In physical geography and oceanography, what is the primary cause of a tsunami, the series of large ocean waves that can cause severe flooding along coastlines?

Introduction / Context:
Tsunamis are powerful, long wavelength ocean waves that can travel across entire ocean basins and cause devastating flooding when they reach shallow coastal areas. Unlike ordinary wind generated waves or daily tides, tsunamis are usually triggered by sudden disturbances that move large amounts of water quickly. Understanding the main cause of tsunamis is important in natural hazard awareness and disaster management, and is a common topic in general science and geography examinations.

Given Data / Assumptions:
- The question asks for the primary cause of tsunamis, not every possible cause.
- Options include undersea earthquakes, normal tides, gentle tectonic drift, and regular wind waves.
- We assume standard textbook explanations of tsunami generation.
- We focus on the most frequent and significant trigger of destructive tsunamis around the world.

Concept / Approach:
Most destructive tsunamis are generated by undersea earthquakes occurring at tectonic plate boundaries, especially in subduction zones where one plate is forced beneath another. When such an earthquake causes a sudden vertical displacement of the sea floor, the water above is lifted or dropped, creating a wave that spreads outward as a tsunami. Normal tidal forces from the Moon and Sun cause regular rising and falling of sea level but do not generate tsunamis. Gentle tectonic drift is slow and does not move water suddenly. Wind driven waves from storms can be large but typically have much shorter wavelengths and are different in character from tsunamis. Therefore, the primary cause of tsunamis is sudden undersea earthquakes that displace large volumes of seawater.

Step-by-Step Solution:
Step 1: Recall that many famous tsunamis, such as those in the Indian Ocean in 2004 and Japan in 2011, were triggered by powerful undersea earthquakes. Step 2: Understand that a tsunami requires a rapid vertical movement of the ocean floor that pushes or pulls the water column above. Step 3: Recognise that normal tides are predictable, gentle changes in sea level and do not involve sudden vertical displacement of the sea floor. Step 4: Note that gentle tectonic drift occurs over long periods and cannot create the abrupt shock needed to generate a tsunami. Step 5: Differentiate tsunamis from ordinary wind waves, which are generated by wind blowing over the sea surface, and select undersea earthquakes as the primary cause.

Verification / Alternative check:
Geography and earth science textbooks describe tsunamis as “seismic sea waves” when they are generated by earthquakes. They provide diagrams showing a fault under the sea, sudden uplift or subsidence of the sea floor, and wave propagation. While tsunamis can also be caused by undersea landslides or volcanic eruptions, the majority of major events recorded historically are linked to earthquakes along plate boundaries. Normal tides and everyday wind waves are treated separately and are not classified as tsunamis. This confirms that sudden undersea earthquakes are correctly identified as the primary cause.

Why Other Options Are Wrong:
- Normal daily tidal forces caused by the gravitational pull of the Moon produce tides, not tsunamis, and are predictable and gentle compared to tsunami waves.
- Gentle tectonic drift without sudden movement of the sea floor does not move water abruptly enough to generate tsunami waves.
- Regular wind driven surface waves generated by storms can be high but have shorter wavelengths and are not tsunamis; they are called storm waves or swells.

Common Pitfalls:
Some students confuse tsunamis with ordinary “tidal waves” and may believe that tides themselves cause tsunamis, but the term tidal wave is misleading and outdated. Others may think any large wave from the sea is a tsunami, without considering its cause. A helpful memory aid is to associate tsunamis with seismic activity at the bottom of the ocean. Remembering the link between undersea earthquakes and tsunami generation makes it easier to answer related questions accurately.

Final Answer:
The primary cause of a tsunami is Sudden undersea earthquakes that vertically displace large volumes of seawater.