23 votes
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How much of a mountain is below the surface?

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 ...
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11 votes
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What causes intra-plate faults, such as the New Madrid fault?

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 ...
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  • 1,276
8 votes

What is the reasoning behind the statement "Faulting will occur along the plane where the shear stress is the highest"?

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 ...
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8 votes

What is the reasoning behind the statement "Faulting will occur along the plane where the shear stress is the highest"?

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 ...
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  • 3,780
8 votes
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Measuring bed dip angle from contour lines, which lines to pick?

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 ...
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8 votes
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Strange circular structures found on a photo from Mars rover Curiosity at Dingo Gap

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 ...
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  • 2,625
8 votes
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Terracing in Mahad

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 ...
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  • 1,206
7 votes
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What causes these circular swirls of islands?

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 ...
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  • 5,861
7 votes

Megathrust earthquakes on subduction zones

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 ...
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  • 2,201
6 votes
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In Geology - True Dip versus Apparent Dip: When dip is estimated along a line of section, how is that dip determined in relation to true dip?

This is an excellent question. The answer requires that one visualize a 3-dimensional space with a dip plane, a vertical plane coincident with, and aligned along, our line of interest, and the trace ...
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5 votes

Megathrust earthquakes on subduction zones

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 ...
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5 votes
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Why should this be a thrust fault?

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 ...
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  • 6,464
4 votes

How to measure stretching direction on chocolate-tablet boudins?

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 ...
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4 votes
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How does Vertical Electrical Sounding Work?

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 ...
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4 votes
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Do discordants dykes ever travel concordantly (are transgressive dykes a thing)?

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 ...
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4 votes
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Does flow banding only occur in silicic rocks?

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 ...
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4 votes
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Plotting stereonets in R

You might try the RockFab package. I am not a structural geologist but I use other R packages for geological endevours. Documentation.
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  • 316
3 votes

Can we determine strike when dip is given? What about the reverse?

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 ...
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3 votes
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How to determine if this fault is normal, sinistral?

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, ...
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  • 441
3 votes
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Gravitational Anomaly of a subsurface body

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 ...
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  • 1,663
3 votes

What did tunnel digging teach us about the formation of chains of mountains?

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"...
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3 votes

What is meant by Massif?

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 ...
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3 votes
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Do earthquakes produce folds on rocks?

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 ...
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3 votes

Could the LA County Museum of Art fall into the LaBrea Tar pit during an earthquake?

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 ...
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  • 292
3 votes

Is there any intuitive and easily understandable way to help students understand the triangular facet of outcropted stratigraphy?

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 ...
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3 votes

How can smartphones help geologists in the field?

Part of the answer lies in where the geologist will be working and if there will be coverage by cellular/mobile telephone networks. It is my experience that a number of geologists, particularly ...
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3 votes
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How can smartphones help geologists in the field?

I know a young geologist using his smartphone as a compass. When we were in the field doing structural measurements together, I would: Record a waypoint using my hiking GPS Take the measurement with ...
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2 votes
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What geologic features could this video game environment be modeled from, and how do they form?

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.
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