MINNESOTA’S NORTH SHORE
A Natural Laboratory for Igneous Rocks
MMEW
2006
Jim
Miller
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 Creation
by Partial Melting
Melting the mantle makes
MAFIC Magma
Melting the crust makes
FELSIC Magma
Igneous rocks
crystallize over a
range of
temperatures
Mafic Magma Formation at Mid-Ocean Ridges
Mafic Magma Formation at Subduction Zones
Magma to Rock
Mag
Mafic Magma
T - 1250-1050 C
Felsic Magma
T - 1050-800 C
Magma
Differentiation by
Fractional
Crystallization
Mafic magmas at divergent boundaries
Volcano Types
f (Magma type =
Plate Boundary)
Mixed magmas at convergent boundaries
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
Pillowed Basalts
Submarine Eruptions
Shield Volcanoes
Basaltic Volcanism
in Oceanic Settings
Basaltic Volcanism
in Continental Settings
Mantle Plumes
and
Hot Spots
LARGE IGNEOUS PROVINCES
Manifestations of Plumes and Hot Spots
_______________
THE
MIDCONTINENT
RIFT
_______________
An attempt at
continental rifting
1.1 billion years
ago
Plume
Head
A. Rift Magmatism
Basalt
Flows
Gabbro
Crust
Mantle
The Evolution of
Lake Superior
Part I - FIRE
1,109-1,086 Ma
Mantle
Plume
B. Sediment Infilling
Sandstone
1,090-900 Ma
C. Compression
1,000-900 Ma
Massive
Basalt
Amygdaloidal
Basalt
Amygdules – Gas bubble fillings
Chlorite & Zeolite
Epidote & Feldspar
Zeolite
Thompsonite
LAKE SUPERIOR
AGATES
Gemstone of
Minnesota
Formation of
Lake Superior
Agates
BASALT FLOW SURFACES
AA
AA
Pahoehoe
HAWAII
Pahoehoe
NORTH SHORE
High Falls on the Baptism River
Shovel
Point
RHYOLITE
The other volcanic rock type
Palisade Head
INTRUSIVE ROCKS
Magmas trapped in
the Crust
Duluth
Complex
Coarse-grained
Gabbro
Layered Gabbro
Beaver
Bay
Complex
Volcanics
Present-day
Surface
Intrusions
Mt Josephine
Eagle Mtn
Hovland/
Brule River
Reservation
Devil’s Track
River River
Grand Marais
Cascade
Lutsen Mtns River
Carlton Peak
Temperance River
Sugarloaf
Finland Air
Base
Cove
Tettegouche
Split Rock
Gooseberry
Falls
Lighthouse
Silver Cliff Tunnel
Two Harbors
City Park
Tischer Creek
Twin
SkylinePonds
Parkway
Ely’s Peak
HAPPY
HUNTING!