If a space rock the size of a fist were to impact the Earth at the speed of light, the consequences would be catastrophic and far-reaching. However, it's important to note that an object with mass cannot actually travel at the speed of light according to our current understanding of physics. Therefore, the scenario you describe is hypothetical and doesn't align with the laws of nature as we know them.
However, let's consider the effects of an extremely fast-moving object impacting the Earth. Such an impact would release an enormous amount of energy, leading to a cataclysmic event. The specific effects would depend on factors like the mass, composition, and angle of the impact.
Assuming the space rock somehow managed to reach speeds close to the speed of light, it would unleash an unimaginable amount of energy upon impact. The energy release would result in an enormous explosion and generate an intense shockwave. The impact site would be instantly obliterated, and the surrounding area would suffer from devastating effects, including:
Massive Crater: The impact would create a crater with an extremely large diameter and significant depth, likely wiping out everything within its vicinity.
Ejecta and Debris: The force of the impact would cause a tremendous amount of debris to be ejected into the atmosphere, leading to widespread destruction in the surrounding area and potential global consequences.
Shockwave and Airburst: The shockwave from the impact would radiate outward, causing a powerful blast capable of leveling structures and causing severe damage over a wide area. The airburst would generate a massive shockwave similar to a nuclear explosion.
Fire and Thermal Effects: The impact would release an enormous amount of heat and ignite fires, causing further devastation in the surrounding regions.
Tsunamis and Seismic Activity: Depending on the location of the impact, the energy release could trigger tsunamis if the impact occurs in or near a body of water. Additionally, seismic activity would be generated, leading to earthquakes and ground disturbances.
Atmospheric Effects: The release of enormous amounts of dust, debris, and gases into the atmosphere could have global consequences, potentially causing changes in climate, severe cooling, or other long-term effects.
It's important to reiterate that the scenario you've described, with an object the size of a fist traveling at the speed of light, is not physically possible according to our current understanding of the laws of nature.