Earth Science Today
Russ Colson
Minnesota State University Moorhead

Historical Geology:

Topic 2: Stratigraphy, plate tectonics, and geological change

Particular rocks and fossils tell us what the Earth was like at one time in the past, a snapshot of Earth at that time.  But geologists are not only interested in what things were like at one time, they are interested in how things changed through time.  Layers of rock tell stories of change, converting individual snapshots into a motion picture of the Earth as it changes through time.  For example, a crinoid-rich limestone may tell us about an ancient ocean, but the sequence of layers of rock types reveal what came before and after that ocean, whether it advanced or retreated for example.

Stratigraphy comprises many of the key tools for reading the stories of change.  Plate tectonics comprises the grand theory explaining what has caused much of that change.
 

• Stratigraphy:   Consider the following marine environments:  sand at and near the beach, clay and silt (mud) farther offshore where the water is deeper and the bottom movement of water more gentle, and limestone still farther offshore where mud and sand don't reach and CaCO3-forming creatures live.  Although this is not a universal near-shore sequence, it has been very common in Earth's history and we will use this sequence as an example in learning how to read rock layers.

Imagine an ocean coming into North Dakota where it had not been before.  Let’s think of a particular spot, say Bismarck, the capital of North Dakota near the center of the state.  The sea-level rises and the ocean advances over the area where Bismarck is now.  So, let's reason together.  Which has to get to Bismarck first:  the offshore region, with its fine muddy bottom, or the beach with its sand?  Which type of rock will then be on the bottom; shale (the muddy rock) or sandstone (the sandy one)?
      Unless the ocean advances very rapidly (residing in one place for only decades perhaps), such that the working of the waves does not have time to develop a sandy beach, the sandy sediments that are found at the beach have to be deposited before (and therefore underneath) the muddy deposits.  This gives us a means of figuring out past sequences of events, whether oceans were advancing or retreating for example.  This illustrates the key concept of stratigraphy, the older rock representing the older environment will be on the bottom originally (principle of original superposition).  The more recent environment will be represented by the rock originally on top.  Thus, we can figure out sequences of events and tell stories of the geological past.  (Clay stratigraphy activity, with illustration and puzzles, est2b1.html)  (Stratigraphy thought puzzle, est2b2.html).

You can tell a story of the Red River Valley:  Most of us have heard that there was once a huge lake in the Red River Valley.  But how do we know that, and when was it here?  Was the lake here before or after the glaciers?  Was it here before or after the dinosaurs?

Here is the stratigraphy beneath our feet in the Red River Valley.  From it, you can read the story of the Red River Valley over the geologically recent past (the older sediments are listed at the bottom).  Discuss it with someone else and try to read the story before checking the answer key.
   mixture of silts, sands, and plant debris
   well sorted clay-rich mud
   mixture of silts, sands, and woody tree fossils
   well sorted clay-rich mud
   poorly sorted mixture of mud, sand, and gravel
   well sorted clay-rich mud
   poorly sorted mixture of mud, sand, and gravel
(Key to story, est2b3.html)

One of the truly astonishing and wonderful realities of Earth is that there are many, many stories written in it.  In fact, there are so many stories as to dwarf our ability to comprehend them all.  The sheer number of events recorded in rocks reveals stories that transpire through deep time, a time that has grown from the confines of our experience to be as immense and unfathomable as the rest of creation. There are stories of mountains where there are now seas, seas where there are now mountains, tropical jungle where there were once glaciers and glaciers where there were once tropical jungles.  For the convenience of our finite minds, the immensity of geological time has been divided into periods.

   ERA                             PERIOD
                                     Quarternary (Q)
Cenozoic                       Tertiary   (T)
                                     Cretaceous (K)
                                      Jurassic (J)
Mesozoic                       Triassic (Tr)
                                      Permian (P)
                                      Pennsylvanian (P)
                                      Mississippian (M)
                                      Devonian (D)
                                      Silurian (S)
                                      Ordovician (O)
Paleozoic                       Cambrian (C)
Precambrian (pC)

 North Dakota has one of the simplest-to-read stories, because the rocks here are flat-lying and have not been greatly affected by mountain building events that have affected other parts of the country.  Because of oil interests in the western part of the state, subsurface layers of rock have been extensively explored by drilling.  Consider the stratigraphic column of Western North Dakota (~ 3 minutes to load at 14.4K baud) and try to spot the rocks that indicate the coming of seas, or their retreat.  Consider how many different events have happened just here in North Dakota where things have been geologically quiet.

Stratigraphy deals with stories and sequences of events.  It does not deal directly with how long these stories took to transpire.  However, when these stories began to be read in the early 1800's, it was clear to most people who read them that they must have taken an immensely long period of time.  However, it was not until the discovery of radioactivity, and its use as geological clocks, that confident ages of rocks and ages of events could be determined.   (Activity and puzzle to illustrate how radioactive dating works, est2b5.html).
 
 

• Plate Tectonics:  People have known for a long time that the outlines of the continents can be roughly fit together like a jig-saw puzzle.  They have known that earthquakes and volcanoes do not occur randomly just anywhere, nor do they occur in circular clusters, rather, they occur in linear or curved arrays (activity to plot plate boundaries, est2b6.html).  Mountain ranges are also linear or curved, not circular clusters.  As we have learned more about the ocean, we have found deep ocean trenches paralleling volcanic and mountain chains, and long underwater mountain ranges paralleling the jig-saw outline of opposing coastlines of continents.  Until the 1900's there was no theory that could unify all these observations under the umbrella of a single concept.  Plate tectonics, accepted by most scientists in the late 1960's, provided that unifying theory.  Plate tectonics has provided the theoretical underpinnings for our understanding not only of the modern geography of volcanoes, mountains, earthquakes, and ocean trenches, but it provides an explanation for why the geological past is so filled with change.  It explains how Antarctica could have once been tropical and the Sahara once covered by glaciers.  It explains how North Dakota was once home to coral and crinoid reefs.  It explains how and why mountains formed where they did in the past, sometimes with repeated episodes of mountain formation over long periods of time.   It explains many formerly inexplicable distribution of ancient creatures (such as how land creatures had distributional ranges that overlap what are now separate continents).

What are plates?
What is the source of energy for plate movement?
How does plate tectonics make mountains or basins?
What kind of plate boundaries are possible and what are they like?
How can we explain ancient mountains, and mountains in the middle of continents (Urals)?
 

Alternate Stratigraphy and Plate Tectonics Lab Activity, considering predictions as a way to test theories.
 

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