Minnesota's Rocks and Waters

Principles of Earth History

Much of the remainder of the course will examine Minnesota's geologic history. It is therefore important for us to learn how we can use geologic principles to decipher that history. First, however, we must understand the nature of geologic time.

Relative time

• Telling relative time simply involves placing events in a sequence relative to one another. This sort of time says nothing at all about age in years before the present.
• The geologic calendar that we use to place events in a sequence of relative time is the geologic column.
  • The largest scale division of time is into two eons.
    • Precambrian eon (older)
    • Phanerozoic eon (younger)
  • Eons are divided into eras.
    • Archean (older Precambrian <2.5 Ga)
    • Proterozoic (younger Precambrian 2.5 Ga - 543 Ma)
    • Paleozoic (oldest Phanerozoic 543 Ma - 248 Ma)
    • Mesozoic (middle Phanerozoic 248 Ma- 65 Ma)
    • Cenozoic (younest Phanerozoic 65 Ma - present)
  • Eras are divided into periods (see column for divisions)
  • Periods are divided into epochs (see column for epochs of the Tertiary Period of the Cenozoic Era (Paleocene, Eocene, etc.)
  • A more complete geologic column and time scale can be found here

Absolute time

• Absolute time is the age in years before the present of a rock or a particular geologic event. Ma refers to millions of years, and ga refers to billions of years.
• Absolute ages are generally assigned to events in the geologic column by radiometric dating of rocks formed during the events.
  • Radiometric dating depends upon the constant rate of decay of radioactive isotopes to produce stable daughter products. During the decay process, the isotope gives off energy in the form of subatomic particles and radiation generating heat.
  • Basically, the geologist measures the ratio of parent isotope to daughter isotope in a rock, and knowing the rate of decay to form the daughter product, is able to calculate an age in years before the present.
  • For this procedure to work, the rock must be a closed system, that is, no material can have been added or lost following the formation of the rock.
  • Dates on igneous rocks give the age of crystallization
  • Dates on metamorphic rocks give the age of the last metamorphic event.
  • Dates on sedimentary rocks are meaningless if they come from minerals that predated the formation of the rock (i.e., minerals in a siliciclastic or terrigenous rock).
    • sedimentary rocks are usually dated by knowing their relationships to igneous rocks of known age, using superposition and cross-cutting relationships described below.
    • once the age of a sedimentary rock is known, the age of characteristic fossil assemblages in that part of the sedimentary sequence is known. Henceforth, other sedimentary rocks of the same age and with the same fossil assemblage can be correlated to one another using comparison of fossils.

Telling relative time

• Principle of Superposition - In a layered or stratified sequence of rocks, such as sedimentary rocks or lava flows or ash deposits, the rock at the bottom of the pile is the oldest and rocks successively higher in the pile are successively younger.
• Principle of Original horizontality
  • Layered or stratified rocks are generally deposited horizontally.
  • Departure from horizontality generally indicates some sort of deformation, such as tilting or folding.
  • Deformation usually occurs during mountain building, but gentle regional tilting of rocks can occur during regional uplift of otherwise stable parts of the continent (the craton).
• Cross-cutting relationships
  • When one body of rock, such as a dike, cuts across another rock, such as a sequence of layered sedimentary rocks, the rocks being cut are older and the rock doing the cutting is younger. (Click here for another example of cross-cutting relationships.)
  • This also applies to situations where a fault cut across rocks. The rocks being faulted are older than the fault.
• Principle of faunal succession
  • Organisms change or evolve through time and species do not repeat themselves during this evolution.
  • Certain fossils may be good indicators of certain intervals of time. The best index fossils
    • are short-lived as a species
    • are rapidly dispersed over wide-geographic areas
    • are easily fossilized (generally possessing hard parts)
    • are easily recognized
  • Index fossils enable geologists to correlate sedimentary rocks that may be widely separated from one another geographically. Correlation is the establishing of time-equivalence between two rocks.
• Unconformities and their significance
  • an unconformity is a surface in the rock record along which rocks are missing and time is therefore not recorded.
  • the most obvious type of unconformity is an angular unconformity, where rocks beneath the surface are more strongly deformed and those above the surface are less strongly deformed.
  • unconformities often represent an interval of mountain building, where rocks are deformed, intruded by plutons and uplifted. Erosion removes rocks from the uplifted region, and subsequent subsidence (sinking) of the region is followed by deposition of sediment atop the unconformable surface.
• Principle of Uniformitarianism or Actualism
  • This principle underlies all of the work we do in geology. This is the way it works. Natural laws are unchanging. Therefore, the way natural laws govern geologic processes that are operating today is the same way that natural laws governed geologic processes that operated in the past. We can therefore use our understanding of present-day geologic processes to interpret past geologic events.
  • Sometimes this principle is paraphrased as "the present is the key to the past", but be careful.
  • Note, however, that the principle does not imply that past rates of process, or conditions under which the processes work, were the same as today. Nor does it imply that Earth's processes work at constant or uniform rates. Consequently, to avoid confusion, we often refer to the principle of uniformitarianism as "actualism."

The north shore of Lake Superior is a good place to apply these principles in working out a relative sequence of events. Study of this cross section of the North Shore will also introduce us to the Precambrian geology of northern Minnesota. You should try to work out the sequence of events in this cross section for yourself. This will serve as a test of your skill in telling relative time. A solution to the problem of the sequence of events will be posted here after the warmup is completed so you can check yourself.

Calibrating the Geologic Column

Let's assume that we have worked out a relative sequence of events in a geologic cross section, and now we want to know the absolute age of a particular event or series of events. This will require us to calibrate the geologic record.

Or, let's say that we have studied all the rocks all around the world, and have stacked them in the order in which they were formed. And let's also say that we have used fossils in the sedimentary rocks to identify the era and period in which all the rocks formed. Furthermore, let's say that we have obtained radiometric dates on the igneous and metamorphic rocks in this world-wide stack. Now, the problem is to use this information to assign absolute ages to various events represented by rocks in the stack. In other words, we want to calibrate the record of rocks, the geologic column.

We use our principles of superposition and cross-cutting relationships, and the concept of an unconformity, to do so. Click here to see two examples of how this is done.