G145 Week 2

 

Review:

      Catastrophism vs uniformitarianism

      Geologic time & the Geologic time table

      Relative vs absolute dating (Stratigraphic superposition, cross cutting relationships etc…)

      Earth’s structure (Core (inner & outer), Mantle (mesosphere & asthenosphere, Crust (continental & oceanic))

      Continental Drift vs. Plate tectonics 

            Evolution of each idea and evidence used

 

Plate boundaries (divergent, convergent, transform)

 

Hot spots

 

Mechanisms of plate tectonics

      Slab pull

      Heat convection cells

            Radioactive decay

      Ridge push

 

Rock types

      Igneous:  Formed from magma

            Intrusive:  magma cooled below the surface

            Extrusive:  magma cooled on the surface

 

      Sedimentary:  Forms from compressed, eroded sediments

 

      Metamorphic:  Forms when heat and/or pressure is applied to a pre-existing rock causing the minerals in the rock to change

 

Earth’s crust is made up of rocks

      Rocks are made up of minerals

      Minerals are made up of atoms of elements (more on this later)

 

Rock Cycle

 

Igneous rocks

      Environment:

            Intrusive:  Magma cools inside the earth

            Extrusive:  Volcanic…lava cools on the surface of the earth

      Composition

            Felsic  (minerals are rich in silicon and aluminum)

            Mafic (minerals are rich in iron and magnesium)

            Intermediate

 

Mafic magmas are hotter than felsic magmas.

      How does this affect the viscosity of the magma?

 

Felsic magmas have more silica and water

      How does this affect the magma?

 

 

Volcanic structure based on Magma

      Felsic/intermediate magma:

            Cooler temperature (~900oC)

            Very thick (viscous)

            Very explosive

 

      Mafic magma:

            Hotter temperature (~1200oC)

            Low viscosity

            Not very explosive

 

Felsic/intermediate Volcanoes

      Examples:

            Mt. St. Helens and the rest of the Cascade Moutains, Mt. Pinatubo, Mt Unzen

            Occurs primarily at subduction zones on over-riding plate

            Magma generated by melting subucting slab and the over-riding plate

            Common rocks:  Rhyolite, andesite, obsidian, pumice

 

Mafic Volcanism

      Examples:  Hawaiian volcanoes, Larch mountain, Newberry volcano (part of it)

            Found at divergent boundaries and any place crust is being thinned

            Magma is generated by melting the upper part of the mantle (asthenosphere)

            Common rock:  Basalt

            Other features:  Flood basalts & hot spots

 

Sedimentary rocks

      Clastic (Lithic)

            Sandstone

            Siltstone

            Mudstone

            Conglomerate

            Breccia

      Chemical

            Limestone

            Chert

            Agate

            Jasper

            Opal

            Coal

 

Metamorphic Rocks

      Minerals Align in planes

      Often Banded in appearance or layered

      Not that common in OR or NW

            Bandon

            Wallowa Mtns.

            Ochoco Mtns.

 

 

 

 

Terranes

      Sea-Floor

            Formed along mid-ocean ridges

                  Composed of mostly:

                        Basalts (extrusive)

                        Gabbros (intrusive)

                        Some peridotite (upper mantle)

                        Thin sediments (mud, chert clay)

 

            Ophiolite complex

            Seamount Features

 

Fore-arc Basin

      Between subduction zone an volcanic arc

      Fills with volcanic-arc sediments (graywacke sandstones)

      Forms an accretionary wedge

      Many slabs of material angled downward

 

Back-Arc Basin

      Between volcanic island arec and the continent

      Fills with volcaic sediments seaward and continental sediments landward

      Folded and faulted by collision with overlapping thrusts landward

 

Island-arc Volcanoes

      Aleutian type volcanoes

      Composition is Andesite/Diorite

      Tuffs (volcanic ash flow materials)

      Andesite Lava flows

      Diorite intrusions

      Linear distribution of volcanoes and rocks

 

Exotic Terranes

      Generally Mesozoic age rocks or older

      Formed in an area far away

      Often composed of continental fragments of other regions

      Easy to recognize as Exotic by

            Fossils

            Rock type

            Paleomagnetic

            Structural…..

 

Why does all this matter to the Pacific NW?

      The Pacific NW was created by the accretion of various types of terranes

            Accreted terranes were “attached” to the West coast of N.America by igneous plutons

 

Accretion IS the history of this area!

                        (see handout)

http://usinfo.state.gov/products/pubs/geography/map2.htm

Regions of the Cordillera

 

http://geoinfo.amu.edu.pl/wpk/geos/GEO_2/GEO_PLATE_T-1.HTML

Physiographic Regions of OR

      Blue Mountains

      Eastern Blues

      Western Blues

      Klamath Mountains

      Owyhee Uplands

      High Lava Plains

      Basin and Range

      Columbia Plateau

      Cascade Mountains

      Willamette Valley

      Coast Range

 

Overview of NW geology

      Precambrian to Paleozoic (1 bybp to 400mybp):

            Wide continental shelf across Western N. Amer.

            Shallow seas cover NW at this time

            Subduction zone located far off shore creating island vol.

      Paleozoic-Mesozoic (400-70 mybp):

            Island volcanoes begin colliding with N Amer

            Subduction zone starts and shuts down many times as accretion occurs

            Western margin is folded due to collisions

            Granitic batholiths are emplaced and weld terranes to continent

      Cenozoic: N. America basically in present geographic location

      66-35mybp:  warm and tropical, many basins created, Subduction zone active, first Cascade Mts. build and change the climate

      35-24mybp: John day volcanics

      24-5 mybp: Uplift of coast range, Columbia river flood basalts, more basins form

      5-2mybp: 2nd stage of Cascade volcanism, great rift basin develops

      1mybp to present:  High Cascade volcanism, glaciers cover much of the area, floods