Auckland Volcanoes: Mangere Mountain
Huge eruptions have buried the Auckland region in volcanic ash in the past. Geologists have warned that there is a significant risk of ash fall in our lifetime. The simulation at the Auckland Museum gives you some idea what a bad day at the beach might look like!
There are 48 small volcanoes in the Auckland region all of which have erupted in the last 150,000 years. The rock about 100Km below your classroom can melt. A huge molten bubble of magma can gradually make its way towards the surface through the denser surrounding rocks a little like a hot air balloon lifting off the ground. When the magma nears the surface eruptions follow a similar sequence that is over rather quickly.
If you hear the sound of a large explosion and suddenly find yourself several hundred metres up in the air you will remember reading this. The molten rock eventually came in contact with water as it neared the surface and built up a huge pressure of steam. This pressure blasted an area of real estate 2 kilometres across and a football field deep into the air.
What goes up must come down and a ring of shattered rock and dust gathers around the volcano’s throat. This is called a Tuff ring
If the lava contains a lot of gas it froths like a bottle of coke that has been shaken before removing the cap. Frothy lava is ejected from the vent and a steep sided scoria cone formed. This process is called lava fountaining.
When the gas content falls lava can flow up the vent and burst through the crater wall. Large amounts of debris raft away on the free flowing lava
Finally as the volcano finally loses its fizz a small scoria cone can form inside the vent
Mangere mountain came into existence about 18,000 years ago. Can you see evidence for:
- volcanic eruptions that got no further than the intial explosive phase with a tuff ring and a small scoria cone
- More than 1 crater in mangere mountain
- smaller scoria cones formed as the magma ran out of fizz
- A breach in the volcano when the wall was washed away by a lava flow.
The youngest Auckland field volcano, Rangitoto, erupted 600 years ago. A new volcano could erupt in a neighbourhood near you sometime soon.