What led to the end of the Snowball Earth?

• A. Volcanism
• B. Increased solar radiation
• C. Massive meteor impact
• D. Global cooling and glaciation

Answer: (A)

Deglaciation occurs when enough greenhouse heat accumulates in the atmosphere to melt the ice, and the key driver in this scenario is volcanic outgassing of CO2. While the Earth was frozen, volcanic activity continued to release CO2, but weathering and removal of CO2 from the atmosphere slowed or halted under the ice. That meant CO2 could build up in the atmosphere over long timescales. Once CO2 reached high enough levels, the greenhouse effect trapped enough heat to raise global temperatures, overcoming the high albedo of the ice and initiating melting. As ice receded, the surface became darker and absorbed more solar energy, accelerating the warming and leading to a complete melt of the ice cover.

Other options don’t fit as the primary trigger. Increased solar radiation alone helps but isn’t enough to thaw a globally ice-covered planet with its strong reflectivity; a massive meteor impact would likely inject dust that cools the surface rather than warms it; and global cooling and glaciation describes the state during Snowball Earth, not how it ended.