Thursday, 31 May 2012

Thursday video: structural geology I

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Today's video introduces structural geology (part I). It is presented by Jonathan Bergmann and includes an interview with Dr. Randall Marrett. It is a good introduction to our little corner of the geological room, and it may be useful if you are deciding now what do you want to have as a profession later on. Be aware: structural geology is cool!!

If you are studying geology in school, or in college, this video is also useful for introducing the basic concepts of structural geology. If you have any questions... well, just ask, that is why there are comments in this blog!

Enjoy!



(yep, I know... I forgot to upload the video yesterday... ahem...)

Thursday, 24 May 2012

Thursday video: Difference between crust and lithosphere

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Mechanical and chemical division of Earth.
Today's video deals with a topic of tectonics: the difference between crust and lithosphere. Many people mix these two things, and hopefully this video will help you to know when to talk about crust, and when to talk about lithosphere. It is very easy, but many people just don't pay enough atttention!

This video has been produced by Khan Academy, a non-profit organisation which aims "to provide a high education to anyone, anywhere". A really good initiative. Visit their site here, where you will find many resources of virtually anything!: www.khanacademy.org 

Enjoy!

Source: http://www.youtube.com/watch?v=f2BWsPVN7c4

Thursday, 17 May 2012

Thursday video: the Variscan orocline in Iberia

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An orocline is an orogenic belt where a change in horizontal direction occurs, characterising the mountain belt by a horizontal curvature or a sharp bend. We can find examples of oroclines in pretty much everywhere. A famous orocline are the Carpathians Mountains, formed by the subduction and continental collision between the European and the Apulian plates during the Alpine orogeny.

The Variscan orogen in Iberia also forms a remarkable orocline (but much older than the Carpathians!), and this video shows its evolution, with emphasis in the different timing of granitic bodies emplacement. Very interesting indeed!






This video is the companion of a paper published in Tectonics last year by a series of researchers From the ODRE group (University of Salamanca), Universidad Complutense de Madrid, Natural History Museum of London, University of Victoria and St. Francis Xavier University, both in Canada.

Gutiérrez-Alonso, G., J. Fernández-Suárez, T. E. Jeffries, S. T. Johnston, D. Pastor-Galán, J. B. Murphy, M. P. Franco, and J. C. Gonzalo (2011), Diachronous post-orogenic magmatism within a developing orocline in Iberia, European Variscides, Tectonics, 30, TC5008, doi:10.1029/2010TC002845.



U-Pb (zircon) crystallization ages of 52 late-Variscan granitoid intrusions from NW Iberia (19 from new data, 33 from previous studies) constrain the lithospheric evolution of this realm of the Variscan belt of Western Europe and allow assessment of the relationship between oroclinal development and magmatism in late-Carboniferous-early Permian times. The U-Pb ages, in conjunction with a range of geological observations, are consistent with the following sequence of events: (i) oroclinal bending starts at 310--305 Ma producing lithospheric thinning and asthenospheric upwelling in the outer arc of the orocline accompanied by production of mantle and lower crustal melts; (ii) between 305 and 300 Ma, melting continues under the outer arc of the orocline (Central Iberian Zone of the Iberian Variscan belt) and mid-crustal melting is initiated. Coevally, the lithospheric root beneath the inner arc of the orocline thickened due to progressive arc closure; (iii) between 300 and 292 Ma, foundering of the lithospheric root followed by melting in the lithospheric mantle and the lower crust beneath the inner arc due to upwelling of asthenospheric mantle; (iv) cooling of the lithosphere between 292 and 286 Ma resulting in a drastic attenuation of lower crustal high-temperature melting. By 285 Ma, the thermal engine generated by orocline-driven lithospheric thinning/delamination had cooled down beyond its capability to produce significant amounts of mantle or crustal melts. The model proposed explains the genesis of voluminous amounts of granitoid magmas in post-orogenic conditions and suggests that oroclines and similar post-orogenic granitoids, common constituents of numerous orogenic belts, may be similarly related elsewhere.



Thursday, 10 May 2012

Thursday video: strike and dip

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If you don't have clear what is dip and strike, then take a look to this video. I am planning to post more advanced stuff, but this blog is open to everyone and of course, many different levels of knowledge.

BUT... have in mind that this video could be more precise when she talks about anticlines and synclines in the first slide, and about the direction of strike (which... I wrote some months ago about it). In any case, it is a good introduction for students and the general public.

Enjoy!




Thursday, 3 May 2012

Thursday video: Debunking expanding Earth

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Where the mass comes from?
Have you ever heard about the "Expading Earth" hypothesis? Have you ever watched or even listened to the videos posted in YouTube by comic artist Neal Adams? Well, then you may be as amazed as I am by the imagination and persistence of this man.

Neil defends the idea, and even calls it "theory" (sorry Neil, but yours is not a theory... check out in some dictionary what is the scientific meaning of theory), that our planet Earth is in constant expansion, and there is a conspirancy and cover up of these "facts". Well... this is funny up to some point. The truth is that this is quite sad, because YouTube is a website that many kids use and it end up spreading this unscientific positions. But, how a 12 year old kid may know that?

If you have ever had to answer to some friend why the Earth doesn't grow, if you have ever found a person who thinks that Neil* is right and the rest of the world is wrong... Well, watch this video, produced by "potholer54", an Australian journalist with many interesting videos. Send it to your friends, to the ones who think science is about covering facts!

Have fun... and I will try to post something more serious next Thursday. Or perhaps we will discuss why Earth is not flat :0)


*Well, he and a few more: http://en.wikipedia.org/wiki/Expanding_Earth

Tuesday, 1 May 2012

How to calculate an apparent dip from a real dip (and viceversa) using orthographic projection and trigonometry

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Any student of geology in any university in the world learn during its degree the relationship between the real dip and the infinite apparent dips that a plane contains. Most of students learn how to calculate a real dip from a couple of apparent dips or, inversely, how to work out an apparent dip given the real dip and another direction using the stereonet

Stereographic projection provides an awesome graphic method for these calculation, which is very useful if one has to do a bunch of calculation, but it is not very useful if we have to deal with a large collection of data, or if we need to have a high degree of precision. Then, it is time for orthographic projections and the always useful trigonometry. 

Let's imagine that we have a known plane, and we need to calculate an apparent dip. We know the dip angle and the dip direction or strike, and obviously we know the direction along we want to know the apparent dip. In this context, 

δ = real dip. Note that the real dip is always measured along the maximum slope direction for a plane. No apparent dip can be larger than the real dip. 
α = apparent dip. This is the dip measure along a line which is not the maximum slope direction.
β = angle between the strike direction of the plane and the apparent dip direction.

You could think that it is difficult, but it is actually quite easy. The "trick" lies on relating the three triangles involved in the diagram (one containing a and b, another containing c and b, and another containing a and c). (Note that c is the hypotenuse of the horizontal triangle)

 The following trigonometric relations are quite straight and don't need much explanation:

(1)           sin β = a/c;  a = c sin β
(2)           tan δ = b/a; b = a ∙ tan δ
(3)           tan α = b/c; b = c ∙ tan α

(4)           b = a ∙ tan δ
(5)           b = c ∙ tan α
(6)           a = c ∙ sin β


Clear so far? Now, if you equal (4) and (5), and substitute a by (6),

(7)           a ∙ tan δ = c ∙ tan α
(8)           c ∙ sin β ∙ tan δ = c ∙ tan α
(9)           sin β ∙ tan δ = tan α

what you obtain is a direct relation between  α and δ. If you want to know the real dip from an apparent dip, use (11). If you want to calculate the apparent dip from the real dip, then use (10)

(10)          α = arctan (sin β ∙ tan δ)
(11)          δ =  arctan (tan α / sin β)

Easy, isn't it?

Why you would need to use that? Well, for example, I need it sometimes; I work interpreting satellite images, focusing on structural geology. When I measure fracture lengths on a plane, I cannot really measure their length: What I measure is an "apparent length". That means, I measure the projection of a line on a horizontal plane. For example, a 100 m fracture on a plane dipping 80 degrees will look very short if the direction of that fracture is the dip direction of the plane, but it will look as 100 m if the fracture is oriented along the strike. Any direction in between, will be variable. If it is variable, how can we correct it? knowing the apparent dip in that direction.

This method provides a way of correcting this distorsion, simply using any spreadsheet. You know the length of every single fracture, and the length you have measured. You also know the real dip of the plane (well, I can measure it on the DEM!), the strike, and that is all you need to know. But this will be another explanation, coming soon :-)

Feel free to make any comments, or perhaps any correction of suggestion. Hopefully this has been useful for you.