The three main types of rock (igneous, metamorphic, sedimentary) all represent genetic (origin-based) classes.
Can think of these as three ways of forming rock, rather than three types of rock:

• Igneous: any rock cooled from a molten state
• Metamorphic: any rock transformed due to intense heat and/or pressure
• Sedimentary: any rock formed by bits of previously existing rock and/or organic material consolidated into a solid mass

Each type of rock gives some evidence of its history.
Thus, Every Rock is a Record of History.

Within igneous rocks:

• Can tell if formed at depth (plutonic) due to intrusion between previously existing rocks or at surface (volcanic) due to extrusion onto surface:
  • Intrusive rocks cool slowly and form larger visible crystals (phaneritic texture).
  • Extrusive rocks cool quickly, and form microscopic crystals (aphanitic texture).
  • So gross crystal size can be used to as a proxy to infer environment of cooling
• Chemical composition can indicate the magma/lava source:
  • Primarily from mantle rock (very high in Mg and Fe, ultramafic)
  • Oceanic crust, or otherwise with very little mixing with crustal rock (high in Mg and Fe, basaltic or mafic)
  • Considerable mixture of mantle rock with continental rock, or formed by melted continental material (low in Mg and Fe, high in Si and Al, granitic or felsic)
  • So gradiations in the degree of concentration of Mg, Fe, Si, and Al indicate aspects of initial magma body and degree of mixing on way up
• Also, igneous rocks can be most easily dated by radiometric means; represent our best clue to numerical age of rock units.

Within metamorphic rocks:

• Determine whether due to contact metamorphism (e.g., baked region around igneous intrusions) or regional metamorphism. The latter may indicate large-scale tectonic events in the past, and will often be foliated (i.e., show recrystallization perpendicular to forces involved).
• Also, metamorphic grade (observed by particular combination of metamorphic minerals present) indicate the pressure/temperature conditions under which they formed: the best clue to previous depth of that batch of rocks. (For example, can use to see how deep the ancient Appalachians were).
• Classification combines aspects of type and grade of metamorphism involved

Within sedimentary rocks there is considerable information concerning the surficial environments (including fossils), so we will spend a lot of time in class and lab looking at them.

• Formed by accumulation and lithification of bits of previously existing rock and/or organic matter
  • Bits of previous rock and/or organic matter are called sediment
  • Because sedimentary rocks form by deposition, they naturally create horizontal beds called strata (singular stratum)
  • Major divisions of sedimentary rock reflect the type of sediment:
    • Biogenic sedimentary rocks: sediment made of solid bits of organic material (whole or broken up) that gets deposited
      • Coal: buried remains of plant life
      • Many types of limestone are made primarily of the shells of once-living things
        • Made of calcite, aragonite, or dolomite
        • Typically form in salty water (and thus marine or brackish lakes)
        • Chalk is a type of limestone formed by microscopic algal plates, common in the warm shallow seas of the later part of the Age of Dinosaurs
        • Most of the carbonate mud in the shallow marine environment (and the white beaches of places like the Bahamas and the Caribbean) is the broken down remains of the internal skeleton of marine algae
    • Chemical sedimentary rocks: sediment is in the form of dissolved bits (ions) that precipitate out of water:
      • Some limestones form from crystals of calcium carbonate that precipitate directly out of warm salty water, rather than being biogenic
      • Evaporites (rock salt, gypsum, and related rocks): form when salty water evaporates leaving a residue of salt
    • Detrital (also called "siliciclastic" and "clastic") sedimentary rocks: sediment is grains of various sizes weathered from previously existing rock, cemented together by minerals in the ground water
      • Very commonly produced in terrestrial and near-shore environments
      • By far the most common in which dinosaur fossils are found
      • Basic detrital sedimentary cycle: Uplift to erosion to transport to deposition to lithification (typically cementation):
        • A region experiences uplift, pushing up once-buried rocks and exposing them to the surface elements
        • This source rock (or more likely, rocks) experiences erosion: weathered away and broken up by wind, rain, water, plant roots, gravity, etc.
        • The broken fragments (sediment) are transported by water, wind, glacial ice, etc.
        • As sediment is transported from the host rock, it undergoes changes
          • As distance increases, roundness increases (edges get worn way)
          • As distance increases, sorting increases (different sized particles get winnowed out)
          • As distance increased, maturity increases (softer and more easily dissolvable minerals breakdown, leaving only clay and silica (aka sand and silt) in the end)
        • Sediment is deposited at some location (stream bed, banks of a river, desert, delta, lake bed, ocean, etc.)
          • These locations (deserts, flood plains, rivers, lakes, swamps, coastlines, continental shelves, etc.) are called depositional environments
          • The particular environment of deposition will leave different types of sedimentary structures: see below
        • Sediment is lithified (turned to rock): sometimes simply by compression, but more often by cementation:
          • Ground water percolates between the grains of sediment
          • Dissolved minerals in the ground water precipitate out, glueing (cementing) the grains together
      • The major types of detrital rocks are based on their sediment size, shape, and mixture:
        • Breccia: big angular chunks mixed in with smaller sediment; deposited very close to the source rock (and thus not rounded or sorted)
        • Conglomerate: large rounded chunks surrounded by smaller sediment; deposited further from source than breccia, commonly form in channels of rivers
        • Sandstone: formed by relatively well-sorted, well-rounded particles; deposited in many environments (deserts, beaches, river beds, nearshore marine, etc.)
        • Various sorts of mudstones: very well sorted with very small particles; deposited in quiet water (lakes, floodplains, off shore, lagoons, etc.)

The Rock Cycle: any rock can be transformed to any other major class of rock, because rocks are classified by the process in which they are formed. So if you melt an igneous, metamorphic, or sedimentary rock, and it cools down, you form a new igneous rock; if you recrystallize an ingneous, metamorphic, or sedimentary rock, you form a new metamorphic rock; and if you erode an igneous, metamorphic, or sedimentary rock and deposit the sediment from it, you form a new sedimentary rock.

Because sedimentary rocks form where animals and plants lived and died, these are the rocks in which fossils are common. One of the main categories of information sedimentary rock contain is the paleoenvironment (the conditions that existed when that rock was formed). The different environments of deposition represent different paleoenvironments. Some of the clues to discover paleoenvironments:

• The roundness, sorting, and maturity of sediment in a detrital sedimentary rock indicates the relative distance to the source rock (i.e., breccias form right near their source, mudstones at a great distance)
• The energy of the environment (how fast the water or wind was moving) is reflected in different sized particles of sediment (and, since fossils are buried by the sediment, different types of fossils):
  • Quiet water (lagoons, lakes, deep ocean, etc): very fine grained sediments (mudstones, fine-grained limestone, etc.), preserve small details, but unlikely to contain fossils of large animals (which would not be buried before decay sets in)
  • Faster moving water, wind, etc.: deposit large amounts of sediment (esp. sand) quickly, more likely to bury large objects (such as large dinosaur bodies)
• Environment of deposition often indicated by sedimentary structures: traces left in the sediment by various processes before lithification. Some common sedimentary structures include:
  • Mud cracks: indicate mud that was wet, then dried, then buried by later sediment
  • Ripple marks: indicate moving wind or water (current in stream; wave action along shore; etc.)
    • Symmetrical ripples (aka oscillation ripples) represent water moving back-and-forth, such as at a shoreline
    • Asymmetrical ripples (aka current ripples) represents wind or water moving in a single direction
  • Crossbeds, aka cross- stratification, are basically the cross-sectional representation of ripple marks, although the tops of the ripples are often lost (truncated) by erosion during the next ripple. Crossbeds are formed by currents (wind or water) pushing sand off the back of a ripple and onto the front (indicate streams, small dunes, etc.)
  • Trough crossbeds, cross-section of big sand dunes where shifting winds produced complex movement of sand (deserts, beaches)
  • Raindrop marks: indicates a wet surface (but exposed to air) that was rained on, then buried
  • Coal beds: indicates abundant plant life that was buried faster than it could decay
• Putting the sedimentary structure and rock type (lithology) evidence together allows you to reconstruct the paleoenvironment. For example, this set of dinosaur tracks are found associated with the impressions of halite (rock salt), indicating that the dinosaur was walking in an arid environment.

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Last modified: 17 January 2024