it more bluntly, this amounts to saying that it is unfalsifiable and therefore not helpful (in Popper's words, "A theory that explains everything, explains nothing."). Erik Lundin, a Norwegian geologist, is a fairly ...

20

Trench has a specific meaning in plate tectonics. It doesn't just refer to any valley; it specifically refers to the features formed at subduction zones by the flexure of the downgoing oceanic plate. A classic example is the Marianas Trench which is the location of the deepest point in the ocean Fast spreading ridges do indeed have valleys at their centre ...

19

Is there a theory on how the Earth's plates were initially formed? The answer to this is has roots in another question you asked about the differences between continental crust and oceanic crust. Carlson et al. (2014) just put out an excellent review of what we know about Earth and how it formed through time. I am using it as a guide and source for much of ...

19

The Cascades (the volcanic range that Mt. St. Helens and Mt. Ranier are a part of) are "arc volcanoes" (a.k.a. "a volcanic arc", etc). Volcanic arcs form at a regular distance (and fairly regular spacing) behind subduction zones. Subduction zones are areas where dense oceanic crust dives beneath more buoyant crust (either younger oceanic crust or ...

18

The plates are not as rigid as you think. You seem to be imagining the situation as something like this: I boil an egg and take the shell off in pieces, but I can't take a piece of shell from the end and make it lay flat on the side of the egg. However, rock is not that rigid on scales of thousands of kilometres and millions of years (I don't think there ...

18

First, a correction. Most climate scientists are climatologists rather than meteorologists. Climatology and meteorology, while related, are quite distinct sciences. Meteorologists and climatologists don't see eye to eye. The consensus amongst meteorologists regarding climate change is not nearly as strong as it is amongst climatologists. Many meteorologists ...

17

According to one survey using GPS mounted on a plateau below the summit, Mount Everest is increasing its height approximately 4mm each year. As for plate techtonics, this site describes it pretty simply: Much like a car crashing head-on with a truck, crumpling occurred when the two continents met and the result was the creation of the Himalayan Mountains....

17

The Ural mountains are one of the oldest mountain ranges on Earth. They started forming about 300 Ma ago by the subduction of the oceanic crust once attached to the Kazakhstania plate under the ancient Laurussia continent. A subduction process that finished about 240 Ma ago. The tectonic plates are far from fixed, some of them disappear over time other new ...

16

No, lava does not push the plates apart. The mid-ocean ridge basalt is passively filling the space left by the plates moving apart. Plate motion is driven by gravity (image by Vic DiVenere at Columbia; see note below): There are thought to be two main effects at play: ridge push, and slab pull. We don't know much about their relative importance, but it ...

16

Mountain ranges are usually formed as orogeny where tectonic plates collides, known as convergent boundaries. The continental plates have less density than the oceanic plates and the buoyancy results in that they are mostly above sea level. Continental sea floor is known as continental shelf, but usually, it doesn't reach far from the coastline. Therefor ...

15

Of course it isn't "absurd", and looking at the ball-park energy budget figures you'll see why: First, I don't think anyone is claiming the Earth is completely frozen. More of a "slushy at the Equator" scenario. But let's assume an average 1 km thickness of ice for arguments sake (i.e. probably an exaggeration although polar ice would be thicker). The ...

15

I think you are confused about the timescales and the magnitude of the impact that is being talked about here. The collision between the early Earth and a roughly Mars sized body, Theia is thought to have happened between 4.4 and 4.45 billion years ago. The Pangaea supercontinent had fully assembled by around 250 million years ago. Previous to this there ...

14

I don't recall this being considered a real 'mystery'. For decades after Tuzo Wilson's revolutionary Plate Tectonics the accepted explanation was of a change in direction in the plate motion over the mantle plume. I am pretty sure this was still the case when I read Plate Tectonics: How It Works (by Cox and Hart, Wilson's former students - I encourage you to ...

14

The oceanic plates are themselves formed from the divergent boundary, so probably not. Even if a new rifting occurred exactly at the boundary, the result would eventually be that the ocean floor surrounds the divergent boundary. A very simplified model of an ocean would have a divergent boundary in the middle. This oceanic spreading center might have ...

14

Such forms tend to be created by glacial activity, which, ahem, the ice-covered continent is known for. Much discussion of this in the related question in Skeptics: Are there three pyramids in Antarctica? Here's the generic answer in Wikipedia: A pyramidal peak, sometimes in its most extreme form called a glacial horn, is an angular, sharply pointed ...

14

As it was explained to me at university there are two factors; buoyancy and erosion. Rock buoyancy is a major factor, fresh Basalt is hot and dry and has a much lower density than older oceanic crust. This is why the mid-Atlantic ridge rises above the surrounding seabed. The effect is even more pronounced with seamounts because they're composed of even ...

13

Please take into consideration that I am not a specialist of plate tectonics, just a paleontologist. Although this cycle is often nicknamed the Wilson cycle (probably because of Wilson, 1966), the idea that supercontinent formed cyclically every 440Ma was advanced by Worsley et al. 1984 (see review on the subject by Nance & Murphy 2013). The mechanism ...

13

One permanent threat to plate tectonics is the oceans vanishing. The scientific jury may still be out on this matter, but most geologists and geophysicists consider water to be the lubricant that makes plate tectonics possible. In a billion years or so, the Sun will have become 10% more luminous. This is conjectured to make the Earth to undergo an ...

13

Why was the initial theory of plate tectonics so controversial? Plate tectonics was anything but controversial. A mere four or five years expired between its original proposal in 1963 by J.T. Wilson and its almost universal acceptance in 1967 or 1968. What was controversial was Wegener's theory of continental drift. His theory was not plate tectonics. ...

13

To the best of our knowledge, sea-level is rising because the volume of water is increasing. There is substantial local variation in sea-level change; it's falling in some parts of Canada. But of the dozens of controls on local and global sea-level, the net effect is currently an average global rise of about 3 mm/y. It's not really 'a riddle'. The ...

13

Crustal plates are not homogeneous, uniformly continuous rock masses with uniform stresses. They are a mixture of rock types with variable stresses and stress concentrations. They are also discontinuous. There are fault zones within plates. Some faults are active & others inactive. When movement occurs along an active fault, usually due to in-plate ...

12

Plate tectonics appears to require multiple factors all arraigned in feedback loops. The planet must be large enough that it's surface to volume ratios is low enough to trap enough heat from radioactive decay to power tectonic motion. Convection requires a minimal threshold of energy trapped in the material before that trapped energy causes mechanical ...

12

There isn't a concentration of tectonic plates - rather the opposite! The "Ring of Fire" is a pattern really. It is marked by subduction zones. So the question becomes, "Why is the Pacific Ocean surrounded by subduction zones?". Think back to Pangaea. This was a supercontinent that formed in the late Palaeozoic. Virtually all of the Earth's land masses ...

12

This is a very simple answer, and it depends on the what the seismometer is measuring. I'm assuming you mean the time derviative/intdegral $\frac\partial{\partial{}t}$ or $\int{dt}$ Most seismometers measure displacement over time or velocity over time (series). So taking the time derivative will give you the spectral velocity or the spectral acceleration ...

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