#igneous rocks

“Yooperlite” on Lake Superior! These rocks are sodalite in syenite and the sodalite part glows when a long wave UV light is shined onto them! Hunting these is so much fun!!

📽️ By: Thecrystalcollector

Volcanic igneous plumbing systems or VIPS are interconnected channels and chambers that magma travels through and is stored in. Generating VIPS starts with partial melting. Partial melting is dependent on temperature, pressure, flux addition, and composition of the source rock. The magma then migrates out of its source region by the process of segregation and extraction.

Segregation is the separation of melt from its source rock through gravitational compaction. When the source rock experiences compaction, minerals start to melt at grain boundaries. Melt droplets then build up and connect into melt pools until they are being extracted. There are two end members of melt extraction: melt can be extracted in pulses if the development of magma channels are rapid and the network is highly interconnected, or melt can be constantly drained from the source if the magma channels are developed in a continuous and steady manner.

Extraction controls the chemical composition of the melt, the amount transported and the volume contained in plutons. For instance, if the magma channels are not well connected, the source may not be drained successfully, and dykes may freeze before propagating far enough to feed plutons. If the source rock could not initiate dyke ascent with sufficient melt, the source rock may remain undrained, favoring diapiric ascent of the source rock.

What is a diapir? It forms when a blob of hot, ductile magma ascends through brittle overlying rocks. It is the main mechanism of magma transport from the lower to middle crust. Diapirs are often fed by a series of dykes and sills.

When magma stops ascending, a magma reservoir is formed. The kind of reservoir is determined by the depth and geometry. Emplacement in the lower crust are classified as plutons. Thickness can range from one km to tens of km and is more than the length of the reservoir. It takes 100,000 to 6 million years to form.

Emplacement in the middle to upper crust are sills. These are sheet intrusions that normally are emplaced at least 3km below the surface. When sills become stacked, the form a dome shaped structure known as a laccolith.

Thanks for coming to today’s lesson! Tune in tomorrow one last igneous trivia! Fossilize you later!

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Pink or White?

Both of these rocks are GRANITE which means they are coarse-grained (phaneritic) rocks that contain at least 20% quartz and no more than 10% mafic minerals. The rest is some kind of feldspar. Mafic minerals are the black grains you see in the samples above and are most likely the minerals pyroxene, amphibole, or biotite.

(Want to learn more about igneous rocks and how we classify them? Click here for an igneous rocks introduction)

The distinction between these two granites comes with the type of feldspar contained in the rock.

The feldspar content in alkali granite is almost entirely orthoclase (aka k-spar or potassium feldspar). The pink grains in this rock are orthoclase crystals. Fun fact, my dad's headstone is an alkali granite and the specific variety used is called "North American Pink."

Other alkali granite samples.

The feldspars that make up plagiogranite (also known as tonalite) are almost entirely plagioclase feldspar, which contain calcium or sodium instead of potassium. These are typically dark grey to white in color. In this particular sample, the white grains are sodium feldspar (aka albite).

Guys today in Petro I saw an Amazing Thing™ that even the professor told me was amazing to catch; Sericite and Carbonates in a thin section of a Quartz rich Syenitoid rock (XPL)!!!!!!