Mount Saint John is an extinct volcano in the Auckland volcanic field. Its peak is 126 metres above sea level and the crater is around 150 m wide.
The Maoris built a fortress here called Te Kopuke. It was built in the crater. They formed the external slopes for good defence. The higher rim was fortified by transverse ditches, banks and scarps. You can find many rectangular storage pits for sweet potatoes (kumara), where the main occupation area was.
The Auckland volcanic field
The Auckland volcanic field has formed above a mantle hot spot. The source of its magma is about 100 km beneath the ground. The magma is basalt magma. In total there are about 50 hot spot volcanoes in this field. They formed over a period of 250000 years and all produced scoria cones, lava flows and explosion craters. Mount Saint John's Lava Flow ran until the Waitemata Harbour.
Each of the volcanoes usually only erupt once. The volcanoes erupt until the run out of magma.
All Auckland volcanoes have erupted very hot, fluid and with basalt magma.
The type of eruption depends on whether the magma got in contact with water. Generally speaking there are only two types of eruptions:
1. Magma came in contact with water
If the rising magma came in contact with ground water, a lake or the sea it is cooled by the water directly. The magma breaks up into tephra fragments ( ash or rocks). The water heats up very quickly and expands. This creates a superheated stream. Steam, gas, tephra and rocks form a rapidly expanding explosion, which is called a phreatomagmatic eruption. Explosive pyroclastic flows surge outwards rapidly. These explosions can throw material to a height of 500 metres and create often tuff rings. Characteristics of these eruptions are large circular craters, which are surrounded by tuff rings.
2. Magma didn't come in contact with water
If the rising magma didn't come in contact with water, the volcano erupts less explosively. The gaseous magma erupts in the form of a fire fountain. When the magma has lost its gas, the rest of it just runs out as a lava flow, which can travel up to 10 km. Left behind is a scoria cone.A scoria cone is steep hill. Around the cone there is tephra with small holes, which are the remains of the gas bubbles in the lava.
In Auckland two types of lava occur: pahoehoe and aa. Pahoehoe is much more fluid lava than aa. Its flows are thinner and faster moving.
To claim this Earthcache you have to:
Answer the following questions and send the answers to me:
1. Which type of eruption was the eruption of Mount St. John?
To answer this question consider if you can see tuff rings and the shape of the volcano, as well as the size of the crater. Please mention these in your answer.
2. Describe the crater.
3. Take a photo on the volcano and attach it to your log.
First to find: funkymunkyzone and boneybruce