Back to Conner Bay Rocks Parcourse
Answers to the Conner Bay Geology questions.
1. Cooling would be slow if the molten body was deep in the Earth. (The Earth gets hotter the deeper you go.) Crystals could slowly grow to relatively large sizes within such a melt. If this mixture of crystals and melt was then forced towards the Earth’s surface (volcanism!) where the rock is relatively cool, it would complete its crystallization rapidly, forming a fine-grained groundmass.
2. Most igneous rocks are composed of two or more different minerals. As a melt (magma) containing many different elements cools, crystals of one mineral (containing only a few of these elements) generally form first. With continued cooling, other minerals can form. (Like evaporating a water solution containing a lot of salt and a little sugar...the salt would start to crystallize first, eventually to be joined by the sugar.) If an early-formed mineral is more or less dense than the surrounding magma, it will tend to sink down or float up. This is one way crystals of all one type may collect together and eventually become a rock composed of essentially only one mineral. The plagioclase accumulations on the moon may have floated up through a dense ocean of darker lunar magma.
3. There are 3 classes of rock: Igneous, Sedimentary, and Metamorphic. The crystals that completed the “freezing” of this igneous rock interlock like pieces of a jigsaw puzzle as would happen in igneous rock, which “freezes” from a molten condition. The black crystals here have elongate shapes that would not be expected in sedimentary rocks, where grains would be more rounded due to transportation by wind, water, or ice. In metamorphic rock, crystals often have a squashed appearance with elongate grains parallel to one another due to re-crystallization at high temperatures under directed pressure.
4a. I can!
4b. These are the routes taken through a limey mud by worms before the mud was converted to rock. Both the
shells and worm marks are fossils.
5. As a crack spreads from its point of origin, these little ridges form at right angles to the spreading direction. They roughly encircle the point where the crack started (towards the right side of the photo). That is, the crack spread towards the left from its point of origin near the right side of the photo, forming “speed bumps” as it went.
6. The spiral mark is a shell of a fossilized animal known as a gastropod (snail). (This snail died 450 million years ago!)
7. It shows layering (bedding), typical of sediment accumulated at the Earth’s surface. Also, the individual grains are rounded because they were rubbed together in stream or wave currents.
8. It has a clear rim, whereas the interior of the crystal is crowded with tiny black grains. (The black grains probably unmixed from the crystal as it cooled.)
9. Garnet’s main use is as an abrasive, as in high quality sandpaper. It is also used as a semi-precious gem. Its presence at the Earth’s surface is only possible if it has been unearthed from its former deep burial by erosion.
10. It was cracked as it hit bedrock or other boulders while it was being carried by glaciers. The cracks are convex towards the relative direction of motion of the impactor. Thus, this boulder would have been moving from right to left if these cracks had formed when it hit a stationary bedrock knob.
11. This rock may have undergone folding before it was consolidated (e.g. it might have been part of a landslide)
12. This is probably a rock that was originally igneous (basalt lava) that has been re-crystallized during a metamorphic event too mild to have coarsened grain size much. (The formation of new minerals has altered the texture, however.)
13. The least ambiguous indicator of relative age occurs where the 1 inch thick white vein meets a thinner white vein. The thicker vein cuts across the thinner one and spreads it apart. It is thus younger. The same one inch white vein appears to only partly penetrate the spotted vein, so their age relation is unclear. At the top of the photo, the spotted vein appears to cut across the thin white vein, but the thin vein continues off the top of the photo without evidence it has been spread apart (dilated) by the spotted vein. My guess is that the spotted vein is oldest, then the thin white vein formed, and finally the one inch white vein formed. The white mineral is, in all cases, quartz.
14. The irregular bedding may be due to irregular compaction of this sediment. Perhaps it was exceptionally watery when buried by more sediment.
15. They were gently folded during the collision of tectonic plates in Vermont, where deformation is greater.
16a. Layering is unusual in igneous rocks. It is known to occur however, where crystals have settled through molten rock and collected on the floor of a magma chamber. This is not the case here, because this boulder came from a vertical cylindrical pipe in Canada, and there we can see the layering is also vertical. It is thought the layering developed by a plastering-on process as downward!)-flowing magma froze against the vertical margins of the pipe.
16b. It was torn off previously crystallized igneous rock and carried by the flowing magma. As this inclusion probably is related to the magma that enclosed it, it may yield clues as to how the magma changed composition during volcanism. (Note...The rounded shape of the inclusion suggests that it was partly melted by the magma.)
17. The slicken lines continue down beneath the bedrock surface where no recent glaciation could form them. Fault movements were parallel to the scrape marks as pointed by the hammer pick. The underlying rock
probably moved relatively towards the left (assuming faulting was a response to the horizontal compression that caused the folding).
18. A small thrust fault (marked by the white zone at the base of the lens) cut up into the layered rocks above. The extra layers carried upward made the total rock thickness above the fault greater than the thickness to either side. Doming thus occurred above the fault. Hard to picture, huh?
19. Fault movement (the grinding of one rock mass across another) resulted in the fragmentation of rock in the fault zone. The open spaces between the fragments were then filled by percolating watery solutions, from which white calcite was precipitated (came out of solution). Eventually, all the spaces were filled by this natural cement.
20. Faulting here is indicated by offset layers, the crushed rock zone, and the slicken-lined scrape marks
where one fault block slipped along and abraded the other block. Calcite was formed after faulting provided
open spaces for its deposition, However, fault movements apparently continued, as some of the
calcite has been grooved and offset by faulting.
21. The offset could be due to faulting (unlikely...there is no evidence of slip along the irregular black crack). It could be due to solution along crack walls. Or it could be due to a changing direction of the vein crack as it
approached the black crack. I favor the latter explanation for reasons requiring field observations. Then the vein might be continuous at some depth below the present rock surface.
22. The whiter layers were deposited during “calm” times when the seawater was relatively clear. The darker layers could represent storms, submarine slides (caused by earthquakes?), changes in water depth, etc.
23. These are glacial striations, formed a bit more than 10,000 years ago when glaciers dragged rock fragments
across this outcrop. Motion was from the hammerhead towards the butt of the hammer handle.
24. The crystals were oriented parallel to one another by flowage of the surrounding molten rock (magma).
An analogy would be the way logs carried by a stream tend to become oriented parallel to the stream current.
(Parallel orientation of crystals is more commonly interpreted as a result of compression during tectonic
collision and metamorphism, but the field relations described in the question eliminates this explanation. Also, crystals oriented by squashing rarely have retained their flat crystal faces as well as crystals that completed their growth in contact with a liquid.
25. Possibilities might include sunspot cycles, Earth orbital variations, or thresholds of tectonic stress build-up and release. The thicker white layers (times of calm waters and calcite deposition) probably took much longer to deposit than a single season.
26. Probably the glaciers did it. Later, when waves undercut the cliff, the boulder rolled down onto the wave-cut platform. Waves wore down and carried off the smaller particles, but this boulder was too big and thus was left behind.
27a. Sedimentary. Most of the grains have irregular shapes and appear to have been broken. A few have flat edges, but this mineral tends to break with flat edges anyway (it exhibits cleavage).
27b. The darker rock has more small-sized dark particles and size sorting is relatively poor, indicating this
sediment was deposited beyond the reach of currents that could winnow out the smaller grains.
28a. The melt (magma) could have filled a crooked crack along which there was movement. Alternatively, the magma might have been generated by irregular melting of the adjacent rock....or maybe the adjacent rock was irregularly mushy rather than brittle
28b. The crystals appear to have been crushed and strung out, as would occur if re-crystallization had occurred
under directed pressure.
29. Yes. The leading hypothesis for such rocks is that some of their minerals melted, with the melt segregating into thin lenses and layers. This must have happened far below the Earth’s surface where the rocks
get really hot.
30. In the field (not obvious in this photo), it can be seen that the bedrock here occurs as nearly horizontal layers (the wave-cut platform is essentially a bedding plane). Photo15, for example, shows both the
sedimentary layering and the “cracks”.
31. The seams show where solution has been concentrated. They are thus enriched in insoluble minerals. The ½ inch seam in the photo occurs where there once was several inches of rock! The part of the rock which has
disappeared has been dissolved and carried away.
32. Perhaps bending was slow, so that grains could be dissolved where elastic stress was greatest, causing each bit of elastic strain to become permanent. (Folded rock won’t spring back to its original shape when dug out.) Also, some slippage probably occurred between beds, analogous to the slippage that occurs between cards when you bend a deck of cards.
33. There are at least two possibilities. Perhaps the “sponge” boulders were subjected to a long interval of salt water weathering which produces pits just like this in siltstones or sandstones. Alternatively, these rocks may contain abundant chunks of limestone that have weathered out because of their greater solubility. Neither explanation is entirely satisfactory. If the first explanation is correct, why do so few boulders show this feature? If the second explanation is correct, chunks of un-weathered carbonate would still exist in the interior of the
boulders...but the boulders would then be a highly unusual rock type.
34. The pits occur where relatively soluble veins have been selectively dissolved during weathering.
35. Quite a mystery! Such a large boulder is unlikely to have been carried very far, particularly as several
similar boulders occur nearby, but not elsewhere. One possibility is that it was carried here from beneath Lake Champlain, perhaps on an iceberg. It might have undergone long-term weathering in an arm of the ocean that occurred in the Champlain Basin before the most recent glaciation.
36a. It is entirely different from the underlying bedrock. Unlike the bedrock, it is coarse-grained, contains abundant feldspar and pyroxene, is hard, and is pinkish colored.
36b. Cooling is relatively slow at great depth in the Earth, as all rocks are very hot there. Even at shallow depth, the interior of large bodies of magma would also cool slowly.
37. Fossils...remains or imprints of plants or animals preserved in the Earth’s crust (but not plants in this case, as plants hadn’t evolved when these rock formed!)
38. In the deep ocean made murky (turbid) by landsliding. The other environments would result in sediments having better sorting of particle size.
39a. There has also been squeezing at some angle to the overall squashing, causing small folds or kinks.
39b. The narrower dike formed first, as it had to first be there in order to have been later cut and offset by the thicker dike. The thicker dike represents the intrusion of new matter, forcing the adjacent walls apart (evidence it didn’t form by a replacement process).
40a. The grains tend to be elongated in the same direction, as would happen at high temperatures under a directed pressure.
40b. Squashing was from the upper right and lower left. Note also the oval shape of the larger grains, typical of squashed metamorphic rocks.
41. Weathering-out of scattered grains of a relatively soluble mineral (probably a carbonate)
42a. The boulder is quite rounded, suggesting that it came from far away, giving it time for any sharp corners to be worn off.
42b. This texture does not show any preferred alignment of grains. Its interlocking texture and the “fragile” crystal shapes (particularly the black skeletal grains) indicate an igneous origin (crystallized from a molten condition).
43. These look like shrinkage cracks, perhaps due to expulsion of water from an extremely fine-grained mud (syneresis). They do not prove the mud dried out in the air. After shrinking, the mud was compressed to form solid rock.
44. This rock appears to have been thoroughly mucked up by worms or other organisms before consolidation.
45. They were pushed here by waves and lake ice. They came from nearby bedrock, as they are nearly all the same type of rock as the bedrock.
46. A glacier would have deposited a great variety of rock types. Also, the fragments are all about the same size, indicating water sorting. Finally, the ridge crest has a uniform elevation and approximately parallels the present shoreline.