Question

Explain, in your own words, why a tsunami often causes a rapid withdrawal of water from beaches before the first surge.

Short answer

The withdrawal is due to the wave's trough reaching the shore before the crest, causing the water to draw back before the surge.

Step-by-step solution

Understanding Tsunami Formation

Tsunamis are typically caused by underwater earthquakes or volcanic eruptions that displace a significant amount of water. This displacement creates waves that travel across the ocean.

Initial Wave Movement

When the ocean floor initially moves, it pushes water outward in all directions. As a result, the energy travels in powerful waves across the ocean at high speeds.

Wave Approach to Shoreline

As these waves approach the shoreline, the water depth decreases, and the wave's speed reduces, causing the wave height to increase greatly. However, before hitting the shore, this energy causes water to be pulled back from the beach.

Drawback Effect

The rapid withdrawal of water from the beach happens because the trough (the lowest point of the wave) reaches the shore before the crest (the highest point). This causes a significant amount of water to be pulled away from the shoreline, leading to the 'drawback' effect.

Resulting Surge

After this drawback, the crest of the wave follows and hits the shore with immense force, leading to the damaging surge associated with tsunamis.

Key concepts

Underwater Earthquakes

An underwater earthquake occurs when the Earth's tectonic plates shift beneath the ocean. These plates are giant slabs of the Earth's crust that float on the molten rock beneath them. When these plates grind against each other, they can get stuck due to friction.
When they finally release, the energy of this sudden movement is transferred into the surrounding water, causing a powerful disturbance that can lead to a tsunami.

• The energy released from the earthquake shakes the ocean floor.
• This energy is transferred to the water above, causing waves to form.
• The initial waves can spread across the ocean within hours, covering great distances.

Understanding underwater earthquakes helps us realize the scale and power behind tsunamis. The energy involved is immense, capable of traveling vast distances with minimal loss of power until reaching coastlines.

Volcanic Eruptions

Volcanic eruptions can also cause tsunamis, although they are less common than those caused by underwater earthquakes. When a volcano erupts underwater, the eruption is powerful enough to displace a large volume of water quickly.
This displacement creates waves that radiate outward from the eruption site.

• Eruptions can create a sudden, forceful explosion.
• Massive amounts of rock and ash can be ejected, displacing water.
• The resulting energy forms waves that travel just like those caused by earthquakes.

These waves travel rapidly across the ocean, sometimes reaching speeds over 500 kilometers per hour. As with earthquakes, this highlights how volcanic activity beneath the sea can have dire impacts above water.

Wave Energy

Wave energy is the driving force behind tsunamis. When something like an earthquake or volcanic eruption causes a disturbance in the ocean, energy is transferred to the water, forming waves.
These waves carry energy over vast distances without actually moving water long distances horizontally. Instead, the waves are a moving form of energy.

• The energy travels rapidly through water, barely noticeable at sea.
• As waves reach shallower water near shore, they slow down.
• This slowing down leads to an increase in wave height, intensifying the energy's effect.

Wave energy is both fascinating and terrifying due to its relentless power and speed. The way it propagates and increases in force as it nears land explains why tsunamis are so destructive.

Drawback Effect

The drawback effect is a noticeable phenomenon that occurs just before the main impact of a tsunami. As the tsunami wave approaches land, the trough, or lowest part of the wave, arrives first.
This results in water being pulled back from the beach, exposing parts of the seabed that are not usually visible.

• The trough leads the way, drawing water away from the shore.
• This creates the illusion of a rapidly receding tide.
• Following the trough, the crest of the wave hits, causing the powerful surge.

The drawback effect is an important warning sign of an approaching tsunami. Recognizing it can provide precious time for evacuation and can help save lives in coastal areas.