1. Commonly, very uniform rock is preferred for building stones. This rock is quite non-uniform. Some parts are much coarser-grained and pinker than other parts. Personally, I prefer non-uniform rocks as they are usually the most interesting. This rock may have been heated so much that it partly melted. The coarser pinkish areas could represent zones where the partial melt segregated and eventually slowly re-froze. Other interpretations are possible, however.
2. Alternative #3 is correct. Simple compaction of sedimentary beds couldn’t create the wild patterns in this rock. Similarly, slow freezing in a large body of molten rock would produce a more uniform product. This rock looks like it was deformed at such high temperatures that it flowed (particularly the paler layers). It has been altered so much that it is uncertain whether it was originally sedimentary or igneous.
3. Yes, each grain can be distinguished as a separate entity. Each grain is a single crystal of some mineral and is about ¼ inch wide. Three minerals predominate; pink orthoclase, gray plagioclase, and black hornblende. The crystals aren’t bounded by flat crystal faces because they interfered with one another as they completed their growth.
4. Are you kidding? These bricks are surrounded by person-made cement! Manufactured items are not rock. (Of course, however, all the constituents here were derived from rock.)
5(a). The light-colored fragments were picked up by molten rock, which has since solidified to form the darker parts of this boulder. (The texture and mineral composition of the enclosing rock indicates that it is igneous.)
5(b). Yes, the way minerals weather often helps in their identification. The feldspathoid minerals, for example, typically weather away more rapidly than the common mineral, feldspar. To make a positive determination of the precise identity of the gray mineral would require laboratory study.
6. It appears that the bluish plagioclase formed first, at it is surrounded by the whitish plagioclase. The blue plagioclase grains are probably relics that survived crushing during metamorphism, but the more crushed plagioclase recrystallized to become white plagioclase.
7. No. The grains have been squashed out parallel to the pen so the rock was probably once shorter in this direction. (This assumes no volume loss during deformation.) (Squashing during metamorphism rarely makes rocks more compact. This rock is probably no denser than the rock it formed from.)
8(a). The feature of this rock that indicates its sedimentary origin is its combined layering (bedding) and very fine grain size. Each bedding plane (separating one bed from the next) was once the sea floor. The beds, barely discernable here, are parallel to the pen.
8(b) The impacting rock’s motion relative to the dolostone was towards the right. This answer is guessable! Where a rock hit the dolostone a glancing blow, it would pull the dolostone behind the rock’s movement direction towards the impact point. This would open a crack roughly concentric about that point. The rock in front of the impactor rock would simply be compressed and would not crack (Rocks are stronger in compression than tension.) The net result would be the formation of a semi-circular crack concave in the direction of motion of the impactor. (Research project…What dictates the radius of the semi-circle?)
9. The boulder could have been carried here by glaciers or floated here on an iceberg. (It also could have come from the Laurentian mountains of Canada, where Adirondack-type rocks are also common.)
10. These fossils are trails left in sand, probably by worms.
11. The zone, marked by the ballpoint pen, is where part of the rock shifted relative to the other part (i.e. it’s a fault zone). It’s apparent the faulting occurred when the rock was mushy (because it was very hot?). The portion of the rock under the pen moved relatively downwards, as indicated by the way layers have been dragged as they approach the fault zone.
12. Slow crystallization of melts commonly produces rocks containing crystals ½ inch or less in diameter (like the fine-grained rock of the photo). If this rock contained less water then the melt, the remaining melt would get increasingly rich in dissolved water. Watery melts are relatively fluid so that atoms moving towards growing crystals in them can move farther. Hence fewer but larger crystals would form. If such a watery melt moved into a crack, it could cool enough to begin freezing on the walls of the crack. If the water started forming separate bubbles during freezing, the quartz might be selectively dissolved in these bubbles. The center of the crack could then represent the deposit from these bubbles. (Note the use of hedge words such as “could” and “might”; the origin of pegmatite is controversial.).
13. Possibly the whitish layer beneath the hammer was squashed so much that it “necked out” (like stretching a heated glass rod till it “necks” and separates into two pieces). Alternatively, this layer may simply have ended at its “nose” (no layer goes on forever). It’s even possible that the layer was dragged by adjacent faulting and that the surrounding layers are not bent around the fold nose.
14(a). Garnet is rare in igneous rocks because it forms only in aluminum-rich rocks subjected to rising temperatures. It is rare in sedimentary rocks as it is generally altered to other minerals before the sediment becomes consolidated. It doesn’t crystallize in sedimentary rocks because they don’t reach high enough temperatures.
14(b). They must have grown at about the same rate in all directions (at least in the plane of the photo), as they form crystals that have about the same diameter as measured in any direction. (They are almost round.) This tells us that their atoms are probably similar distances apart in any direction. (Actually, the atoms of each element in garnet form rows with identical spacings in 3 directions at right angles to one another and therefore belong to a group (System) of crystals known as Isometric.)