TEMP Reddit Journal Entry

Over the last couple of years I’ve become aware of a new way in which I learn things. It is a sort of consolidative learning.

Since I’ve retired, one of my activities has been to learn about geology. That mostly involved taking classes or reading books; occasionally it happens via going on field trips, but those are pretty few and far between. But, over the last few years, I’ve become of a new way I learn things – or perhaps it is better to say consolidate what I already know, or connect the dots…

Contents

Geology Subs

It involves reddit, which I visit nearly every morning, in response to the daily email that alerts me to new activities in the subs I follow. These are primarily geology-oriented subs like “whatsthisrock” “askgeology,” and “rockhounds.” Initially I visited because I wanted to get better at identifying field specimens of minerals, and identification requests, and the ensuing discussion, make up a significant portion of the content. After a while, I began weighing in on the debates, and came to recognize areas – such as mafic igeneous rocks – where I had something to contribute.

This morning, for example, in response to a comment I had posted identifying something as vesicular basalt, someone replied asking about the difference between that and iron ore. I knew in general, but as I wrote my response I realized there were details I was unsure of (e.g., whether iron ore is primarily sedimentary), and which minerals that make up basalt commonly contain iron. I looked that information up to make my answer more complete, and that both increased my confidence in what I had supposed (iron ore is indeed primarily sedimentary), and I learned more about iron-containing minerals. And it left me with a hypothesis – that I will probably verify at some later point – about why basalt is not a economically viable source of iron.

Here is the answer I wrote:

Iron Ore

Iron ore is usually formed as a result of chemical precipitation or hydrothermal alteration which produces concentrations of iron-rich minerals like hematite and magnetite. Rocks that form in this way would not usually have all the tiny vesicles that are visible in this rock. Basalt, on the other hand, can (under some circumstances) be this way due to volatiles that expand due to decreasing pressure as the basalt erupts. While basalt can have a lot of iron in it, it is spread more evenly through the rock and bound into different minerals (which may also bind magnesium rather than iron) — this probably makes it harder and more expensive to extract the iron in basalt economically, but I don’t know anything about extracting or refining metals so this is just a guess!

Other Answers

Out of curiosity I looked through the posts I’ve made on reddit, and was astonished at the number I’ve contributed. For my own reference, I’m including the more substantive one’s here;

Anorthosite

While not among the hardest rocks, anothosite — a type of plagioclase (feldspar) is an interesting rock. It is light colored, can have a lustrous surface, and is relatively resistant to weathering and erosion (in contrast to basaltic rocks). Anothosite makes up much of the moon’s surface, and is the reason our moon is unique in the solar system for its shinning white surface, which if your story has magical elements in it could be something you could work with.

Vesicular Basalt

As others have said, this is a vesicular basalt. That’s the very fine-grained black material. The vesicles (bubbles formed by volatiles outgassing as the basaltic magma erupted) are filled with minerals (so now, technically, they should be referred to as amygdules) — quartz, calcite and zeolites are likely candidates for the fillings.

Most likely the vesicles were filled well after the basalt solidified as a result of hydrothermal deposition. I’d guess the greenish minerals are epidote, and the orange-ish are quartz or other minerals that are stained by iron.

I love basalt, and the way it gets altered over time.

Water Ice

Actually, according to the scientific definition of “mineral,” it is a mineral.

     There is a lot of information about how snowflakes form (Ken Lebrecht of Cal Tech has amazing videos on you tube), but I’ve not found anything especially informative about how ice crystals form on water.

     About all I can say is that around 0 to -4 C one of the faces of the seed crystal is more energetically favorable to crystallization and so that’s where the molecules tend to attach most frequently and thus you get the randomly oriented acicular crystals — and then the intervening surface freezes.

     The shapes of snow crystals forming from vapor are extremely sensitive to temperature and humidity, so I presume the same thing is true for crystals forming on water.

     I’d love to know more!

…

I did a bit more reading. Another thing that’s going on is when ice forms it releases heat which, if the ambient temperature is just below freezing, accounts for the spacing between crystals. This is true of any substance that freezes, but water has a particularly large amount. It’s called the latent heat of fusion.

Spreading Zones and the Midcontinent Rift

Spreading zones are what produce oceanic crust, so by definition they are where the newest oceanic crust is; you will see the oldest oceanic crust farthest away from the rift.

     But you’ve got the causality reversed. The rift comes first, and then the ocean. That is, a rift opens somewhere and erupts, and if it continues it widens and lengthens and eventually opens all the way to an existing ocean, at which point it beomes an ocean. Over 10’s of millions of years it produces oceanic crust, and pushes apart the land mass where it formed.

     The foregoing assumes that the rift is forming on a continent — it is also the case that rifts can form in the ocean; and sometimes an existing oceanic rift will ‘jump’ to another location on the ocean floor.

     And, as hinted at above, sometimes rifts start to open and then ‘fail.’ This is the case in North America. About 1.1 billion years ago a rift opened in Laurentia (the archaic landmass that includes most of what is now North America), and gradually widened and lengthened until it was almost two thousand miles long. Had it continued, it would eventually have split Laurentia apart and resulted in an intervening ocean. But after 100 million years, it failed — I believe the leading theory is that a landmass collided with the eastern coast of Laurentia/NA, and shoved the rift closed. Lake Superior is, in part, a surface remnant of this rift (although it is a bit more complicated than that). Look up “Mid Continent Rift” if you want to know more on this….

Weathering (of Granite)

Not at the exact same rate, but your flat granite barren will likely have lichen mechanically breaking down the rock, and pools of water that will accelerate alteration of the plagioclase into clay — and there will be freeze~thaw as well. Maybe a granite outcrop in a desert would weather more slowly (but there will still be wind and thermal contraction/expansion). And, by definition, granite formed deep below the surface under great pressure so by the time it has reached the surface (or rather the surface has reached it) it will be unstable (as geologists reckon it) due to decompression.

Weathering of Basalt and Gabbro

Could be weathered/altered basalt or gabbro. That would fit with the density/location. If basalt the pits are gas bubbles, if gabbro the pits are left by grains of a softer mineral that has weather out. Cutting it open would tell you. Plenty of tectonic activity in California — I would guess Mt San Jacinto is the remnant of a volcano or pluton (solidified magma), but don’t know So Cal geology.

EDIT: didn’t notice the last image — that makes me lean towards an altered gabbro or some other plutonic rock.

Accretionary Wedges and Subduction

The phrase you may be referring to is “accretionary wedge” which refers to the sediment scraped off a plate that is subducting. California is very complex with multiple subductions and arc islands getting sutured onto the ever growing western edge. “Assembling California” by John McPhee is a very readable book on the tectonics.

Weathering into Core Stones

It is most likely the boulders did not come from somewhere else.

The idea is that wherever that mass of rock is, some it weathered away. In any mass of stone, there are likely to be joints and faults, and these erode away leaving chunks of stone. Just as ice cubes become more rounded as they melt, so do the chunks of stone weather more at the ‘corners’ or angles, and gradually become weathered.

Look up spheroidal weathering in Wikipedia for a better explanation.

Mafic/Felsic Magmas/Lavas

Lava/magma ranges in composition from mafic (which contains lots of iron and magnesium and is dark and dense) to felsic (which contains more silicon and is less dense). Mafic lavas, which you find in places like Hawaii and Iceland and erupting in the mid-ocean ridges, are relatively thin and flow like liquids). Although these lavas have pretty similar compositions, they can have a variety of forms depending on the conditions under which they are cooling. Pahoehoe forms beautiful ropey wave patterns; a’a’ is very jagged; if the lave erupts under water it forms pillow basalt; and if you have a larger body of lava exposed to a cooling surface you get the six-sided joints forming perpendicular to the surface. All of these forms have the same composition, though over time they will oxidize and undergo other chemical alteration.

Felsic lavas are very viscous (they have usually melted their way up through continental crust and have incorporated more silicon as a result, though its often more complicated than that) and when they reach the surface the explode out of the vents as ash and tephra and cascade down the volcano in what are called pyroclastic flows, or fall out of the air in ashfalls, leaving layered deposits. If you look closely at rhyolite, it may have a grainy composition with small bits of rock or crystals mixed into it.

Of course, these are different ends of what is a continuum.

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Analyzing Conglomerates by looking at Clasts and Sorting

Much of it is regional knowledge but you can also see some of it in the rock. Obviously it is composed of lots of clasts of different compositions. Notice that they are all different sizes, and that in terms of size they are randomly ordered — that suggests they were deposited by a glacier. If they had been deposited by river, etc, they would show sorting by size, so there would be some degree of layering. The same thing is also suggested by the shapes of the clasts, which are generally only roughly rounded, with some sharp-edged fragments and only a few smoothly rounded coasts — if fluvial action were involved, you’d expect to see clasts that were smoother. I would also guess (I’m just an enthusiast not a geologist) by looking at the positions of the elongated clasts that boulder as shown in picture one is rotated about 45 degrees the orientation or the original deposit). Would love to hear if there is more that experts can learn from just ‘reading’ the boulder.

Midcontinent Rift

Agree about the jointing. And the basalt. Don’t agree about the mountain range spewing lava from it sides — or else it’s a very partial explanation from an odd perspective (no offense intended). My understanding is that that the Lake Superior basin is the surface remnant of the Midcontinent Rift System, a 2000 mile long two-pronged rift valley which formed when what is now North America almost split in two 1.1 billion years ago. The rift erupted for around 20 million years producing vast quantities of basalt and gabbro; then a lot of weathering occurred over the next several hundred million years filling the superior basin; then the recent glaciation scooped a lot of the sediment and left us Lake Superior. It’s a mind boggling story. More on the MRS here: https://en.wikipedia.org/wiki/Midcontinent_Rift_System.