Lecture 4 Igneous Rocks and Volcanoes ... - Members.efn.org

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Lecture 4 Igneous Rocks and Volcanoes ... - Members.efn.org

Lecture 4

October 9, 11, 13, 16

October 10, 12, 17

Igneous Rocks and Volcanoes

CHAPTER 4,5

This is only an outline of the lecture. You will

need to go to class to fill in the outline,

although much of the relevant information is

also in the text.


IGNEOUS ROCKS

Please read or skim Chapter 3. At least study the diagrams and photos.

In a previous lecture, I gave brief definitions of each rock type.

Igneous Rocks are crystallized from magma or lava:

magma

lava

tephra

ROCK CYCLE DIAGRAM

Each type of rock can be transformed into any of the other rock types.

Where does magma come from? The rock cycle diagram shows us


WHY DO ROCKS MELT IN THE EARTH?

Temperature

Percent Silica

H2O

Pressure

TEXTURES OF IGNEOUS ROCKS

Plutonic Rocks:

Phaneritic

Pegmatite

Volcanic Rocks:

Aphanitic

Porphyritic (porphyry)

Vesicular, glassy, fragmental, pyroclastic...


IGNEOUS COMPOSITIONS

The most important variable is percent silica.

Minerals are a guide to percent silica, because

percent silica is related to mineral structure.

Silicate minerals:

low silica............................................high silica

olivine...pyroxene...amphibole...mica...quartz

isolated... chain...dbl chain...sheet...framework

[ Feldspar: aluminum in framework replaces some silica. ]

Mafic rocks:

Felsic rocks:

NAMES OF IGNEOUS ROCKS

A rock's name depends on the minerals in it.

High Si minerals........................Low Si minerals

Felsic......Intermediate......Mafic......Ultramafic

Plutonic rock names:

__________ ___________ ___________ ___________

Volcanic rock names:

___________ ___________ ___________ ___________

Mineral content affects the over-all color of the rock.

High Si minerals........................Low Si minerals

light color rocks........................dark color rocks


HOW DOES MAGMA CRYSTALLIZE?

Crystals don't all form at once. Some form at high temperature, and others form later as the

magma cools.

Low silica minerals crystallize first (at highest temperature).

The approximate order of crystallization, which varies with composition, H2O and pressure,

is

BOWEN'S REACTION SERIES

Discontinuous Series.............Continuous Series

olivine

pyroxene (augite)

amphibole (hornblende)

biotite mica

MINERALS MAKE THE ROCK

muscovite mica

quartz

K- feldspar

Na-plagioclase

Ca-plagioclase

Which minerals are present in the rock depends on the composition of the magma from

which they formed.

Ultramafic rocks form today only in the mantle.

Peridotite is made of

Mafic magma forms gabbro and basalt. The minerals in basalt and gabbro are:

Intermediate magma forms diorite and andesite. The minerals in diorite and andesite are:

Felsic magma forms granite and rhyolite. The minerals in granite and rhyolite are:


WHERE DOES MAGMA FORM?

Magma forms in both the mantle and crust.

Magma forms in various tectonic settings:

1) Mid-Ocean ridges

source of magma:

why magma forms:

magma composition:

2) Subduction zones

source of magma:

why magma forms:

magma composition:

3) Hot spots: Ocean Islands [Hawaii...]

source of magma:

why magma forms:

magma composition:


4) Hot spots: Continental [Yellowstone...]

source of magma:

why magma forms:

magma composition:

IGNEOUS INTRUSIONS

These form when magma cools underground.

Plutons: discordant intrusions

Sills: concordant intrusions

Dikes: (discordant) pathways of magma intrusion

Veins: minerals crystallized from (hot) water solution. Often associated with plutons.


VOLCANISM

Whether magma will stay in the ground, or erupt, is determined by three main factors.

What makes volcanoes erupt?

1) Silica content

2) Vapor content (gasses, including H2O)

Combination of high silica + high vapor content:

3) Crystallization

TYPES OF VOLCANOES

1. Shield volcanoes and lava plateaus

rock name:

volcano shape:

lava characteristics:

examples:


2. Tephra cones (cinder cones)

rock names:

volcano shape:

tephra characteristics:

examples:

3. Stratovolcanos (composite cones)

rock names:

volcano shape:

lava and tephra characteristics:

examples:


4. Volcanic domes

rock names:

volcano shape:

lava characteristics:

examples:

5. Calderas and craters

These may occur in a number of types of volcanoes. Most volcanic mountains have small

craters in the top.

Calderas: definition:

How formed:

Types of associated volcanoes:

Examples:

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