Plagioclase Crystals
Tasks:
- Observe the plagioclase crystals in the rock. Are they small and evenly distributed or larger and irregular? What does this suggest about the cooling history of the lava?
- Look closely at the surrounding rock. Is it fine-grained basalt or coarser andesitic rock? How does the grain size help you determine the cooling rate of the magma?
- Examine the rock surface. Are the plagioclase crystals more resistant to weathering than the surrounding rock, or do they appear more eroded? What does this tell you about the durability of plagioclase in this environment?
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What Are Plagioclase Crystals?
Plagioclase is a common group of feldspar minerals found in many volcanic rocks. These minerals are essential components of basalt, andesite, and other igneous rocks. Plagioclase feldspars range from calcium-rich (anorthite) to sodium-rich (albite) compositions, forming a continuous series of minerals.
At Paihia’s coastline, the exposed volcanic rocks contain visible plagioclase crystals, which appear as white to light-gray mineral inclusions. These crystals formed as magma cooled and solidified millions of years ago. By studying these minerals, we can understand the geological history of this region, including past volcanic activity and the cooling processes that shaped the landscape.
Geological Background: Volcanic History of Paihia
1. The Formation of Volcanic Rocks in Paihia
Paihia is part of New Zealand’s Northland volcanic region, which has undergone multiple phases of volcanic activity over millions of years. The rocks found along the coastline are primarily basaltic and andesitic in composition, formed from ancient lava flows.
- Basalt: A fine-grained volcanic rock, typically dark in color, that forms from rapidly cooling lava.
- Andesite: An intermediate volcanic rock, containing more silica than basalt, often with visible crystals (phenocrysts) of plagioclase, pyroxene, and amphibole.
The presence of plagioclase feldspar in these rocks suggests that the magma cooled slowly enough for crystals to develop before the lava fully solidified.
2. The Formation of Plagioclase Crystals
Plagioclase feldspar forms directly from cooling magma. The size and shape of the plagioclase crystals at Paihia indicate that they crystallized at different depths before being transported to the surface by volcanic eruptions.
- In basalt, plagioclase often appears as small, dispersed white or light-gray crystals embedded in a fine-grained matrix.
- In andesite, plagioclase can form larger, well-defined crystals known as phenocrysts, which stand out against the darker rock.
These variations suggest that the volcanic rocks at Paihia came from different depths within a magma chamber, with some magma cooling slowly underground (allowing large crystals to form) and others cooling quickly at the surface (creating fine-grained rock).
3. Weathering and Exposure of Plagioclase at Paihia
Over thousands to millions of years, the coastline has been shaped by erosion and weathering, exposing the plagioclase-rich volcanic rocks. Mechanical and chemical weathering processes break down the rock, sometimes leaving plagioclase crystals more visible due to their different resistance to erosion.
- Mechanical weathering: Wave action and temperature fluctuations cause the rock to fracture and break apart.
- Chemical weathering: Rain and seawater react with minerals, sometimes altering plagioclase to clay minerals like kaolinite.
Because plagioclase feldspar is relatively resistant to chemical breakdown compared to other minerals, it remains visible in the weathered rock surfaces.
https://www.mineralienatlas.de/lexikon/index.php/MineralData?lang=de&mineral=Plagioklas
https://de.wikipedia.org/wiki/Feldspat
https://www.kristallin.de/Gesteine-systematisch/Basalt/Basalt.html