Geology 209 Lectures 1-5

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Banded iron formations

Different layers composed of iron bearing minerals (wikipedia- alternating layers of iron oxides (usually magnetite or hematite) and iron poor chert). Reddish layers bear mostly silica. Most of the iron in banded iron formations comes from submarine volcanoes. found worldwide. Processes by which they’re formed appear to be restricted to early geologic time.

Formation processes

deposition basin must contain waters that are rich in iron. Waters must be anoxic (goes w/ iron rich), must not be euxinic (rich in hydrogen sulfide). Must be an oxidation mechanism active within depositional basin that steadily converts the reservoir of ferrous iron to ferric iron. Must be an ample source of reduced iron that can circulate freely into the basin.

Waters needing to be anoxic is why they don’t form today- oxygen richness/ deficiency in water depends on the atmosphere. Banded iron formations could occur 4 billion to 1.8 billion years ago, but then atmosphere became too oxygen rich.

Geology data collection methods - Field based observations -

field based information collection in person.

Remote sensing- field observations from satellite based instrumentations- ex; mapping more efficiently than in person on the ground, can get a sense of the grounds actual movements (magma slightly inflates the grounds surface, not visible to human eye but satellites can track the refilling of volcanoes this way)

Geologic map

• Shows geology of the area

• Different colors are different groups of rocks 

• From looking at the map and the legend we get a sense of the age of the rocks- usually ones at the base are oldest, ones at the top are youngest

• Geologic map; shows distribution (spatial) of rock units 

• Can be used by geologists 

• Ex; most of water used in Calgary comes from the bow (surface water), but in rural communities we often rely on groundwater (stored in sediments and rocks) Water bearing qualities of rock formations are shows  on the right side of the geologic map- can show where is a good place to drill for groundwater. 

• Aquifer; good water bearing rock, lets water flow

• Aquitard; dont let water flow. wont be replenished if extracted. Acts as a barrier to groundwater (sometimes people use these in foundations!)

Earthquakes

• P-wave (body wave), S wave (surface/ shear waves). P waves are faster, s waves come after.

• Speed of the waves is determined by the properties of the rock- less dense, slower, dense, faster. Body waves can travel through solid and fluids, Shear waves can only travel through solids

• Recording waves as they travel through different layers tells you how the different layers react, can show things about the formation- ex; where there are breaks.

• Earthquake waves= seismic waves. Deploy sensors which can measure them (from natural or artificial earthquakes if large enough to gather data), response/ interpretation shows layers and such.

• Breaks might be where petroleum is trapped.

• Hazard maps- show potential for large scale movement, dangers, from these breaks and layers and stuff

The crust

< 1% of earth by volume (less than)

2 types

• Continental- under continents. Thicker, average 25-30 km, but can be lower. Varies based on mountains (himalayas- 80 km)

• Oceanic- under oceans. Thinner, average 5-7 km, fairly uniform in thickness

Oceans make up 70% of the crust,, have lower elevation and are made of basalt. continents 30%, higher elevation, usually composed of granite (lighter coloured minerals that are less dense). Basalt more dense than granite- density measured relative to that of water using specific gravity.

• Continents are formed at higher elevations because basalt/oceanic crust is floating on the earth's mantle, continental crust floats higher 

  • Form ocean basins

Hyposometric curve (the crust)

shows the proportion of the earths surface at a certain elevation.

• Distribution of land surface relative to sea level 

• Continents are above sea level 

• Continental shelf: part continent under sea level (submerged)

• Abyssal plain: deepest part of the ocean (4 km depth)

The mantle

Largest and thickest layer- 82% of earth by volume

Solid rock- generally, despite magma, but can behave like a fluid sometimes. The rock can move over long time scales.

Divisible into; Upper mantle (base to 410 km deep), composed of ultramafic (extremely rich in iron and magnesium, even more dense) rock peridodite (most common mantle rock), Transitional mantle (mineralogical changes), and Lower mantle (660-2,900 km)

Basalt is volcanic, magma is formed in the mantle, and as its moved up it carries fragments of the mantle (Xenolith)

Below certain depth, mantle convects similar to a pot of water heated from below. As it heats up, the bottom gets hot and moved up and cold comes down (convection cell). Behaves as a solid over short timescales, can slowly move/ flow over long time scales.

The core

~ 17% of earth by volume. Base of mantle to center (64,000 km)

• Outer core liquid iron and nickel- know because of seismic waves- P wave speed increases, and s waves don’t travel through.

• Inner core solid, primarily composed of iron and nickel (similar to iron meteorite)

• Outer core flows around the solid inner core generates earth's magnetic field (the motion creates the magnetic field)

  • Go deeper it's getting hotter, like pot of water, convects

  • Liquid is hotter, hot goes up and cold goes down 

• Earth's magnetic field shields us from high energy radiation from the sun

  • Also use for navigation 

  •  Interacts with solar waves and protects us 

  • Interaction between high energy particles and magnetic field creates aurora 

Earths layers

Layering based on mechanical behavior 

• Compositional layers: crust, mantle and core 

  • Same composition 

• Behavior of the layers is different how they respond to force 

• The crust and a bit of the upper mantle, behave in a rigid way (break rather than flow)

  • Crust and upper part of the mantle (does not take part in convection because its not hot)=earth's lithosphere (rigid upper layer)

  • Base of lithosphere to transitional crust is called Asthenosphere–convects 

  • Asthenosphere; layer below lithosphere, convects quite a lot, basically churns around

• Lower mantle is also rigid-pressure is too high–Mesosphere

• Outer core-liquid and inner core-solid 

  • But has the same composition

  • Outer core convects, inner is solid rock so doesn't connect because of high pressures  

• Lithosphere under continent is thicker (150 km or more)

  • Diamonds form because its thick and there's high pressure 

• Distinction between lithosphere and crust- crust is the oiter part, mostly composed of pelsic rock, upper mantle is much denser mantle rock)