In earth science and natural hazards, the term "tsunami" is given to which type of large ocean wave phenomenon?

Introduction / Context:
The word "tsunami" is frequently heard in news reports about coastal disasters, but it is often confused with ordinary tides or storm waves. In geography and earth science, the term has a very specific meaning. This question tests whether you can correctly identify what a tsunami actually is and what kind of natural process usually generates it.

Given Data / Assumptions:

• The focus is on the definition of the term "tsunami" as used in earth science and disaster management.
• Several different types of ocean waves and water movements are listed, including wind waves, storm surges, tides and waves from undersea disturbances.
• We assume a general knowledge level similar to school geography or general studies for competitive exams.

Concept / Approach:
A tsunami is a series of very long wavelength sea waves usually generated by sudden displacement of a large volume of water. The most common cause is a powerful undersea earthquake that uplifts or drops part of the sea floor. Tsunamis can also be produced by underwater landslides, volcanic eruptions and, rarely, large meteorite impacts in the ocean. They travel across the deep ocean at very high speeds and grow dramatically in height when they reach shallow coastal waters. They are not the same as ordinary wind driven waves or the regular rise and fall of tides caused by the Moon and Sun.

Step-by-Step Solution:
Step 1: Recall that the Japanese word "tsunami" literally means "harbour wave", referring to the huge waves that strike coastal harbours after undersea disturbances. Step 2: Understand that a tsunami is linked to sudden vertical movement of the sea floor, most often from strong undersea earthquakes on plate boundaries. Step 3: Note that ordinary surface waves are generated by wind blowing over the ocean, which is a different process and usually produces much smaller waves. Step 4: Recognise that storm surges are abnormal rises in sea level pushed by strong winds and low pressure in cyclones, not the long wavelength waves of a tsunami. Step 5: Remember that tides are regular daily and monthly changes in sea level due to gravity, not sudden destructive waves triggered by earthquakes. Step 6: Match this understanding with the options and select the one that clearly states that tsunamis are large sea waves generated mainly by undersea earthquakes and similar submarine disturbances.

Verification / Alternative check:
Descriptions of major disasters such as the 2004 Indian Ocean tsunami or the 2011 Japan tsunami always mention a powerful undersea earthquake as the trigger. Diagrams in geography textbooks show the sea floor suddenly moving, displacing huge volumes of water and sending waves outward in all directions. These explanations do not refer to ordinary tides or local wind waves, which confirms that the correct description must involve undersea earthquakes and related submarine events, not everyday tidal or wind phenomena.

Why Other Options Are Wrong:
Ordinary surface waves from local winds are common and relatively small; they do not match the scale and cause of tsunamis. Storm surges are linked to cyclones and hurricanes and involve a broad rise in sea level, not the very long wavelength waves produced by undersea earthquakes. Regular tidal waves are caused by gravitational forces and follow predictable cycles; they are not sudden destructive events triggered by seismic activity. Small ripples on calm days are harmless and clearly do not match the destructive, large scale nature of tsunamis.

Common Pitfalls:
Many people casually use the phrase "tidal wave" when they actually mean tsunami, which leads to confusion. Another mistake is assuming that any very big wave is automatically a tsunami, even if it is actually a cyclone related storm surge. To avoid these errors, focus on the cause: if the wave is generated by an undersea earthquake or similar sudden displacement of water, it is a tsunami. If it is produced by wind and pressure in a storm, it is a storm surge. If it is regular and predictable, it is part of the tide. Remembering the cause will help you pick the correct definition in exams.

Final Answer:
A tsunami is large sea waves generated mainly by undersea earthquakes and similar submarine disturbances.