Download Below is a review of some common mistakes that PNW Geology

Below is a review of some common mistakes that PNW Geology students make when they talk (or write)
about these three rock types: basalt, porphyritic andesite, and granodiorite. These are mistakes about
how they are defined, mistakes about their properties, and mistakes about how they form from Earth
processes.
In a learning process, it's good to make mistakes, it's necessary, because only by making mistakes,
processing them, and correcting them, does one learn. So if you made any of these mistakes, that's
good, because you learned. (At least, I hope you did.)
This does not mean that you promulgated any of these errors - everyone is seeing this same review,
whether or not they made mistakes - but if you did, you should check your thinking.
1. Basalt is not harder than the other rocks. For some reason, some people think basalt is a harder rock.
The rocks are a mixture of minerals, each mineral having its own hardness. Of the three rocks,
granodiorite is the rock with the hardest mineral in it, because it is the only rock of the three that has
some quartz in it, which is harder than the rest of the minerals in these rocks.
2. It is not true as a general characteristic that basalt has holes in it. (The holes are called vesicles, and
are from gas that formed bubbles in the lava before the lava solidified). Most pieces of basalt do not
have vesicles (holes) in them.
3. Big-grained is not "course-grained." It is coarse-grained. One of the meanings of the word coarse is big
or rough. The word course does not apply.
4. The melting of the Earth's mantle that leads ultimately to the formation of andesite or granodiorite is
not caused by heating, not caused by addition of heat to rocks or raising of their temperature. Both
andesite (volcanic) and granodiorite (plutonic) are common igneous rocks formed at subduction zones.
Down in that part of the subduction zone where the rocks initially melt to set the processes off that lead
to granodiorite and andesite forming higher up, the rocks down there deep in the subduction zone are
already hot. It is the addition of water, given off by the subducting plate to the hot, overlying mantle,
that sets off the melting process and generates the magma.
It is like the addition of salt to snow or ice, which causes the snow or ice to melt. Adding water to the
hot mantle does the same thing, it causes it to melt.
In general, the oceanic plate does NOT melt. It is a common misconception that the subducting plate
melts. This incorrect understanding even shows up in displays in state parks and national monuments
(though I have been involved in getting some of those displays fixed - or one of them anyway).
5. Granodiorite does not have more dark minerals than granite. Or at any rate, it does not have to. What
makes a granitic-type rock a granodiorite instead of a granite is that it has very little of the potassium
feldspar (the pink feldspar) in it, but does have lots of the plagioclase feldspar (the white-ish mineral)
and some quartz in it. The dark minerals, how much of them are there, don't matter for it to be a
granodiorite. There is a tendency for granodiorite to have a higher amount of dark minerals than granite
does, but it is not a necessary condition, not part of the definition of what makes a rock a granodiorite.
6. Lava is not involved in the origin of granodiorite. Lava is molten rock that has flowed onto, is flowing
on, the surface of the Earth. Granodiorite is solidified magma. Magma is molten rock down inside the
Earth.
7. Granodiorite is not volcanic and does not necessarily have anything to do with any volcanoes. It may
be that some granodiorite forms down inside the Earth without its magma plumbing system connecting
to any volcanoes up above.
8. Granodiorite is not made of granite and diorite. It is its own rock, with its own characteristics, not a
mixture of the other two.
9. Rock names, such as granodiorite or porphyritic andesite, are not capitalized. They are not proper
names or titles like Portland, Oregon, or George Washington, or "To Kill a Mockingbird," they are just
the names of rocks.
10. These rocks do not have characteristic textures in terms of how they feel. Each can be rough,
smooth, or bumpy on the surface, sharp or rounded in shape.
____________
Scores are based largely on how much you engaged in this conversation, as evidenced by:
1. posted early in the week
2. posted in the middle of the week
3. posted late in the week
4. posted original contributions that did not just repeat what had already been posted
5. responded to at least three other students, at least a couple of them in some depth rather than just a
brief sentence
6. said some interesting, at least partly on target and informative, things in your posts.
7. and of course, you cited your sources