Lecture 7: The Rock Cycle

Rocks

  • Naturally occurring
  • Coherent (holds together), but not always as you see beach sand is not held together
  • If it is not lithified, then it is not a rock
  • Composed of multiple mineral grains all the same material or many different types

Igneous - formed from cooling of molten (melted) rock

Sedimentary - formed at the surface of the earth by deposition and cementation of particles of rock transported by wind, water, or ice

  • It could also be (bio) chemically produced debris

Metamorphic - formed when sedimentary, igneous or metamorphic rocks are heated and/or squeezed

  • It does not melt, but becomes harder

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Igneous Rocks

  • If the rocks cool below the surface: plutonic or intrusive
  • If the rocks cool above the surface: volcanic or extrusive
  • Big body of rock can take a long time to cool, and when they come above the surface, the cool really fast
  • Speed of cooling affects the texture of the rock
  • Volcanic rock cools very quickly, therefore the crystals are small
  • Intrusive rocks cool very slowly, therefore the crystals are large

Lava flow - flow of material outside a volcano

Ash - particles thrown into the air

Dikes - rocks that vertically cut across other rocks

Sill - rocks that cut across parallel to layering

Batholith - large underground magma chamber that cools

Hawaiian Basalt Flow - extrusive volcanic rock, lava flow, cool quickly, contains heavy materials containing iron and magnesium

Granite - light colored, silicic minerals, contains quarts and really big crystals, intrusive rock which cools slowly

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Sedimentary Rocks

Clastic - sandstone, if smaller than sand, then it is called silt or clasts

  • If it larger than sand, it is called conglomerate
  • Example - ripples are formed on the sand by waves

Chemical - formed through chemical processes, naturally occurring salt underground which is mined

  • Example - Salt Lake City, water evaporated due to the wind, salt accumulates over time and formed halite

Biochemical - limestone contains little clasts or organisms in the rock, formed underwater and preserved and form sedimentary rocks

  • Example - coral reefs

Organic - Carbon-rich remains of once living organisms

  • Example - coal


Metamorphic Rocks

  • Form through the application of heat and pressure
  • Rocks in the Canadian shield mostly formed at the base or roots of mountain ranges
  • Formed a billion years ago, but has been eroded away

Migmatite - transitional rock between igneous and metamorphic rock, hard to categorize but is still called a metamorphic rock

  • Rocks increase in pressure as you go deeper into the ground


Thin Sections

  • Hand specimen of rock is sawed into a block
  • A block chip is then shaved and polished by grit (sandpaper)
  • Rock is thinned down to the point where it is translucent, light can pass through it
  • Process helps identify the characteristics of the mineral
  • To tell apart a sedimentary rock, look for rounded grains
  • Te tell apart an igneous rock, look for interlocked grains
  • To tell apart a metamorphic, look for aligned grains (face the same way)


Identification of the 3 Rock Types

Igneous - large crystals can be seen because interlocking took a long time, so crystals are large

Metamorphic - you will see a flow

Sedimentary - lots of clasts (grains) will be seen


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