The day that our class went to the lighthouse at the southern tip of the island was one of the few days that it rained on us. It was only sprinkling, but I frantically kneeled down trying to use my upper body to cover my notebook as I wrote down everything being said by our professor Lisa. As the rain stopped and the wind blew the clouds on their way all of us noticed a giant darker grey block of rock in the middle of a bunch of white pumice. This block had many crystals of minerals with brilliant colors in it. We all wondered, how it got there and what made this rock so different from the rest?
Our friend, the big block, was actually thrown through the air around 5 km (3 miles) from the vent of the volcano during the Minoan eruption. It occurred at the same time as the pyroclastic surges of phase 2 (see Jessica’s blog The Forces that Drive Rock for more on pyroclastic surges). The story is a little bit more complex than it simply being a piece of rock being torn from the vent with the explosion. The pumice around it is felsic while the block itself is mafic. Felsic means it is 70% silica or greater while mafic means it is 50% silica or less. How did these two chemically different rocks come to be in the same deposit?
When new magma is being made it starts out mafic, and then slowly becomes more felsic as the mafic minerals crystallize out first in addition to incorporating some surrounding crust. To picture this, imagine when you boil salt water and the salt precipitates out of the water. The salt is the mafic minerals and the water is the magma.
The terms mafic and felsic though largely depend on the percentage of silicate minerals which are based on silicon and oxygen for their basic elemental components (1). The “MA” in “mafic” specifically means magnesium and the “FIC” is from a Latin word for iron (1). Then the “FEL” in “felsic” is based on the term feldspar, and the “SIC” is to specify the higher percentage of silica (1). I’ll come back to feldspar later, but the question still remains of why this big block of mafic basalt is here surrounded by felsic pumice.
There was actually a shield volcano covering the entire bay long before the Minoan eruption. Shield volcanoes spread out over long distances because it is made from mafic magma that is very high in temperature and low in silica content (around 1200 degrees C). The higher temperature and lower silica content equals a less viscous lava flow while a lower temperature and higher silica content means it will be more viscous.
If something is less viscous it will flow more like water and if something is more viscous the flow will behave more like honey. When the Minoan eruption occurred it blew straight through the overlying shield volcano sending chunks of mafic basalt through the air along with the felsic pumice from the vent. Now we know how the block of basalt got there, but why does it look so much more different than the pumice?
The block is such a darker grey compared to the pumice around it because mafic minerals are usually darker in color due to it being made of heavier elements like magnesium, iron, calcium, and sodium (1). The pumice in comparison on Santorini is a much lighter color because it is more felsic due to the lighter elements like silicon, oxygen, aluminum, and potassium (1). Why though were there such brilliant crystals within this big block?
That’s because different minerals crystallize at different times based on the composition of the magma. Bowen’s Reaction Series tells us which minerals will crystallize first based on composition and temperature. If you look at figure 3, mafic magma is hotter than felsic magma.
This is because of the shorter length of time the mafic magma has been sitting in the crust compared to the felsic. Now, I only put the minerals that are in the rocks at this deposit so this Bowen’s Reaction Series is incomplete.
If you look at the reaction series you will see that the green crystal in figure 2 is pyroxene and olivine, and the white crystal in figure 4 is plagioclase. This is why I wanted to wait to explain feldspars though because as you see there are two types of plagioclase outlined in the series.
Feldspars refer to the family of silicate minerals in igneous rocks which are rocks that form from the cooling of magma (1). Plagioclase starts out as calcium feldspar and overtime it absorbs more sodium. This is just to give you a better picture of what’s going on in figure 3.
We know that the big block is mafic because it is covered in these mafic crystals, but why is the majority of it black and not just all crystal? The crystals just had more time to crystallize than the rest of the rock which is actually made up of the same minerals. Those minerals are microscopic though so they aren’t visible to the naked eye.
Some of these minerals were crystallized by the time the eruption occurred but most of them were still a part of the magma and the shield volcano above it. When it finally erupted, blowing through the remnants of the shield volcano, the minerals that hadn’t crystallized yet cooled very fast so they are very small. If you will, picture pouring sand into a bunch of honey and then putting it into the refrigerator. The honey hardens and has these sand grains spread throughout it. The black part of the block is the honey while the crystals are the sand grains.
There is just one more item and that is the pumice surrounding the block. We know that its felsic because the pumice on Santorini is mainly biotite and quartz which is on the felsic side of Bowen’s Reaction Series in figure 3.
If you’re still reading this you probably love science at least a little bit. The evolution of the magma chamber gets pretty complex with being mafic and then felsic along with all these different minerals and crystals too. It reminds me how much I still don’t know about Mother Nature, but it also drives me to keep digging for the answers until I can’t dig anymore.
- Strickler, Mike. “Geomania.” GeoMan’s Home Page. N.p., n.d. Web. 16 June 2016. <http://jersey.uoregon.edu/~mstrick/>