Earth's Geological and Atmospheric History

Statement 1 is correct: Earth's magnetic field has undergone reversals, with evidence showing reversals occurring roughly every few hundred thousand to million years (with variable intervals). Statement 2 is incorrect: When Earth was created ~4.5 billion years ago, the early atmosphere (primordial atmosphere) consisted mainly of hydrogen, helium, methane, and ammonia with virtually no free oxygen or the current composition. The current oxygen-rich atmosphere (21% oxygen) developed later due to photosynthesis. Statement 3 is correct: Living organisms, particularly photosynthetic organisms (cyanobacteria), fundamentally modified Earth's atmosphere by producing oxygen through photosynthesis, creating the Great Oxidation Event around 2.4 billion years ago. Statements 1 and 3 are correct. > Earth's Evolution: Magnetic reversals documented; Early atmosphere was oxygen-free; Life created oxidizing atmosphere.