Free ACT Practice Question

About 66 million years ago, roughly three-quarters of species on Earth, including the non-avian dinosaurs, died out in the mass extinction that marks the Cretaceous–Paleogene (K–Pg) boundary. A thin clay layer, found worldwide and unusually rich in the metal iridium (rare in Earth's crust), separates rocks of the two periods. Geologists date boundary rocks radiometrically and count fossil species in layers above and below the clay. Two scientists debate the primary cause of the extinction. (Ages in Table 1 are constructed, illustrative estimates.)

Scientist 1 The extinction was caused primarily by a single large ASTEROID IMPACT. The boundary clay is enriched in iridium at sites on every continent and in deep-ocean sediments; iridium is scarce in Earth's crust but abundant in asteroids and comets. The same layer contains shocked quartz—grains whose internal structure is fractured in a pattern produced only by the sudden, extreme pressure of an impact, not by volcanism. A buried crater roughly 180 kilometers across, dated to the boundary, lies beneath the edge of the Yucatán Peninsula, matching the size expected from a body roughly ten kilometers wide. Such an impact would have blasted dust and vapor into the upper atmosphere, blocking sunlight for months to years. This "impact winter" collapsed photosynthesis and the food chains built on it. The abruptness of a single event explains why the extinction appears sharp and nearly simultaneous right at the clay layer.

Scientist 2 The extinction was caused primarily by massive VOLCANIC ERUPTIONS spread over a much longer period. Around the boundary, one of the largest volcanic events in Earth's history erupted enormous "flood basalts"—lava provinces hundreds of meters thick covering vast areas, preserved today in the Deccan region of India. Over tens to hundreds of thousands of years, these eruptions released huge volumes of carbon dioxide and sulfur gases, driving repeated swings in climate and acidifying the oceans. This is why the fossil record shows some marine and land species already declining in the layers below the iridium clay, before any impact horizon. A prolonged environmental stress, not a single instant, best fits a staggered pattern of loss. The impact, if it occurred, may have been only a final blow to ecosystems that volcanic disturbance had already pushed toward collapse.

Table 1: Constructed estimates for the timing of key events near the K–Pg boundary