GEO1115_2021_Activity8

THEME 8 – IGNEOUS ROCKS [ 53.5/54 : A+ ] A CTIVITY 8.2S – T ECHNICAL QUESTIONS ( /54) Reference material: - Lecture of associated theme (Brightspace) - Glossary ( https://mysite.science.uottawa.ca/dumas/ ) - Reference document “Section of the lab manual” in Brightspace Activity: 1. In general, lava flows and shallow intrusive igneous rocks have ( finer / coarser ) crystals than deep intrusive igneous rocks. ( /1) Finer 2. Many dikes, sills, and plutons have chilled margins, i.e. smaller crystals at the contact with their wall rock than in their interiors. Explain how this happens. ( /3) It occurs when the magma or lava comes into contact with a cooler surface, such as rock, air, or water, and begins a more rapid crystallization and hardening along this cooler surface. 3. Give the precise name of the different textural components of a porphyritic texture, and explain how this texture is formed (i.e. describe the cooling history of the rock – the cooling stages). Information on this texture can be found in the Igneous Rocks Classification Table (see “Section of the lab manual”) and on the following website: https://en.wikipedia.org/wiki/Porphyritic . ( /4) Porphyritic means coarse crystals, called phenocrysts, within in a finer matrix. This means a rock with more than one distinct size of crystal, a smaller fine texture with larger, more visible, crystals interspersed inside this fine texture. This occurs as rising magma cools in two stages, first lower within the Earth’s crust it cools slowly and forms the larger crystal pieces. Then as it rises and reaches a much shallower depth it cools much more quickly and crystallizes into extremely fine crystals that are invisible or nearly invisible to the naked eye. 4. What is the most abundant aphanitic igneous rock on the Earth’s surface? Where do we find it mostly, and at what type of tectonic plate boundary is it created? ( /3) Basalt is the most abundant aphanitic igneous rock on the Earth’s surface. It is found mostly in the uppermost oceanic crust and is created at divergent tectonic plate boundaries as the plates separate and reveal new ocean floor. 5. What is the name of the dominant mineral forming rocks very rich in iron and magnesium, and what is the arrangement of the SiO 4 4- tetrahedra in this mineral? ( /2) The dominant mineral is olivine and it has simple, isolated tetrahedra structures. 6. What is the relationship between the order in which the silicate minerals crystallize according to the Bowen’s Reaction Series and the complexity of the silicate structure of these minerals? ( /4) The first minerals to crystallize according to Bowen’s reaction series have a simple silicate structure, such as simple isolated tetrahedral structures. As you get further along the series to the last minerals to crystallize they get more complicated silicate structures, such as single chains, double chains, sheets, and framework.