Questions tagged [structural-geology]

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25
votes
1answer
9k views

How much of a mountain is below the surface?

Is there some kind of math rule for how much does a mountain extend (depth) below the surface? I found an article like this http://www.livescience.com/6595-depth-himalayan-mountain-roots-revealed....
20
votes
1answer
2k views

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

There's been news (some recently) about the New Madrid fault and other active intra-plate faults. For those living in the midwest of the United States, it's been a bit of a shock to learn they have ...
13
votes
1answer
201 views

How to measure stretching direction on chocolate-tablet boudins?

This has always bothered me during field mapping: A boudin with two stretching directions has the form of an M&M candy (or almond shape). What do I do to measure the two stretching directions if ...
12
votes
1answer
575 views

What causes these circular swirls of islands?

I was following the boarder of the US and Canada on Google Maps, because, and found these interesting circular patterns of islands near the Northwest Angle. I have heard of geologic folding but I have ...
12
votes
1answer
650 views

What is the definition of a continent?

Is there a scientific definition of a continent? Is there an academic reference for such a definition? There's a tendency to correlate with tectonic plates, but of course lots of plates don't have ...
9
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2answers
354 views

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

In "An Introduction to Seismology, Earthquakes and Earth Structure", authors Seth Stein and Michael Wysession made the statement: "Faulting will occur along the plane where the shear stress is the ...
8
votes
1answer
2k views

Measuring bed dip angle from contour lines, which lines to pick?

Measuring dip angle of a bed, I choose any 2 contour lines that cross the bed, and then project lines off and measure the angle (I'm assuming this is standard technique that doesn't require ...
6
votes
2answers
304 views

What geologic features could this video game environment be modeled from, and how do they form?

I have been playing this game for a while and I like the city's setting. It seem to be placed on a rock platform, or a mesa maybe, connected to the mainland on the bottom-left. It also has some ...
5
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2answers
628 views

Megathrust earthquakes on subduction zones

I am a beginner in geology and earthquake studies, and learning about subduction zones recently. Question is: do large earthquakes (sometimes referred to as megathrust quakes) occur on any point in ...
5
votes
1answer
638 views

How does Vertical Electrical Sounding Work?

I recently conducted a brief training exercise with VES equipment, although it had to be called off due to rain. I can't help but wonder about the basic physics of it, why is it that increased ...
5
votes
1answer
304 views

What is meant by Massif?

In India, Panchchuli, Kun Nun and Shilong only these mountain are claimed to be massif. Why other mountain formations are not termed as massif...? What are the characteristics of massif..? Anybody ...
4
votes
2answers
1k views

Plotting stereonets in R

Is there a package for R to plot Schmidt nets, like it is done in geology? So far, I only know software like Stereonet. However I think it would make a lot of sense to handle these data in R, to have ...
4
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1answer
213 views

Gravitational Anomaly of a subsurface body

How does the gravitational anomaly measured at the Earth's surface and produced by a subsurface body depend on its depth and on the density contrast of the body relative to its surroundings?
4
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1answer
711 views

What are the differences in terms of appearance between sandstone and limestone?

On the field, I find it very difficult to differentiate the two types of rocks just by looking at them.I know that you should do some analysis of the sample before any conclusions. But, for example in ...
4
votes
1answer
61 views

State of Stress in rotated 2D system

I'm working on the rotation of the state of stress using the book Geodynamics of Turcotte page 101. The 2D coordinate system $x'$ and $y'$ is inclined at an angle $\phi$ with respect to the $x$, $y$ ...
3
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2answers
3k views

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

In any geological plane, strike gives the direction of a line formed by intersection of horizontal plane and geological plane. And dip shows the orientation of the geological plane. While studying, I ...
3
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1answer
131 views

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

Transgressive sills "jump" between bedding planes, following joints: ...
3
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1answer
690 views

Does flow banding only occur in silicic rocks?

I made a video about what I learnt in my A Level Geology course so far, about igneous rocks. One thing I mentioned was that flow banding is mostly only found in silicic rocks (and so mostly rhyolite) ...
3
votes
1answer
1k views

How to determine if this fault is normal, sinistral?

Studying for an exam tomorrow and stumped on this question (parts d and e). The region show in Map 2 consists of a series of sedimentary strata that have been tilted and then eroded, producing an ...
3
votes
1answer
103 views

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

LACMA (the Los Angeles County Museum of Art) is built right next to the famous La Brea Tar Pits and that keeps me up at night†. Question: What is the subsurface structure like adjacent to the Lake ...
3
votes
1answer
120 views

How did these dark lenticular features form? (Near Hoganekkal Falls India.)

These photos were taken near Hoganekkal falls in south India. The darker lenses appear to be the same rock type but have varying thickness and do not form a continuous layer. How did this structure ...
3
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2answers
1k views

what is a transverse isotropic rock?

I know isotropic means equal properties in all directions , but the term "transverse" is not making it easy for me to understand.
3
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1answer
188 views

How can oil can escape from a shale source rock into a reservoir rock?

How can oil can escape from a shale source rock into a reservoir rock if the shale itself is impermeable?
3
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0answers
491 views

How to differentiate between normal and reverse fault, given a stress tensor?

I want to generate synthetic fault slip data using a known stress tensor. I am using a rotation matrix to get the orientation of the principal stress axes, and rotating the tensor [1 0 0, 0 phi 0, 0 ...
2
votes
1answer
3k views

Why should this be a thrust fault?

Why should this figure represent a thrust fault? I know this might be a stupid question, but I really want to understand. Since thrust faults are reverse faults why in the figure above if we turned ...
2
votes
1answer
128 views

Do earthquakes produce folds on rocks?

Earth rocks are frequently folded at surface. Folds in alternating layers of limestone and chert in Crete, Greece. Source: commons.wikimedia Are earthquakes the main mechanism ...
2
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1answer
53 views

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

Since tunnelling is like making a giant horizontal core sample, what did the analysis of the extracted rocks teach us? Is the inside homogeneous or layered, are there "deformations", etc?
2
votes
1answer
75 views

Why is Venus so cracked?

Wired.com's Space Photos of the Week: Perfectly Safe Celestial Coronas includes radar images of the surface of Venus taken from spacecraft in orbit around it. One of them shown below. What causes ...
2
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1answer
1k views

Which type of convergent boundary creates the tallest mountains? [closed]

Which type of convergent boundary creates the tallest mountains? Continental-Continental Ocean-Ocean Ocean-Continental
2
votes
1answer
39 views

Where can I find information about the basic concepts of sedimentary basins?

There are a lot of papers on the web but they don't explain the concepts, they just mention them, taking for granted that the reader knows the concepts. I mean piggyback basin, forearc, back arc, ...
2
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2answers
1k views

How to understand Mohr diagram?

Structural geologists use Mohr diagram to evaluate stress on rock's surface, but I find it difficult to understand how this diagram works (Though I am good at math). Where can I find a good ...
2
votes
1answer
80 views

Can we separate a sandy, clayey & salt layer on basis of their rheological properties?

Since, given enough compaction, salt is able to behave like a fluid & buoyant too if it's overlain by a higher density rock (it could be fine grained clay or a sandy sequence). But in another case,...
2
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0answers
14 views

Does fold tightness correlate to deformation intensity?

I'm studying a migmatite with two overprinting fold structures. The first (Dn+1) is tight and the second (Dn+2) is gentle. Would it be fair to assume that, since Dn+1's tightness is, well, tighter, ...
2
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0answers
185 views

How Big Were the Siberian Traps ORIGINALLY?

252 million years ago, Siberia was subject to a mass series of flood basalt eruptions, creating a large igneous province called the Siberian Traps. Today, it measures three million square miles in ...
2
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0answers
1k views

Drawing strike lines when given strike and dip

Map 1 shows a hillside with topographic contours at 50 m intervals. At Locality A a coal seam is exposed at the surface. The strike and dip of the seam is as indicated on the map. Use this information ...
2
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0answers
275 views

measuring fracture length and width using PKN and KGD models for hydraulic fracturing?

I am trying to understand PKN and KGD models for hydraulic fracturing. I am wondering why equations for fracture width and length does not include 'breakdown pressure' (pressure at which fracture ...
1
vote
1answer
86 views

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

What are all these gaps / circular structures found on a photo taken by Mars rover Curiosity at Dingo Gap? Have a look at the image I made to see what gaps I mean. (I know the quick paint job looks ...
1
vote
1answer
92 views

Structural geology, regarding cross section of two outcrops with similar characteristic but very distinct dip

I found an outcrop of sandstone interbedded with claystone which strike/dip is N 195 E/42. Then, around 4 km at the south of previous outcrop, i found another outcrop with similar characteristic, but ...
1
vote
1answer
36 views

Have cosmic ray muons or solar neutrinos been used to probe inside the Earth?

By looking at the attenuation of cosmic ray muons passing through pyramids as a function of their direction, it seems it is possible to "look inside" by mapping their 3D density distributions. ...
1
vote
1answer
794 views

are laccolith, lobolith, dikes and sills formed of porphyritic texture?

Laccolith, lobolith and batholith are shapes of igneous rocks which exist below the surface of the earth, dikes and sills too. And the way they formed by makes me think that they have a porphyritic ...
1
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0answers
29 views

Is there a way to convert from Dip direction to Strike direction?

I have some set of recorded data in dip direction/dip format. I wish to transform then to strike/dip. I am currently having a hard time imagining how to convert from dip direction to strike direction. ...
1
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1answer
26 views

regarding four-way closure in structural geology

I am a little bit confused on the two concepts in structural geology, i.e., what is the relationship of anticline and four-way closure. My understanding is that anticline belongs to four-way closure. ...
1
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0answers
30 views

How can one find the true dip angle and quadrant when an apparent dip and strike is provided?

I am trying to solve the following question but cannot figure out how: A layer of limestone has a strike of East to West on a hilltop, and an apparent dip of 40 degrees on the vertical hillside ...
1
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0answers
54 views

Calculate normal and shear stresses with fault plane dimensions, slip, orientation and regional stress tensor

I'm trying to verify that some code I have is running correctly when calculating the Coulomb stress change. So for a example I have a fault with : Length = 70 m, Slip= 0.004 m, Orientation(strike/dip/...
1
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0answers
22 views

ALOS Palsar SLC data products for geological mapping

I have some ALOS palsar SLC data products which are downloaded from ESA database and they are processed with ESA ALOS-PALSAR IPF processor which is different from JAXA. Formerly it was not possible to ...
1
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0answers
194 views

Do reverse faults tilt the underlying layers of rocks to form an angular unconformity?

I came across this question while studying in my text book: What does the existence of strata containing a reverse fault and above them parallel strata refer to? I thought that referred to an angular ...
0
votes
0answers
30 views

What local formation am I drilling?

I'm having a water well drilled on my property. The drilling crew has been working since Wednesday. They are using an airlift rig with a 6in bore. They are at 300ft and only have 4gpm. I would really ...