The Mount Maunganui group form a tight cluster in the ocean off Mount Maunganui town beach. The lava remnants of Motuotau and Moturiki Islands and Mount Drury/Hapukiore are all genetically related to the lava dome of Mount Maunganui.
This Earthcache will look at two types of lava flow to determine what type of cooled lava formed Moturiki Island which has had a number of uses over the years, even being known as Leisure Island in the 1980s, with a water park and aquarium.
Igneous rocks form when magma (molten rock) cools and crystallizes, either at volcanoes on the surface of the Earth or while the melted rock is still inside the crust. All magma develops underground, in the lower crust or upper mantle, because of the intense heat there.
Igneous rocks can have many different compositions, depending on the magma they cool from. They can also look different based on their cooling conditions. For example, two rocks from identical magma can become either rhyolite or granite, depending on whether they cool quickly or slowly.
The two main categories of igneous rocks are extrusive and intrusive. Extrusive rocks are formed on the surface of the Earth from lava, which is magma that has emerged from underground. Intrusive rocks are formed from magma that cools and solidifies within the crust of the planet.
When lava comes out of a volcano and solidifies into extrusive igneous rock, also called volcanic, the rock cools very quickly. Crystals inside solid volcanic rocks are small because they do not have much time to form until the rock cools all the way, which stops the crystal growth. These fine-grained rocks are known as aphanitic—from a Greek word meaning “invisible.” They are given this name because the crystals that form within them are so small that they can be seen only with a microscope. If lava cools almost instantly, the rocks that form are glassy with no individual crystals, like obsidian.
Intrusive rocks, also called plutonic rocks, cool slowly without ever reaching the surface. They have large crystals that are usually visible without a microscope. This surface is known as a phaneritic texture, the best-known phaneritic rock is granite.
So, basaltic magmas tend to be fairly fluid (low viscosity), but their viscosity is still 10,000 to 100,0000 times more viscous than water. Rhyolitic magmas tend to have even higher viscosity, ranging between 1 million and 100 million times more viscous than water.
Basalts are usually dark grey to black in colour. Basalts are formed by the rapid cooling of basaltic lava, from the interior of the crust and exposed at or very close to the surface of Earth. These basalt flows are quite thick and extensive, in which gas cavities are almost absent.
Rhyolite is a volcanic rock. It is fine-grained because it forms by the rapid cooling of magma, usually when it erupts onto the Earth's surface. When rhyolite erupts quietly it forms viscous lava flows which can form thick blocky lava flows or steep-sided piles of lava called lava domes. If it erupts explosively it often forms abundant ash and pumice.
Rhyolite is an extrusive igneous rock with a very high silica content. It is usually pink or gray in color with grains so small that they are difficult to observe without a hand lens. Rhyolite is made up of quartz, plagioclase, and sanidine, with minor amounts of hornblende and biotite.
The rocks at the edge of the beach on approach to Moturiki are readily approachable at most tide levels and observations are visible from really close up. Using the information above please answer the following questions, which should be sent to the CO at your earliest chance. You are welcome to log your find following your vist to GZ. If you need any help with any questions please contact me.
Question 1. What type of rock are you observing at the listed coordinates; Basalt or Rhyolite?
Question 2. Please give your reason for your choice for question 1. Do you think it formed quickly or slowly?
Question 3. With natural erosion of the rocks at GZ inevitable from ocean movement and tides, and the rock type, what do you think this island will look like in the future, say 100,000 years time. What other natural factors may influence this appearance?
Please message me your three answers to the questions above, if you need any help please let me know.
Question 4. Please provide a photograph (with Moturiki island in the background) of yourself or a personal item to prove you visited the site. A personal item such as a handwritten name, or trackable is an option for those who do not want to photograph themselves. This picture should be uploaded with your found it log.