Igneous Rocks Pages 129-134

Objectives Summarize three factors that affect whether rock melts. Describe how the cooling rate of magma and lava affects the texture of igneous rocks. Relate cooling time to crystal size Classify ig rx according to their composition and texture. Describe intrusive and extrusive ig rx structures.

Origin of igneous rocks Igneous rocks are formed by molten material from a volcano or molten material within Earth Temps are 1600 at 60-200 km beneath the surface

Why is it so hot? Know these! The deep rocks are under pressure from the rocks on top Also … radioactive elements produce heat Leftover heat from formation of the planet

Formation of magma Three factors affect whether rock melts Temperature Pressure Presence of fluids

This means they crystallize (and melt) at different times Rocks have different melting points based upon the chemical composition of their minerals This means they crystallize (and melt) at different times Remember Bowen's

Partial Melting Different minerals have different melting points, and minerals with lower melting points are the first to melt This creates lava with a specific composition As the magma heats up, other minerals melt and the magma’s composition changes

Where do Igneous Rocks form?

Magma—Inside Earth Lava—Outside Earth

Cooling time=crystal size Magma trapped underground is insulated so it cools slowly As it cools, the atoms arrange themselves in patterns to form crystals If the magma cools slowly, the atoms have time to arrange large crystals. These crystals are called mineral grains.

Rocks form when these mineral grains grow together.

Intrusive Igneous Rocks Rocks that form beneath Earth's surface are called intrusive igneous rocks. They have crystals large enough to see. We call this texture/grain size: phaneritic See handout

How am I ever going to remember phaneritic? Think of large “ph”annies

Granite

Granite is an intrusive igneous rock It cooled slowly inside the earth It has a phaneritic texture

Diorite—salt and pepper look Typical Minerals Na/Ca PLAGIOCLASE & MAFICS (amphibole) about 50/50 Quartz - absent to trace

Gabbro A mafic, phaneritic, igneous rock from the top of Bowen's Reaction Series.

Extrusive Igneous Rocks When rocks form from molten lava above Earth's surface we call them extrusive igneous rocks. They cool quickly so the atoms don't have time to arrange themselves into patterns to form crystals. They have a fine-grained texture. This texture is called aphanitic.

Rhyolite

Rhyolite is an extrusive igneous rock It cooled quickly,It has small grains This is called an aphanitic texture

Andesite Porphyry This is an andesite porphyry A porphyry has big chunks in a matrix Matrix—fine-grained background

Basalt Basalt makes up the ocean floors It is a mafic igneous extrusive rock

Common Igneous rock terms Type of lava Felsic Intermediate Mafic Texture term Intrusive Granite Diorite Gabbro Phaneritic Extrusive Rhyolite Andesite Basalt Aphanitic Vesicular-Frothy Pumice Scoria Obsidian Hyaline-Glassy

Cooled slowly cooled quickly Pairs Granite Rhyolite Cooled slowly cooled quickly

Others-- Scoria formed when lava cooled quickly above ground. You can see where little pockets of air had been. Scoria is actually a kind of glass and not a mixture of minerals. Vesicular—bubbly texture

Pumice Vesicular texture Uses lava soap Stone-washed jeans Glass. Pumice has cooled too quickly for minerals to form. Chemical composition, however, will be from the bottom of Bowen's Reaction series (i.e. felsic).

Obsidian Hyaline texture is also called a glassy texture Obsidian--Volcanic glass Notice the conchoidal fracture

Pegmatite Rock cools extremely slowly, so very big mineral grains form.

Classification of Magmas Mafic—high in iron and magnesium Felsic—high in feldspar and silica (quartz) Mafic—50% silica Colors-- black, green, red Intermediate—60% silica Felsic—70% silica Colors-- pink, white, light gray Remember, are granitic rocks mafic or felsic?

Composition of Igneous Rocks Felsic—high in silica (quartz and feldspar) Light colored Mafic—high in magnesium and iron Dark colored Intermediate A mixture of the two In between colored

Review Igneous Rocks

Melting point is also affected by two factors The pressure As pressure melting point The water content High water content=lower melting point Low water content=higher melting point

Remember a rock is made of different minerals When rocks melt, those with lowest melting points melt first Partial melting When rock crystallize, those with highest melting points crystallize first as the magma cools Fractional crystallization

Bowen’s Reaction Series See handout.

Bowen’s Reaction Series

Igneous Rock formation beneath the Earth's surface Pluton-large intrusive igneous rock body Dike—cuts across rock layers-vertical Sill—horizontal –parallel to existing rock layers

Caldera

Pillow Basalt

Igneous Rock Structures