Geology 104 - Spring 2007

Lecture #2 - Earth Materials

Earth Systems

• Picture of landscape - as a whole, then parts ("science"), then put back together ("story").
• Earth = lithosphere + hydrosphere + atmosphere + biosphere
• System = flow of energy and materials from one sphere to another
  Ex. CO2 in atmos. > raindrop to ocean in hydro. > clam shell in bio. > fossil in lith.
• We will consider processes involving the hydrosphere - rivers, glaciers, groundwater - later. Today we look at the lithosphere.
  

Minerals

Def: naturally occurring, inorganic solid with a crystalline structure and a definite chemical composition

• Structure gives it characteristic shape that we see.
• Composition gives it other characteristics, as you will see in lab this week.
• Ex. Feldspar & quartz - homogeneous substance
  

Rocks

Def: a consolidated aggregate of minerals

• There are 100's of different rock names. We'll learn a few of the most common.
• Rocks are defined by composition and texture.
  • Composition = what minerals are present
  • Texture = size, shape, and arrangement of mineral grains
• Ex. Granite, sandstone, gneiss - made up of minerals (= building blocks of rocks)
  Granite & gneiss have >1 mineral, Sandstone has 1 mineral
  

Igneous Rocks

Formation

• "igneous" = fire-formed - cooled from liquid rock
• Liquid rock forms deep beneath the surface (100-200 km down)
• More buoyant, so it rises: called magma while underground, lava at surface

Classification

• When liquid rock cools, mineral crystals form (composition), in an interlocking pattern (texture).
• Rock name depends on size of crystals and which minerals form.
• 2 main compositions:
  • Silicic minerals contain lots of silicon (e.g., quartz) > light-colored
  • Mafic contain lots of magnesium & iron > dark-colored

Grain size	Composition
	Silicic	Mafic
Fine	rhyolite	basalt
Coarse	granite	gabbro

Stories from rocks

• coarse grains => long time to cool => underground (intrusive)
• fine grains => fast cooling => at surface (extrusive)
  

More details about igneous rocks in text, and in lab manual.

Sedimentary Rocks

Formation

• Accumulate in layers at or near the surface
• Clastic rocks start as loose grains (= sediment) at the bottom of a lake or ocean
  • Grains are compacted (squeezed together by weight of overlying layers), and
  • cemented (held together by minerals that form in between grains) & turned into stone.
• Others form by precipitation from solution, or by accumulation of organic material

Classification

• Clastic rocks are named according to grain size:
  • Sandstone - sand-sized grains
  • Shale - very small grains
• Non-clastic sedimentary rocks are named according to composition:n size:
  • Limestone - contains the mineral calcite - often contains fossils made of calcite
  • Evaporites - form by precipitation from solution

Stories from rocks

• Sedimentary rocks tell us about conditions at the surface at the time they formed.
  • Sandstone => beach or desert
  • Lmestone with fossils => warm shallow ocean
• More details about sedimentary rocks in text and in lab manual.

Metamorphic rocks

Formation

• Form by changes in the solid state, due to heat & pressure, but without melting.
• During metamorphism, new minerals may form, old ones may rearrange themselves.
• Many met. rx have a texture called foliation = layering (because flat mineral grains stack up, or because light & dark minerals form alternating bands)
• Foliation develops because of pressure.
• If metamorphism occurs because of heat only, no pressure, then no foliation develops.
  

Classification

• Foliated met. rx are named according to grain size:
  • Slate - fine
  • Schist - coarse
  • Gneiss - coarse with bands
• Non-foliated rocks are named according to composition:
  • Marble - contains calcite

Stories from rocks

• Foliation => heat + pressure => deep burial & forces associated with mountain-building
• No foliation => heat alone, say around a body of magma
• More details about metamorphic rocks in text and in lab manual.
  

The Rock Cycle

All classification schemes are arbitrary divisions of a continuum.

• Ex. Heat a metamorphic rock enough and it will melt. The result is an igneous rock, not metamorphic
• Bury sediments deeply enough, and they don't just stick together. They begin to change, and the result is a metamorphic rock, not sedimentary.

Geologists express this as the "Rock Cycle" - any type of rock (ig, sed, met) can be change into any type of rock (ig, sed, met) if the appropriate processes act upon it.

Examples: Sandstone (sed) > heat & pressure > quartzite (met)
Granite (ig) > erosion, accumulation, compaction, cementation > sandstone (sed)
Gneiss (met) > melting > Granite (ig)