Earth Science Today
Russ Colson
Minnesota State University Moorhead

Historical Geology:

Topic 1: Geological time and life of the past, reading stories in rocks and fossils.

Often classification of rocks substitutes for geology.  One student told me once "I always thought geology was just about rocks."  But, in fact, geology is about telling stories, not knowing names of rocks.  However, understanding the different types of rocks and how they form is crucial in knowing how to read their stories.

• Igneous Rocks:  We learned about igneous rocks while addressing a previous topic.  These are rocks that have formed by cooling from a molten state.  Therefore, these rocks will tell a story about melting and cooling.  The fact that it is an igneous rock reveals that it got hot enough to melt.  The size of crystals in it tell us how quickly it cooled.  Fast cooling corresponds to cooling at or near the Earth's surface.  Slow cooling corresponds to cooling deeper in the Earth's crust.  Suppose we find a coarse-grained igneous rock at the surface of the Earth (the entire Sierra Nevada mountain range consists mostly of coarse-grained igneous rock).  Think of the stories it tells!  How did it come to be at the Earth's surface since we can see it must have cooled deeper in the Earth's crust?  Suppose you find an igneous rock that is made mostly of crystal too small to see (it looks massive and uniform), but there are a few big, sparkly crystals in it.  What story does it tell?  (answers, est2a1.html).  A table of some of the most common igneous rocks is shown below.

             Less SiO2               gabbro                   basalt
 
 

• Metamorphic Rocks:  Metamorphic rocks form as a response to changes in temperature, pressure, or other parameters of the rock system.  In general, high temperature and pressure reflect deep burial, such as in the roots of a growing mountain range, or exposure to an extra-hot place such as near a magma intrusion.  Both the phase of the rock and the texture change in response to changes in the conditions of the rock.  Phase refers to the type of minerals found in the rock (think about the phase of water, which changes under different conditions of temperature or pressure to be either solid, liquid, or gas).  The atoms of the rock reorganize into minerals which are chemically stable at the new conditions of temperature, pressure, and other parameters.  Texture refers to the size and orientation of the minerals in the rock.  In general, minerals grow larger with increasing temperature, pressure, and duration of metamorphism.  In addition, flat, plate-like minerals become aligned like sheets of paper stacked all together.  This orientation is called foliation.  A table of some common metamorphic rocks is shown  below.

Thought Puzzle:  You find a gneiss at the top of a large mountain.  What story does it tell us of the past? (answer, est2a3.html)

• Sedimentary Rocks:  Sedimentary rocks are ones that have formed by the weathering of preexisting rocks (chemical and mechanical breaking up of sedimentary, igneous, or metamorphic rocks) to form sediments that are transported (moved by wind, rivers, ice, or other means), and deposited (when the wind or river, for example, becomes too slow to carry the sediment further and it settles out), and then lithified (minerals, often deposited out of pore water, cement the sediment grains together to make a new rock).  Sedimentary rocks tell us about past environments at Earth's surface.  Because of this, they are the primary story-tellers of past climate, life, and major events at Earth's surface.  Each type of environment has particular processes that occur in it that cause a particular type of sediment to be deposited there.  This can be illustrated by a simple experiment.  Think about putting a pile of mud, sand, and gravel in a pile on one side of a plastic basin, and putting water at the other side.  Gently slosh the water against the "shoreline" of the mud, sand and gravel (making waves).  Do this for a while.  What happens? (answers, est2a4.html).  This simple experiment illustrates the key way that many sedimentary rocks tell stories:  The size of particles reveals the energy present in the environment of deposition.  Think of this other example:  Fast mountain rivers have gravel in them because smaller sand and mud is washed away.  Slow meandering rivers often have mud in them because the slow water allows the mud (clay and silt) to settle out.  Water, ice, and wind each carry sediment in a different way, with water and wind sorting the sediment out by size according to the speed (energy) of the water or wind, whereas ice carries sediment indescriminately, resulting in an unsorted mixture of gravel, sand, and mud which we call glacial till.  Finding an unsorted sediment or rock can therefore be a clue to a long-vanished glacier.  Finding shale (made of tiny clay and silt particles, try scraping them off with a knife to see the powder that forms) is a clue to a body of very quiet water with little movement, like a deep lake or an ocean distant from the waves.

       We can also interpret sedimentary rocks by considering the concept The Present is the Key to the Past.  For example, if you find a sandstone, where might that rock have formed?  Where do you find sand today? (beaches, rivers, dune deserts for example).  If you find coal, made of abundant plant material that has accumulated, consider what kind of environment we find plant material accumulating in today  (swamps and bogs for example).  By comparing the rocks we see with modern environments, we can figure out what type of environment that rock formed in.  By considering details of the rock, such as the texture (how the grains fit together and their size) and structures in the rock (cross-bedding of layers, ripple marks, etc) and comparing these to the textures and structures in modern sediments, we can identify more details about the environment that a rock formed in.
     Some sedimentary rocks form not by deposition of particles carried by water, wind, ice etc. (called detrital rocks) but rather by crystallization from water (chemical sedimentary rocks).  For example, table salt is mined from rock halite, a rock deposited as water becomes very salty (such as when sea water evaporates).  Such a rock tells the story of a body of sea water that has restricted access to fresh sea water (which would dilute the water and prevent salt from crystallizing) and possibly arid or hot conditions.

Detrital rocks:
particle size too small to see       sand sized particles      pebbles sized particles
        Shale                                      sandstone                     conglomerate

Chemical rocks:
        rock halite (NaCl), rock gypsum, limestone

Biochemical rocks (crystallization or formation aided by living things):
        limestone (mostly CaCO3), coal (mostly C).
 

• Fossils:  Fossils are not found willy-nilly all mixed together in peculiar rocks as though simply dumped there by aliens from space.  Rather, fossils are found together with other fossils and rocks in groupings that make sense in terms of the living communities we see in modern environments.  For example, some types of creatures are suspension feeders, meaning they eat by extracting particles of food suspended in the water.  Their "filtering" apparatus would be quickly overwhelmed and plugged in muddy water, thus, in the modern world, we find these creatures in clear, clean water.  Likewise, in ancient rocks, we see communities of creatures and rock types with the suspension feeder fossils that suggest clear, clean water.

    Fossils also give us clues to past climate.  Polar bear fossils (or their equivalent from the geological past) tell us about cold climates.  Coral often tell us about tropical climates (this is because CaCO3 which coral make their homes out of is more easily dissoved in cold water, thus making cold water less hospitable to them).  Some species of coral (made of CaCO3) have a higher concentration of Sr if the sea temperature was higher. 
    There are many things which the features of a fossil can reveal about the environment the creature lived in.  For example, the thickness of the shell of a bivalve (sea shell) can reveal wether the environment was quiet or turbulent (a thick shell would be needed to survive a turbulent environment).
    One of the most significant questions about an environment is whether it was marine (ocean) or terrestrial (rivers, lakes, etc).   The type of creature can reveal whether water was fresh or marine (think about walleye vs sharks).  Fossils can reveal whether there was nearby land.  For example, petrified wood indicates the presence of trees which don't grow out in the middle of the ocean.  Also, all dinosaurs were terrestrial and are found mostly in terrestrial rocks although sometimes in marine rocks where they got washed out to sea.
    The number of fossils (abundance) and the variety of creatures (diversity) give clues to the environment as well.  Abundance indicates an abundance of food in the environment, diversity indicates that the environment was stable (because large variations in temperature, salinity, oxygen etc create conditions that only a few types of creatures can survive in).  It is interesting that there is a similar relationship present in human societies.  Where resources are abundant, there are more people (in general), where conditions are stable, people become increasingly specialized in their work.

Rock Cycle puzzle:  knowing the processes by which each type of rock forms, and knowing that one rock is changed into another rock through geologic time, you can complete the following schematic diagram of the Rock Cycle.  In each circle, I have written the name of a type of rock.  On each arrow write a process by which one rock can be changed into another rock.  (answer, est2a6.html)

Lab Activity:  Rocks and stories they tell (in MSWORD)
Lab Activity: Invertebrate fossils. (in MSWORD)

A dinosaur identification activity is available in MSword.

Also, a 12-15 hour unit sediment stories activity for kids age 9-14 is available in MSword. This unit teaches kids about how to read the stories written in simple sediments and sedimentary rocks.

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