THE LANGUAGE OF THE
EARTH – PART I
IGNEOUS
ROCKS
Average composition of the Earth’s Crust
(by weight, elements, and volume)
Layers of the Earth
OCEANIC
CONTINENTAL
CRUST
CRUST
SiO2
47%
56%
Al2O3
16%
18%
FeO
13%
9%
MgO
10%
3%
CaO
10%
4%
Na2O
2%
5.5%
K2 O
0.7%
2.5%
TiO2
1.1%
1.3%
P2O5
0.2%
0.7%
MANTLE
SiO2 – 45%
MgO – 37%
FeO – 8%
Al2O3 – 4%
CaO – 3%
others – 3%
CORE
Fe – 86%
S – 10%
Ni – 4%
Compositional Layers
Structural
Layers
THE
ROCK
CYCLE
Plate Tectonics
The Engine Behind the Rock Cycle
Definition
IGNEOUS ROCK - An aggregate of minerals
crystallized from molten rock (magma).
Major distinctions in rock type are based
on two criteria:
1) the chemical composition of the magma
(mafic to felsic) and
2) the environment of magma emplacement
(plutonic or intrusive vs. volcanic or extrusive)
Igneous Environments
Fine-grained
= Volcanic
Coarse-grained
= Plutonic
IGNEOUS ROCK TEXTURES
f (rate of cooling = crustal environment)
Plutonic---Volcanic
1 mm
Degree of Undercooling
1 mm
Porphyritic Texture
Indicative of
two-stage cooling
Magma Composition
Related to Partial
Melting of Source
Melting the mantle
makes MAFIC Magma
Melting the crust
makes FELSIC Magma
Igneous rocks
crystallize over a
range of
temperatures
Igneous Rock-forming Minerals crystallized from
Various Magma Compositions
Generation of Diverse Magma Compositions in Various Geologic
(Tectonic) Settings
Volcanism and Earth’s Systems
Atmosphere – originally created from gases released from
volcanic eruption
Hydrosphere – produced by condensation of volcanic water
vapor
Biosphere both positively and negatively influenced by
volcanism
• lava flows and ash weather to produce fertile soils
• violent eruptions can destroy nearly all life in their paths
• large amounts of ash and volcanic gases in atmosphere can
trigger rapid climate changes and contribute to mass
extinctions
Properties of Magmas
Cinder
Cone
Fissure Eruption
Basalt
Lobe
Mt Pinatubo
Pahoehoe Crust
Mt Fuji
Mt St Helens
New Zealand
Effusive Eruptions
• Mafic magma
• Relatively low gas content (<1%)
• Fountaining followed by flow as
gas content diminishes
• Creates vesicular to massive lava
flows
Photos from USGS
Explosive Eruptions
• Mostly involves intermediate to felsic magmas
• Driven by degassing of magma as it rises up the neck of the volcanic vent
• The dramatic increase of volume resulting from degassing causes the magma
to be violently thrust out the neck and shattered into fine fragments –
VOLCANIC ASH
• Creates pyroclastic deposits
http://www.geology.sdsu.edu/how_volcanoes_work/
Water solubility (carrying capacity) in rhyolite as function
of pressure; from Yamashita (1999)
Eruptive Styles and Volcanic Landforms
Effusive - magma fountains or oozes from vents
• Fissure eruptions (flood or plateau basalts)
• Shield Volcanoes
• Submarine Eruptions (pillowed basalts)
Explosive – violent eruptions of gas-rich magmas
• Stratovolcanoes (composite)
• Pyroclastic eruptions – ash and fragments
• Calderas – self-destruction of stratovolcanoes
FLOOD BASALTS
Basaltic Volcanism
in Continental Settings
NORTH SHORE
Flood Basalts of
the Midcontinent
Rift
Shield Volcanoes
Basaltic Volcanism
in Oceanic Settings
Mauna Loa
Cinder Cones –sputtering basalt eruptions
Stratovolcano
Steep, conical volcanoes built by the eruption
of viscous lava flows, tephra, and pyroclastic
flows, are called stratovolcanoes. Usually
constructed over a period of tens to hundreds of
thousands of years, stratovolcanoes may erupt a
variety of magma types, including basalt,
andesite, dacite, and rhyolite. All but basalt
commonly generate highly explosive eruptions.
Mt. St. Helens
Explosive
(pyroclastic)
Eruptions
Costa Rica
Phillipines
Mt. St. Helen’s
Caldera Formation:
The Fate of all
Stratovolcanoes
Pillowed Basalts
Submarine Eruptions