MINNESOTA’S NORTH SHORE�A Natural Laboratory for Igneous Rocks
Jim
Miller
MMEW
2015
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
Mag
Magma to Rock
Mafic Magma
T - 1250-1050 C
Felsic Magma
T - 1050-800 C
Magma
Differentiation by
Fractional
Crystallization
IGNEOUS ROCK TEXTURES
f (rate of cooling = crustal environment)
Plutonic🡪Volcanic
1 mm
1 mm
Degree of Undercooling
Porphyritic Texture
Indicative of
two-stage cooling
Ophitic Texture
Indicative of Late Pyroxene Crystallization
from a Primitive High-T Mafic Magma
Ophitic Basalt
Ophitic Gabbro
Volcano Types
f (Magma type =
Plate Boundary)
Mafic magmas at divergent boundaries
Mixed magmas at convergent boundaries
Mt St. Helens
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.
Explosive
(pyroclastic)
Eruptions
Mt St. Helens
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
Plume
Head
_______________
THE MIDCONTINENT RIFT
_______________
An attempt at continental rifting 1.1 billion years ago
1,109-1,086 Ma
1,090-900 Ma
1,000-900 Ma
The Evolution of
Lake Superior
Part I - FIRE
Massive
Basalt
Amygdaloidal
Basalt
Amygdules – Gas bubble fillings
Zeolite
Thompsonite
Epidote & Feldspar
Chlorite & Zeolite
LAKE SUPERIOR
AGATES
Gemstone of
Minnesota
Formation of
Lake Superior
Agates
BASALT FLOW SURFACES
AA
Pahoehoe
NORTH SHORE
Pahoehoe
AA
HAWAII
RHYOLITE
The other volcanic rock type
Palisade Head
Shovel
Point
High Falls on the Baptism River
INTRUSIVE ROCKS
Magmas trapped in
the Crust
Beaver
Bay
Complex
Duluth
Complex
Present-day
Surface
Coarse-grained
Gabbro
Layered Gabbro
Volcanics
Intrusions
Ely’s Peak
Two Harbors
City Park
Gooseberry Falls
Sugarloaf
Cove
Temperance River
Cascade River
Skyline Parkway
Silver Cliff Tunnel
Split Rock Lighthouse
Carlton Peak
Lutsen Mtns
Hovland/
Reservation River
Mt Josephine
Tischer Creek
Tettegouche
Grand Marais
Devil’s Track River
Brule River
Twin Ponds
Finland Air Base
Eagle Mtn
HAPPY
HUNTING!