Passage:
In dendrochronology, the study of tree rings, researchers occasionally observe anomalous structural damage known as a "frost ring." A frost ring occurs when a sudden, severe freeze strikes during the active growing season, when a tree's newly forming xylem cells are still thin-walled and vulnerable. The rapid freezing of intracellular water causes these expanding cells to rupture and collapse under thermal stress. As normal growth resumes following the temperature drop, the tree deposits a discolored, distorted layer of crushed cells next to the undamaged wood. Volcanic eruptions are the primary driver of such unseasonal freezes. When a volcano erupts with sufficient force, it ejects massive quantities of sulfur dioxide gas into the stratosphere. This gas reacts with water vapor to form microscopic sulfate aerosols, which reflect incoming solar radiation back into space. Consequently, global surface temperatures drop sharply mid-summer, abruptly triggering the intracellular freezing that creates the characteristic frost ring in susceptible trees.
According to the passage, expanding xylem cells in a tree rupture and collapse primarily because of:
- Aphysical abrasion from sulfate aerosols entering the tree trunk.
- the rapid freezing of water inside the developing cells.Answer
- Cthermal stress triggered by the resumption of normal wood deposition.
- Dthe gradual thickening of xylem cell walls during the growing season.