Transcript Porphyritic Fine

Igneous Rocks
Warm-up
 Compare and contrast rocks and minerals.
Overview of Igneous
Rocks
 Form when minerals crystallize
from magma
IntrusivePlutonic
ExtrusiveVolcanic
 Magmas derived from below the
Earth’s surface in the mantle
 Magma is hot and buoyant
Koryakskaya
Sopka Volcano,
Eastern Russia
It’s all about heat and density
 Heat source??
 Composition
 Geothermal Gradient
 Hot stuff
Intrusive Vs. Extrusive
Plutonic
 Formed within the Earth
 Magma
 Reach Surface by uplift
and erosion of the
Earth’s Crust

Volcanic
 Formed at the Surface
 Lava

Igneous Rock Classification
Composition
Texture
Felsic
Intermediate
Mafic
Ultramafic
Course-Grained
Granite
Diorite
Gabbro
Peridotite
Porphyritic
Course-Grained
Fine-Grained
Porphyritic Porphyritic Porphyritic
Granite
Diorite
Gabbro
Rhyolite
Andesite
Basalt
Porphyritic Fine- Porphyritic Porphyritic Porphyritic
Grained
Rhyolite
Andesite
Basalt
Texture
 Related to the cooling history of the rock
Really Fast = no grains glassy
Fast = Small/ microscopic grains Fine-Grained
Slow = large grains Course-Grained
Complex = Mixture  porphyritic
Why??
 Other textures:
Vesicular: trapped gases in lava
Pyroclastic: ash and rock fragments formed
explosively
Texture
 Fine-Grained
Cooled quickly
Crystallized at the
Earth’s surface
Porphyritic Fine-Grained
 Course-Grained
 Porphyritic Course-Grained

Peanut Butter or
Sugar Cookie
Fine-Grained--peanut butter cookie
Texture

Fine-Grained
 Porphyritic Fine-Grained
Two stages of cooling
– 1st cooled slowly within the
Earth (larger cyrstals Phenocrysts)
– 2nd cooled rapidly on the
Earth’s surface (fine-grained
matrix)
Course-Grained
 Porphyritic Course-Grained

Chocolate Chip Cookie
Porphyritic Fine-Grained —Chocolate
Chip Cookie
Texture
Fine-Grained
 Porphyritic Fine-Grained

 Course-Grained
Cooled slowly
Crystallized within the Earth

Porphyritic Course-Grained
Oatmeal Cookie
Course-Grained—Oatmeal
Cookie
Texture
Fine-Grained
 Porphyritic Fine-Grained
 Course-Grained

 Porphyritic Course-Grained
Two stages of cooling
Oatmeal Raisin
Cookie
– 1st cooled slowly within the Earth (larger crystals Phenocrysts)
– 2nd cooled faster but still slow enough that crystals
fully develop – within the Earth (coarse-grained
matrix)
Porphyritic Course-Grained —
Oatmeal Raisin Cookie
Texture
Fine-Grained
 Porphyritic Fine-Grained
 Course-Grained
 Porphyritic Course-Grained

 Vesicular –voids left by
trapped gas
Glassy
 Pyroclastic

Texture
Fine-Grained
 Porphyritic Fine-Grained
 Course-Grained
 Porphyritic Course-Grained
 Vesicular

 Glassy
Very rapid cooling
Ions do not have time to
from crystalline
structures
 Pyroclastic
Texture
Fine-Grained
 Porphyritic Fine-Grained
 Course-Grained
 Porphyritic Course-Grained
 Vesicular
 Glassy

 Pyroclastic – welded
shards of rock & ash
ejected from a vent
during an eruption
Composition of Igneous Rocks
 Silica (Si02) is primary
Composition of Earth's Crust by Mass
ingredient of all magmas
 Viscosity: Resistance to
flow
O-46.6%
Silica content
temperature
All Others-9%
Si-27.7
Ca-3.6%
Fe-5.0%
Al-8.1%
Composition—Silica Content
 Felsic: Feldspar & Silica
>65% silica  High Viscosity
 Intermediate:
53-65% silica  Intermediate Viscosity
 Mafic: Magnesium and Iron (Ferris)
45-52% silica  Low Viscosity
 Ultramafic:
<45% silica  Very Low Viscosity
Composition
 Felsic-rhyolitic:
<900EC; Na, K, Al-rich
Light colored
Composition
 Mafic-basaltic:
>1100EC; Ca, Fe, Mg-rich
Dark Colored
Composition
 Intermediate-andesitic:
900-1100EC; Na, Al, Ca, Fe, Mg, K
Salt & Pepper appearance
Andesite Porphyry
Diorite
Complete the following table by identifying which of the characteristics
in the left-hand column are present in volcanic and/or plutonic igneous
rocks by stating yes or no for the appropriate number. One characteristic
has been completed as an example.
Characteristic
May form from mafic magma
Form at Earth’s surface
Have texture
Made of small grains
Granite is an example
Form as a result of melting
Present at Earth’s surface only after erosion
Contains minerals
Classified based on color
Dark-colored examples have low silica content
Contain visible grains
Volcanic igneous Plutonic igneous
rocks
rocks
1.
3.
2.
4.

5.
7.
9.
11.
13.
15.
17
19.

6.
8.
10.
12.
14.
16
18.
20.
Bowen’s Reaction Series
Hot
Melting
minerals
crystallize from
magmas at
different
temperatures
Crystallization
 Different
Cold
Magmatic Differentiation
 Formation of more than one magma from a single
parent magma
Magmatic Differentiation
 Crystal Settling:
crystallized minerals have a density
greater than the magma and settle to the bottom due to
gravity

Because Fe and Mg are first removed, melt becomes rich in
SiO2, Na, and K
Marbles analogy
Magmatic Differentiation
 Assimilation:
magma reacts with the “country rock” which
is adjacent to the magma chamber
Magma composition is altered according to the composition of the
assimilated country rock
Inclusions are rocks
Incompletely melted
chunks of country
rock
Magmatic Differentiation
 Magma Mixing:
Magmas of different
compositions are mixed together
Resulting magma is of a composition intermediate
between the parents
Magma Mixing
Magma Mixing
Magma Mixing
Fig. 7.21
Composition Quiz
 Which type of lava would flow most easily?
 Mafic/Ultramafic
 Which type of volcano would erupt most
violently?
 Felsic