Step 1: Concept
Identifying the odd item based on its origin or category.
Step 2: Analysis
Volcanoes, earthquakes, and tsunamis are all natural disasters caused by geological processes.
Step 3: Comparison
A chemical explosion is a man-made event or disaster.
Step 4: Conclusion
Since the others are natural and (4) is artificial, (4) is the odd one out.
Final Answer: (4)