Igneous Rock Lab Flashcards
Aggregates of minerals that crystallize from molten material generated within the earth's mantle. The type of rock formed depends on the original composition of the magma and the rate of cooling.
Extrusive igneous rocks
Cool quickly on the Earth’s surface. Fine-grained, lack of crystal growth. Too small to see without a microscope. Aphanitic. Basalt is one of the most common extrusive igneous rocks in the world.
Intrusive igneous rocks
Cool slowly within the Earth. Are course-grained. More stable and larger crystals. Phaneritic.
Felsic - Feldspar/Silica
High in silica. Generally much lighter in color since quartz and feldspar are often white or light in color.
Mafic - Magnesium/Ferric
Minerals are low in silica and generally much darker in color since they contain mostly dark-colored minerals. Mafic minerals crystallize at higher temperatures than felsic.
Ultramafic - made up of mostly Olivine
Dark green colored minerals like peridotite.
Course-grained (phaneritic) texture
Rocks with visible (1mm or larger) mineral crystals. Generally formed in magma that cooled slowly underground, giving crystals time to form. First-formed crystals have regular shapes; later-formed crystals have irregular shapes because they had to fill irregular gaps left over. You can figure out the relative order the minerals crystallized from magma based on regular/irregular shape.
Porphyritic
When magma cools in two stages, the first stage is slow, and large crystals form in the still liquid magma. The second stage is rapid, forming smaller-sized crystals. Rapid cooling can occur during a volcanic eruption or when a rising column of magma reaches the shallow depts in the crust. Ground mass can be fine-grained or coarse-grained.
Phenocrysts
Extra large mineral grains in Porphyritic rocks
Groundmass
Faster cooling mineral (background) in Porphyritic rocks. Sometimes fine sometimes coarse, but generally distinguishable from the larger phenocrysts.
Glassy Texture
Rapid cooling of very viscous magma can produce volcanic glass. They look like shards of broken glass, for example: obsidian.
Vesicular Texture
When gas bubbles get trapped in coling lava they are called vesicles. The rock is said to have a vesicular texture. Examples: Scoria has a spongelike texture and is very light but does not float while pumice has many tiny vesicles and floats in water.
How to identify Igneous rocks
- Texture: The visual overall aspect imparted by the sizes, shapes, and arrangement of the rock-forming minerals.
- Composition: Felxic, intermediate, mafic or ultramafic.
Granite
Rock texture: Course grained (phaneritic)
Mineral Composition: Felsic
Rhyolite
Rock texture: Fine-grained (Aphanitic)
Mineral composition: Felsic
Porphyritic Granite
Rock texture: Porphyritic with a coarse-grained groundmass
Mineral composition: Felsic
