Igneous Rocks

Rock

any solid mass of minerals or mineral-like matter

mineral-LIKE matter CAN be organic. (from a once-living thing)

Types of Rocks:

1. Igneous

2. Sedimentary

3. Metamorphic

Magma

mix of molten rock, gases and mineral crystals that form deep BELOW the Earth’s surface. Magma crystallizes (solidifies) to become rock

Molten rock is called MAGMA or LAVA when?

Called Magma: when it is BELOW the Earth’s surface   
Called Lava: when it is ABOVE the Earth’s surface

Igneous Rock

rock formed from the crystallization of cooled magma. Igneous rocks are considered to be parent rocks.

All the other rock types come from other rocks EXCEPT igneous rocks, which come from MAGMA, making them parent rocks.

Three Ways to Classify Igneous Rocks

1.   Formation - formed above/at surface or below the surface

intrusive vs extrusive

2.   Texture - by the size of crystals

coarse, fine, glassy, vesicular

3.   Composition - by what the rock is made of

felsic, intermediate, mafic

Crystal size determined by rate of cooling

Magma → Slow cooling → Large crystals 

magma cools slowly because it is underneath the ground, where it’s hotter

Lava → Rapid cooling →Small or No crystals

lava cools rapidly because it is above the ground, where it is cooler

Intrusive Rocks

Cools slowly inside Earth’s surface resulting in large crystals.

IN-trusive - inside the Earth’s surface

Examples: granite, diorite & gabbro

(lighter color → darker color)

Granite makes up most of the continental crust

Extrusive Rocks

Cools quickly outside of Earth’s surface, results in small crystals

EX-trusive (exit - outside of Earth’s surface)

Examples: rhyolite, andesite & basalt

(lighter color to color darker)

Basalt makes up most of the oceanic crust

Volcanic Glass (extrusive)

rocks cooled so quickly that NO CRYSTALS FORM

Glassy (Obsidian, little dissolved gases)) and Vesicular (Pumice, Scoria, lot of dissolved gases and float in water)

Texture

Texture refers to the size, shape & distribution of the crystals or grains that make up a rock

Difference between glassy and vesicular texture

the rocks with GLASSY texture that formed from magma contain LITTLE dissolved gases

rocks with VESICULAR texture that formed from magma contains A LOT of dissolved gases

Felsic Rock (Granitic Rock)

Felsic rocks are light colored and not very dense because they are rich in silica (SiO₂  greater than 60%) and don’t contain much iron (Fe) and magnesium (Mg)

Examples: Granite, Rhyolite, Pumice

Mafic Rock (Basaltic)

Mafic rocks are dark in color and have a high density since they have high amounts of iron (Fe) and magnesium (Mg) and low in silica (SiO₂ <50%)

Examples: Basalat, Gabbro, Scoria

Intermediate (Andesitic)

Has a mineral composition of both felsic & mafic rock (Has a silica content between 50-60%) and tend to have a darker gray color

Examples: Andesite, Diorite

When does crystallization occur?

Crystallization (solidification) occurs at the melting point

Intrusive vs Extrusive, Cooling Speed

1. Coarse

2. Fine

3. Glassy

4. Vesicular

1. Intrusive, Slow

2. Extrusive, Rapid

3. Extrusive, Very Rapid

4. Extrusive, Very Rapid

1. What point does crystallization (solidification) occur?

2. What direction do rocks crystallize, when would a substance that was first to melt crystallize as the temperature decreases?

1. melting point

2. Rocks crystallize (become solid) in the opposite direction from which they melt.

The first substance to melt as the temperature increases, will be the LAST to crystallize as the temperature decreases.

As solids, will Felsic or Mafic rocks melt first?

Felsic rocks will melt before mafic rocks as the temperature increases, because felsic rocks have a lower melting/crystallization point than mafic

As liquids, will Felsic or Mafic rocks crystallize first?

Mafic magmas will crystallize before felsic magmas as the temperature decreases, because mafic magmas have a higher melting/crystallization point than felsic magmas

Felsic minerals melt at around ____

Intermediate minerals melt around _____

Mafic minerals melt at about ______

Felsic minerals melt at around 650 ℃

Intermediate minerals melt around 850 ℃

Mafic minerals melt at about 1050 ℃

As the temperature increases, ___ magma will melt first and ___ magma will melt last.

As the temperature decreases, ___ magma will solidify first and ___ magma will solidify last.

As the temperature increases, Felsic magma will melt first and Mafic magma will melt last.

As the temperature decreases, Mafic magma will solidify first and Felsic magma will solidify last.

Bowen’s Reaction Series (Also Silica and Mafic change explanation)

Allows us to predict the way that igneous rocks form from cooling magma.

As magma cools, minerals form in predictable patterns. Different minerals form at specific temperatures as magma cools.

Silica-rich minerals (felsic) undergo a continuous change in composition. (Ca rich feldspar → Na rich feldspar as temperature cools)

Iron rich minerals (mafic) undergo a discontinuous change in composition. 

Why does Mafic and Felsic have different ‘changes’ when they crystallize

Mafic magma undergoes a discontinuous change, when it crystallizes, because it contains many different minerals that have different melting points.

Represented with different arrows (cuz it’s separated)

Felsic magma undergoes a continuous change, when it crystallizes, because it contains many different minerals that have similar melting points.

Represented with one long, continuous arrow

Factors Affecting Magma Crystallization: Mineral Composition

Mafic magmas crystallize at higher temperatures than felsic magmas. Fe and Mg have a higher melting/crystallization point than silicates.

Mafic is higher, Felsic is lower

Factors Affecting Magma Crystallization: Pressure

Increased pressure increases the melting/crystallization point. Increased pressure prevents the atoms from moving further apart, thus the magma will crystallize at a higher temperature. 

More pressure, higher crystallization point

Factors Affecting Magma Crystallization: Percent Water Content

Water lowers melting/crystallization point, because H₂O has a much lower crystallization point (0⁰C) than magma does (at least 700⁰C)

More water, lower temperature

Factors Affecting Magma Crystallization

1. Mineral Composition - Mafic higher temp crystallizes, Felsic lower

2. Pressure - more pressure, higher temp

3. Percent Water Content - more water, lower temp

Obsidian

Pumice

Scoria

Granite

Basalt

Obsidian (glassy)

Pumice (light vesicular)

Scoria (dark vesicular)

Granite (light continental crust)

Basalt (dark oceanic crust)