CEE 437 Lecture 4Igneous and Metamorphic Rocks,

Rock LabOctober 10, 2002

Thomas Doe

Outline

• Igneous Rocks• Metamorphic Rocks• Rock Identification Lab

Northwest Igneous and Metamorphic Rocks

Columbia River Basalts

(miocene)

Snake River Basalts

(pliocene)

Yellowstone Region Acidic Volcanics (Pleistocene to

recent)

Cascade Volcanoes (recent)

Recent Basaltic Volcanism (Newberry Crater)

Cascade Batholiths (Felsic, Cret-Miocene)

Geologic Settings for Igneous Rocks

• Oceanic– Hi Fe, Mg, Ca, low Si– basalt, gabbro

• Continental – Hi Si, Na, K– granite, rhyolite, andesite

Igneous Origins

• Intrusive– Batholithic or plutonic: phaneritic– Dikes or sills that chill rapidly: aphanitic

• Extrusive– deposition as melt (lava)– pyroclastic

• tuff• tephra• pyroclastic flows

Identifying Igneous Rocks

• Chemistry– Acidic: Basic (more Si, less Si)

• Texture– Aphanitic: crystals not visible– Phaneritic: made of visible crystal components– Porphyritic: Larger crustals in aphanitic or

phaneritic ground mass

Igneous Rock IdentificationIgneous Rock Identification

Igneous Rock Classification

SERPENTINITE

Acidic, Felsic Basic, Mafic Ultramafic

Common Types in RED outline

Igneous Structural Features

Magma Generation on Continental Margins

Magma Generation in Convergent Continental Plate Margins

Igneous Structures

Extrusives

• Viscosity varies with Si and water content– Basalt — low viscosity– Rhyolite — high viscosity

• Rhyolite flows relatively unusual as rhyolite does not flow well– Explosive– Tuffs, pyroclastics

Volcano Types

Basaltic: low viscosity — Hawaii, Columbia Plateau

Andesitic/Rhyolitic

Structures of Basalt Flows

• Lava Tubes• Flow Stratigraphy

– collonade– entablature– flow top breccia/scoria

Hawaii Basalt Flows

Basalt Flow Structures

Eruptions of Acid-Rock Volcanoes

Rhyolite Dome

Caldera

Mt. St. Helen’s Blast Zone

Mt. Mazama Ash Distribution

Subduction-Zone Metamorphism

Metamorphism

• Recrystallization of Rock Under Temperature and Pressure

Metamorphic Rock Classification

Metamorphic Classification

• Original Material– sandstone, limestone, shale, basalt)

• Metamorphic Grade (Temperature, Pressure)

• Source of Metamorphism (Regional, Contact)

Basic Metamorphic Types

• Quartz Sandstone Quartzite• Limestone, Dolomite Marble• Shale

– Slate — cleavage, no visible xl’s– Phyllite — foliation, mica sheen but xl’s not visible– Schist — clear foliation, visible mica– Gneiss — like granite but with foliation/gneissosity

• Basalt greenschist, amphibolite

Origin of Foliation (gneissosity, schistosity)

Metamorphism at Continental Collisions

Contact Metamorphism

Metmorphic Grade

Northwest Igneous and Metamorphic Rocks

Columbia River Basalts

(miocene)

Snake River Basalts

(pliocene)

Yellowstone Region Acidic Volcanics (Pleistocene to

recent)

Cascade Volcanoes (recent)

Recent Basaltic Volcanism (Newberry Crater)

Cascade Batholiths (Felsic, Cret-Miocene)

Sauk River Quadrangle

Malpasset Dam