I think wetlands deserve some recognition. Although a little removed from the blog’s hot spot activity on tectonic forces, it’s still connected to the process.
I have always been intrigued to see the Everglades National Park in Florida. Accredited as being the first national park for the protection of an ecological wilderness in the USA, I was keen to check it out early this year. Saving the glades has become a crucial concern as biodiversity rapidly diminishes. The ‘Rivers of Grass” are no more than 8 feet above sea level, yet urbanisation and population pressures have significantly impacted on the water level drainage and supply. On average, 800 people moved to Florida each day. I find that incredible! The Everglades Restoration plan seeks to return water to more natural patterns and distribution throughout the ecosystem. An ambitious plan for the next 30 years.
Christchurch experienced significant liquefaction after the earthquakes. It was a series issue, with silt, sand and water bubbling up in people’s backyards. It happened in the streets, and even through the floors of buildings. The damage was extensive to buildings and infrastructure, particularly in the eastern suburbs of Christchurch, where the water table is very high along with loosely packed sediments in the soil.
So what is liquefaction? During an earthquake, silt and sand grains in the soil are re-arranged as water is squeezed out. They lose their strength. The soil behaves more like a liquid than a solid, as it is forced up through cracks and crevasses. It can no longer support the weight of buildings and roads above it. In fact liquefaction often forms what is called a sand volcano. Liquefaction can cause the ground to subside and also cause lateral spreading on downward slopes. Liquefaction would make for a fun experiment – rather in the lab of course than in the real world. This would relate well for the teaching unit to carry out a practical Earth and Space Science investigation.
Soft sediments beneath Christchurch enhanced earthquake motion. It’s a dangerous recipe. As seismic wave energy passes from stiffer rock through to softer sediments, it deforms the sediments more in order to balance the energy transferred. Continual pulses of seismic energy travel further in the weaker upper layers, than the stronger layers below. As a consequence, upper layers fall under gravity only to be hit again by lower layer earth movements. This produced high impacts of energy and magnified the entire process of liquefaction, separating water and layers of sediment. In effect, the ground was shaking like leaves on a tree blowing in the wind.
Christchurch city has been built upon land that was once a wetland. Travis Wetland Nature Heritage Park is a reminder of what the natural landscape once looked like. Nature can have a powerful way of reminding us what was there before we built cities.
One particular story I have enjoyed reading about was called Rocky. With steep cliffs surrounding the Port Hills region of Christchurch, rock falls were a series threat after the earthquakes, particularly in 2011. Many homes surrounding this area had to be abandoned, as cliff faces collapsed. Rocks came tumbling down, causing significant damage in some instances. One boulder crashed through a roof of a garage in Morgan Valleys Road. With its momentum it continued to roll through a wall and into the entrance hallway of the house. Ironically, the owner put the boulder up for sale in Trade Me’s auction website and called him ‘Rocky’. Thousands of hits turned Rocky into one of the most popular rocks in New Zealand. Amazingly, the auction raised $60,000 for charity. Having been won by NZ Ski, Rocky’s new home will be in Mt Hutt ski field in Methven. Now that is one remarkable rocky story.