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.
Munsell Rock Color Charts To note colors, I use one of these, but they are expensive. In your notes, just use your own judgment. Also note whether the sample is wet or dry
Graded Beds Flysch: old [Alpine] name for thick sequences of turbidites “laid down in a deep trench marking an active plate boundary (like a subduction zone).” PM p17
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
Halokinesis: Differential loading “This may occur when the downward force on the salt layer varies laterally.” PM p.26
Igneous Rocks 3 ways of making primary magma, all basaltic
Plutonic and Volcanic Igneous Structures Laccolith like a sill, but bows up overlying strata to make a dome
Bowen’s Reaction Series Molten- VERY Hot No solids Molten- Not so hot 100% Solid First mineral to crystallize out
Fine crystals Need a microscope Course crystals Easily seen Low silica, HOT, fluidHigh silica, warm, viscousIntermediate
A size comparison of the three types of volcanoes
A Pahoehoe lava flow
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 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
A nueé ardente on Mt. St. Helens
A volcanic bomb Bomb is approximately 10 cm long Some the size of a Bus
Tephra forms Tuff Source: Gerald & Buff Corsi/Visuals Unlimited, Inc. St. Lucia Anecdote
Tephra layers fine away from source
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.
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
Lava Plateau Formation
Formation of a volcanic neck
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.
Why No C-C collisions
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
Basaltic Pillow Lavas
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)
The Cascades, Washington State
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
Notice the potential for a huge lahar during the eruption of this big composite volcano (stratovolcano), Mt. Fugiyama.