Rocks that crystallize quickly at the surface end up with a fine-grained or glassy texture. When lava erupts and cools in the air or in water, there isn’t time for large crystals to form, so the crystals that do develop are very small (fine-grained) or the rock solidifies into a glassy, non-crystalline state (like obsidian). In contrast, slow cooling deep underground yields coarse-grained textures with visible crystals, and porphyritic textures come from cooling in two stages, producing some large crystals within a finer matrix. So the texture that matches rapid surface cooling is fine-grained or glassy.

The region with long, nearly parallel ridges and the valleys between them formed during the Paleozoic tectonic events that folded and faulted large swaths of sedimentary rock in the Appalachian Mountains. As continents collided and compressive forces pushed the layers, they were bent into alternating upward folds (anticlines, which become ridges) and downward folds (synclines, which become valleys). Erosion then wore away the softer rocks more quickly, leaving a repeating pattern of long ridges separated by valleys. This creates the characteristic landscape of the Valley and Ridge province. By contrast, the Blue Ridge is dominated by older, resistant metamorphic rocks that form a high, rugged belt, not a system of long parallel ridges and valleys. The Piedmont is more gently rolling hills carved from metamorphic rocks, and the Appalachian Plateau is a relatively flat-lying, plateau region with fewer long ridges.

The key idea is how different seismic waves move through Earth's materials and what that implies for their travel times. P waves are the fastest and act like compressions that push and pull in the direction of travel. Because they can move through solids, liquids, and gases, they arrive first at a seismic station after an earthquake. S waves, on the other hand, move by shearing the material—shaking rocks side to side perpendicular to the direction of travel. Liquids can’t support this kind of shear, so S waves cannot propagate through liquids at all. Because they travel slower than P waves, they arrive next, making them the second to reach the station. Surface waves (which travel along the Earth's surface) typically arrive after the body waves (P and S) because they move more slowly and are constrained to the surface layer. So the waves that are second to arrive and cannot travel through liquids are the shear (S) waves.

Solstices are the two times each year when the Sun’s path in the sky reaches its extreme height or lowest point because the Earth is tilted on its axis. Around December 21, the tilt is such that the Northern Hemisphere is tilted away from the Sun, making the Sun appear lowest at noon and resulting in the shortest day of the year. This is the winter solstice. After this date, days begin to lengthen as the Sun climbs higher in the sky toward spring. (In the Southern Hemisphere, the situation is reversed, and December 21 is their summer solstice.)