Presentation on theme: "Primary Structures The value of measuring section Measuring section is the process of making detailed notes about each of the layers/intrusions found in."— Presentation transcript:
Primary Structures The value of measuring section Measuring section is the process of making detailed notes about each of the layers/intrusions found in an outcrop. The process really forces you to LOOK. We work up or down section as appropriate. Note rock/sediment type, primary and secondary structures, fossils, note color, orient surfaces and lineations with the brunton, etc.
Cross Beds Ripples, dunes and deltas in cross section, usually truncated tops (younging direction) and current direction indicated Notice the lens in the photo to give scale. Use whatever is the right size and is a known size, coin, a pencil, a ruler, your notebook, etc.
Surface Markings: Load Casts Sand layer over mud, blobs of sand sink into mud.
Flute Casts Vortices dig into the unconsolidated sediment. Shallower and wider downstream.
Mudcracks They curl up when they form, and give the younging direction
Asymmetrical Ripples Steep face on downstream side.
Pillow Lavas Lava erupted under water (MOR, rift valley lakes), or flowed into water (Hawaii)
Halokinesis: Salt layer movements Buoyancy Salt density about 2200 kg/m 3 Sedimentary Rock average density 2500 kg/m 3 “When the positive buoyancy [dense sediment over light salt] is sufficient to upwarp” the sediment above, the salt will rise. PM p 26 http://search.datapages.com/data/open/offer.do?target=%2Fspecpubs%2Fmemoir65%2Fch20%2F0413.htm
Halokinesis: Differential loading “This may occur when the downward force on the salt layer varies laterally.” PM p.26
T ypical a’a’ flow Broken, often further from vent
Fluid basalt forms lava tubes Checking Bowens Reaction Series
Materials extruded from a volcano Pyroclastic materials – “Tephra” Propelled through the Air Types of pyroclastic debris Dust 0.001 mm and Ash < rice sized Cinders or Lapilli - pea to walnut-sized material Particles larger than lapilli Bombs - > 64 mm ejected as hot lava -Surtsey Is. Bombs the size of busses
Pumice Felsic magmas with high water content may bubble out of a vent as a froth of lava. Quickly solidifies into the glassy volcanic rock known as Pumice. http://volcanoes.usgs.gov/Products/Pglossary/pumice.html
Mt Mazama Eruption and Caldera Collapse 4700 BC S Oregon Ngorongoro Crater in Tanzania similar 2 mya
Volcanism on a tectonic plate moving over a hot spot
Hey, the plate changed direction ! Flood Basalts Hot Spot currently forming Hawaii
Flood Basalts Fluid basaltic lava extruded from crustal fractures called fissures e.g., Columbia River Plateau, Deccan Traps in India Cover huge areas Plumes from Mantle
Flood Basalt erupted from fissures - Snake River Plain, southern Idaho Plume Activity
Spanish Peaks and Radiating Dikes (southern CO)
Plutonic igneous activity Types of intrusive igneous features Dike – a sheetlike injection into a fracture Discordant - cuts across pre-existing Sill – a sheetlike injection into a bedding plane Concordant - lies parallel to bedding Laccolith – A mushroom-shaped concordant
A sill in the Salt River Canyon, AZ Sill: Sediments above and below sill are baked. Lava Flow, just baked below.
Plate tectonics and igneous activity Igneous activity along plate margins Mid-Ocean Ridges – Basaltic Pillow Lavas Great volumes of volcanic rock produced along oceanic ridges – New ocean floor – Mechanism of spreading or “rifting” » Lithosphere pulls apart and thins » Less pressure results in partial melting in mantle http://www.archipelago.nu/SKARGARD/ENGELSKA/ICELAND/surtsey.htm
Plate tectonics and igneous activity Igneous activity along Subduction zones – Descending plate partially melts – Magma slowly moves upward – Rising magma can form either » A Volcanic Island Arc if ocean-ocean plate collision (Aleutians, Japan, etc.) » A Continental Volcanic Arc if ocean- continent plate collision (Sierra Nevada)
Plate tectonics and magmatism Intraplate volcanism Associated with plumes of heat in mantle Form localized volcanic regions in the overriding plate called a hot spot – Produces basaltic magma sources in oceanic crust (Hawaii) – Produces granitic magma sources in continental crust (Yellowstone Park) – These differences are predicted by a Crust-Melting model of Granite generation