Gwen is researching the effects of a small asteroid impact in an area that has started to regrow after being hit ten years ago. she considers the event to be a long-term change because the species of plants that were present had to start completely over from seeds after all of the plants in the area were killed by the impact. what is the error in gwen’s thinking?

Gwen’s assumption of asteroid hit as long term change is incorrect. Asteroid hit is not a long term change, instead, it is a short term change.  

Explanation:

Examples of short term changes are drought, flood, volcanic eruption, etc. A short term change occurs quickly and can immediately affect organisms but it doesn’t become a reason for species extinction. The effects of a short term change don’t prevail over a long span of time.

Examples of long term changes are ice age, global warming, deforestation, etc. Unlike a short term change, it takes time but the consequences are far-reaching. It can lead to species extinction.

In this question, asteroid hit is a quick and unexpected hazard, unlike the slow long term environmental changes.

float.

archimedes' principle

denser than water sinks in water.

denser than mercury would sink in it