IGNEOUS ROCKS

Rocks are divided into 3 categories The Rock Cycle- Minerals form rocks All rocks can be transformed into other rock types Rocks are divided into 3 categories Igneous- crystalline- forms as liquid cools Metamorphic- crystalline-forms as rocks are heated and squeezed Sedimentary- non-crystalline- smaller pieces or chemicals from other rocks

Igneous Rocks Formed From Magma and Lava Magma - molten rock below Earth's surface. Lava - magma on the Earth’s surface. Pyroclastic material (pyro = fire clastic = debris Airborne Lava - cools as it falls.

What terms might you use to describe when looking at an igneous rock?

Igneous Rock Textures

Viscosity of Magma/ Lava Viscosity is important for volcanic activity. It is the resistance of a liquid to flow High viscosity is thick and stiff Low viscosity is thin and “runny” Is determined by the amount of: Water in the magma Amount of silica in the magma Mafic is thin and has low viscosity Felsic is thick and has high viscosity. •

Phaneritic Texture Visible Mineral Grains distinguishable as different colored interlocking shapes Aphanitic Texture Mineral grains too small to be seen microscopic

Igneous Rock Textures- how big are the minerals? Porphyritic Texture Two distinct sizes of mineral grains Large and Small Large = Phenocrysts Small = Groundmass or Matrix Groundmass Grains may be either visible or not

Porphyritic texture Aphanitic basalt (mafic composition) constitutes the groundmass

Categories of Igneous EOF Granite Intrusive rocks cool beneath Earth's surface cool very slowly — Phaneritic textures Extrusive rocks Rhyolite cool on the Earth's Surface cool relatively fast Aphanitic textures Basalt porphyry Complex Partially cools below and above Porphyritic textures

Pegmatite A Pegmatite is a very coarse-grained igneous rock. Crystals are >2 cm, often larger. Most are granitic, although mafic pegmatites can form. Biotite mineral grain Feldspar mineral grain Quartz mineral grain

Granite & Rhyolite Felsic magma Phaneritic Texture Granite What are the textures in these two rocks ? Aphanitic Texture

Andesite & Diorite Phaneritic texture Aphanitic texture -forms from intermediate composition magma Aphanitic texture forms from intermediate composition magma What are the textures in these two rocks ?

Basalt & Gabbro Gabbro Basalt aphanitic texture phaneritic texture forms from mafic magma phaneritic texture forms from mafic magma

Peridotite- the abundance of Olivine crystals (more than 40%) makes the rock take on a green appearance

Obsidian- a popular rock among paleopeople for tool making Volcanic Glass

How do we tell where the igneous rocks formed and what can we derive from the rocks about the conditions of formation?

Pyroclastics Lava flow Volcanic neck Igneous dike Igneous Sill Batholith

https://www.youtube.com/watch?v=Q8NXO6YxBmU

A batholith is a large mass of intrusive igneous rock, larger than 100 square miles in area, that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, such as granite, quartz monzonite, or diorite.

Sierra Nevada Batholith- Home to Yosemite National Park and a lot of Granites

Sierra Nevada Batholith

Areal extent of the Idaho Batholith- a huge (15,400 square miles) intrusive body of primarily felsic composition (granites) igneous rocks

Discordant with surrounding rock It cuts across other rocks An igneous dike- Discordant with surrounding rock It cuts across other rocks

Concordant with surrounding rock It runs parallel to other rocks An igneous sill- Concordant with surrounding rock It runs parallel to other rocks

Necks & Ig Dikes Neck Dike igneous sill Igneous sill