Question

Fossil records support the fact that dinosaurs dominated life on Earth for more than 150 million years. During their reign, other animals had little chance of flourishing. Then, 65 million years ago the dinosaurs vanished from Earth. Today, there is no clear answer why this happened although there are several hypotheses.
Here are two viewpoints as to why dinosaurs became extinct.
Viewpoint 1
Massive deposits of ancient lava have been found throughout the world telling us that on Earth, 65 million-years ago, massive volcanic eruptions were common. For over 500,000 years, massive lava flows filled Earth's atmosphere with copious amounts of smoke and ash. Because of this, sunlight was prevented from reaching Earth's surface. Temperatures on Earth dropped significantly. As the sunlight decreased, so did photosynthesis and then the producers that relied on the sun and photosynthesis to make food. Few species could escape the effects of a darkened sky and the huge loss of producers in food webs everywhere on Earth. Dinosaurs as well as many other species could not survive these changes on Earth and 70% of life on Earth died at this time.
Viewpoint 2
65-million-years ago a 10-kilometer asteroid struck Earth near the Yucatan Peninsula in Mexico.
The force of the asteroid is estimated to have been equal to that of three hundred million atomic bombs. Earth's atmosphere filled with gas and dust from the asteroid's impact, as well as with ash and smoke from resulting forest fires. Temperatures on Earth, as well as available sunlight, dropped dramatically. Plant life died and, as a result of this, so did many other species on Earth.
When the dust and ash settled, greenhouse gases, released as a result of the asteroid impact, caused temperatures to rise much higher than average temperatures of pre-impact Earth. This shift in climatic conditions eventually resulted in the extinction of over 70% of all the species once living on Earth.
1x Some scientists believe that the slow repositioning of Earth's landmasses lead to the extinction of dinosaurs. Convection currents within the Earth's mantle caused landmasses to drift, collide, and eventually heave upward. Ocean currents were redirected and sea levels throughout the planet dropped. For example, the interior seaway, which once divided North America, drained away as the Colorado Plateau rose thousands of feet into the air. Without shallow seas like this one, climates became cooler and drier. Such climatic change resulted in new ecosystems that could not provide enough biomass to support dinosaur life.
Summarize the three scenarios, envisioned in these two essays, that possibly led to the extinction of the dinosaurs, and give details of each.

Answer 1

Here are concise summaries of the three extinction scenarios described, with key details for each.

1) Massive volcanism (long-term lava flows)
- Cause: Intense, prolonged volcanic eruptions (large flood-basalt events such as the Deccan Traps).
- Mechanism: For on the order of 500,000 years huge lava flows injected ash, aerosols and gases into the atmosphere. Soot and ash blocked sunlight, producing global cooling and sharply reduced photosynthesis.
- Ecological effect: Collapse of primary production reduced food available for herbivores and then carnivores, causing widespread starvation and cascading extinctions across food webs.
- Evidence: Widespread ancient lava deposits dated to about 65 million years ago; geochemical signatures of large volcanic gas releases.

2) Large asteroid impact (Chicxulub-type event)
- Cause: A roughly 10-kilometer asteroid struck near the Yucatán Peninsula about 65 million years ago.
- Mechanism: The impact released an enormous amount of energy (often compared to hundreds of millions of atomic bombs), lofting dust, vaporized rock and soot into the atmosphere. Immediate effects included global darkness, rapid cooling and massive wildfires. Later, impact-released greenhouse gases raised temperatures above pre-impact levels.
- Ecological effect: Rapid shutdown of photosynthesis and short-term climatic collapse followed by longer-term climatic disruption; widespread plant and animal extinctions (estimated >70% species loss).
- Evidence: Global iridium layer, shocked minerals, the Chicxulub crater, and coincident extinction horizons in the fossil record.

3) Long-term plate-tectonic/sea-level change (continental drift and uplift)
- Cause: Slow repositioning of continents driven by mantle convection—continental collisions, uplift and changing ocean basin shapes.
- Mechanism: Tectonic uplift drained shallow epicontinental seas (for example, the interior seaway in North America), altered ocean currents and produced long-term regional and global climate change toward cooler, drier conditions.
- Ecological effect: Loss of shallow-marine and coastal habitats and overall reduction in productive ecosystems (lower biomass) that could sustain large dinosaur populations, producing gradual declines in diversity and abundance.
- Evidence: Geological signs of uplift and seaway regression (e.g., Colorado Plateau uplift), sedimentary records showing changing sea levels and climates.

Note: These scenarios are not mutually exclusive; many researchers argue a combination of long-term tectonic and volcanic stress plus the abrupt asteroid impact best explains the observed extinction patterns.