How we Deduce Mantle Composition & Structure From Basalts- Igneous Petrology #9 | GEO GIRL

This video covers the importance of basalt on Earth, the 2 main types of basalt (alkaline and tholeiitic basalts), how to read the basalt tetrahedron and the meaning of the plane of critical silica undersaturation and the plane of silica saturation (alkali index composition: alkalic to alkali-calcic vs calcic to calc-alkalic basalts). Then I provide a few optical and hand sample differences that help to tell apart alkali and tholeiitic basalts, and then we touch on the nepheline-quartz phase diagram again (first described in the phase diagram video: ), in which we relate the alkali and tholeiite categories to nepheline vs. quartz composition. Finally, we get into how we can use basalts to determine mantle composition and structure (4 main ways), followed by discussion of the relative thicknesses of the upper mantle, transition zone, and lower mantle layers within Earth, and finally we dive into mantle processes that determine the basalts that will reach the surface and become available for us to analyze directly. Here, we move on to the debate over whether alkalis and tholeiites have the same or different magma source(s). If these two basalt types come from the same magmatic source, the partial melting processes and pressure during melting must differ. However, this is still uncertain, so I encourage you to research this more on your own (very interesting!). Lastly, I list 4 types of magma generating environments (based on the 3 main ways mantle melting in induced we discussed earlier in the video) that I will be making future videos on to discuss in more detail. Hope you enjoy! ;D

References: Essentials of Igneous Petrology by Ronald and Carol Frost: https://amzn.to/37Zs9AL
Earth Materials by Cornelis Klein and Anthony Philpotts: https://amzn.to/3z5sCxe

GEO GIRL Website: https://www.geogirlscience.com/ (visit my website to see all my courses, shop merch, learn more about me, and donate to support the channel if you'd like!)

0:00 why is basalt important?
1:20 2 types of basalt
1:55 basalt tetrahedron
4:14 differences between alkali and tholeiitic basalt
5:50 nepheline quartz phase diagram
7:35 4 ways we determine mantle composition & structure
10:39 layers of Earth's interior
13:06 what induces partial melting of the mantle?
15:10 what composition results from mantle melting?
16:49 different or same source for alkalis & tholeiites?
18:11 4 types of magma generating environments
19:45 upcoming videos!

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