# Tag Info

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Is there some kind of math rule for how much does a mountain extends (depth) below the surface? Definitely! It is called isostasy. When I was a student, the lecture about isostasy started with a slide asking "why don't the mountains fall over?" (it may sound better in my native language..) First, some background: Mountains, and in general the Earth's ...

11

Good question! I'll lay out the prevalent idea, originally (I think) put forward by Sykes, 1978 and supported by a large body of subsequent work by many authors. There are other models, however. In the most general sense: Intra-plate faulting is due to old, pre-existing weaknesses that are reactivated due to the plate interiors being very close to ...

8

This statement is known as the Wallace-Bott hypothesis. It is mostly valid for reactivated faults rather than fresh rocks(where cohesion comes into play). It says that the slip in a fault plane will occur along the direction of maximum shear stress. So we have a stress vector acting on a fault plane(the direction given by the direction of force), it has ...

8

This statement is referring to depths within the crust where the lithostatic pressure is sufficiently high to suppress the formation of dilatant fractures. Rock is a brittle material, and if loaded to failure while under little or no confining pressure, it may fail by the growth of mode-I fractures oriented normal to the least compressive stress. At depth in ...

8

You are correct by saying that the dip angles vary greatly depending on the contours that you pick. In contrast to artificial maps designed to teach basic structural geology to students (such as the one you added in your question), natural maps are rarely "nice and friendly" regarding dip calculation. There are several reasons for this: The dip angle may ...

8

There are no circles, it is an image artifact. Here is another picture from Dingo Gap that shows nothing at the exact same position. Your mind is a well-trained pattern-matching machine that makes up patterns even when they are not actually there. In other words don't jump to conclusions. You are claiming that there are actual structures, which is one step ...

8

The Deccan Traps are the geological formation that make up the Western Ghats in India. They were formed by massive volcanic eruptions that formed flood basalts. These are layers of lava that are extruded from fissures and spread over wide areas of relatively flat ground or over flat sea floor. The terracing is natural. Each of the layers that you see in the ...

7

Near the Lake of the Woods, the island swirl or vortex is due to structures in bedrock beneath the lake. The rocks in this area are old and have been folded by tectonic action. The area has either been folded into a dome or a basin, exposing different layers of bedrock. Glacial activity afterwards has 'flattened'the area, and differential erosion within the ...

7

Megathrust is not a particularly well defined term but in my experience it generally refers to very large earthquakes that occur on the subduction interface. The subduction interface is the main surface which separates the overriding plate from the subducting plate and is where most of the convergence is accomodated. (image source) Earthquakes occur on ...

5

Yes, mega thrust earthquake can occur in any point of any subduction zone. However, some areas areas are much more active than others, meaning that the frequency of earthquakes and energy released per event will vary largely from place to place. Convergence rate is one important controlling factor for that, but not the only. Regarding the difference between ...

5

First don't rotate the whole thing, you are just confusing yourself by rotating the bedding plane. You can't rotate the bedding planes and expect faults to match up like that. Your eye is treating the bedding plane as horizontal which makes it a completely different from of fault. A thrust fault is just a reverse fault with a shallow fault angle. Pay ...

4

If you can't get a macroscopic sense of lineation, try microscopic measurement (thin sections along the supposed plane principal stresses) and transfer them in macro. The Techniques of Modern Structural Geology: Strain Analyses Vol I would be very useful for reference.

4

The diagram is perhaps a little mis-leading. Think of the ground/rock between the electrodes as a big resistor. The surrounding rock can be modeled as a set of parallel resistors. Most of the electrons will take a direct straight line route between the electrodes, but some of them flow through this surrounding rock, so increasing the current and reducing the ...

4

Yes, they are. It's not the sills or dykes that "jump", it's the magma. Depending on the various parameters such as viscosity, stress, temperature, pressure, and local conditions and availability of joints, the magma will flow either as a sill or a dyke (or some other intrusive body). Notice that your two sketches are basically the same thing - it's just ...

4

Flow banding can occur in all igneous rocks, regardless of their composition. Any kind of molten rock that flows may eventually cause some flow banding. Sometimes the flow banding is not even obvious to the eye and can only be detected using geophysical methods such as magnetic anisotropy. Here's an example of flow banding in a mafic rocks from Skaergaard ...

4

You might try the RockFab package. I am not a structural geologist but I use other R packages for geological endevours. Documentation.

4

This is a tale of two three dimensional coordinate systems. You may need to take a bit of a side track if your students are not already familiar with the concepts of three dimensional coordinate systems, three dimensional vectors, the three dimensional dot and cross products, and transformations between coordinate systems. The image below portrays two such ...

3

No. Strike is a line value whose direction is referenced with respect to true north. Dip can be any 0-90 degree angle (below horizontal) perpendicular to that line. It must be physically measured in the field....usually by some seriously underpaid graduate lackey.

3

So, the basic geological history is given. From this, the units named Bed 1, were deposited, then tilted. Bed 2 was deposited and tilted (along with Bed 1). The entire sequence was then faulted. So, how can we tell what the fault did? First of all, notice that there was movement of the eastern block to the north-west. This can be deduced because all the ...

3

The gravitational anomaly (delta_g) changes linearly with the density contrast, and proportionally to the inverse square of depth. That's simply a version of the 'Universal Law of Gravitation'. Here is an example for the anomaly created by an spherical anomalous spherical body with a density difference of delta_rho relative to the surrounding density. G is ...

3

Tunnels have provided nothing more than what was learned from conventional geological drilling and surface mapping. Tunnels provide supplemental information. Very rarely do they provide "make or break" information that is pivotal to geological understanding on a large scale. A tunnel is a lateral large diameter hole. Geological drilling produces small ...

3

A massif is an undisturbed solid rock mass, which is demarcated by e.g. faults. Massifs keep their internal structure and are displaced as a whole. Only mountains of mountain ranges fulfilling this criteria are called massifs.

3

No, earthquakes are sudden, often very energetic events which produce faults and discontinuities. There is a sharp break in the strata when subjected to a powerful earthquake. The folding of strata is a much more gradual process, where rock deep underground is subjected to enormous lateral pressure over millions of years. This pressure is caused by plate ...

3

The two geological faults that may be responsible of large earthquake in proximity of the museum are: Hollywood fault; Santa Monica fault (actually many branches, but they all go together as Santa Monica fault). The museum seems to be enough far away from the fault trace (the surface expression of the fault itself), so it is unlikely that there will be a ...

2

A flatland with occasional huge rocks and abundant rivers and lakes. One possibility might be a plain that experienced glaciation, bringing in the erratic rocks and carving the landscape.

2

Is this a homework question? These are quite steep dips, strongly implying that substantial deformation (folding) has taken place. The interlayering of steep mudstone pretty much rules out your outcrops as being original bedding structure. Draw your examples in plan, using the normal dip and strike notation. Sketch in some possible continuation of the ...

2

In general, a good answer to "where can I learn the basic concepts of X" is "a textbook on X". For sedimentary concepts, you might try Nichols (2009), for instance. Academic papers are a publication vehicle for new research, so you'll seldom find much introductory material in them. For terms like "intracratonic" and "intramontane", you might also find it ...

2

The type of convergent boundaries is not the only factor. From the current configuration of the world one could be tempted to say that Continental-Continental collisions would form the tallest mountains. And they are indeed the most dramatic tectonic collisions and they formed the Himalayas, that are almost 2,000 m taller than any other mountain on Earth. ...

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