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Fossils

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Geologist in ACTION
Fossil Leaf in Concrete
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Location: Near Emerson School, Berkeley, CA
About: Here you can see the imprint of a leaf in concrete that fell onto the concrete just a few moments after it was poured. The concrete dried many years ago and has been very hard since then. Can you make an educated guess about what time of year (season) this concrete was originally poured? The ruler tells you the size of the leaf.
Fossil Fern
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Copyright Bruce Molnia, Terra Photographics

About: A fossil fern in a natural sedimentary rock, with a coin to show you the size of the fossil. Millions of years ago, this fern fell in some mud. The mud was covered by more layers of mud causing it to eventually harden into a rock. The fern leaf decayed away, but this imprint of the leaf remained.
Fossil Leaves with Tree
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Location: Near Emerson School, Berkeley, CA
About: This picture was taken during autumn when leaves fall. The tree by the sidewalk is currently losing leaves that look a lot like the fossil imprint in the sidewalk below it (near the white ruler, which is about 10 cm long). They are the same shape and about the same size. It's a good guess that this tree was here when the sidewalk was paved. In other words, you can tell which is older: the sidewalk or the tree!
Fossil in the desert
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Spain Info Group
http://www.spaininfogroup.com/el_torcal.htm

Location: El Parque Natural del Torcal, Andalucia, Spain
About: A photo of a fossil sea creature (bottom of photo) in a dry desert environment. Unlike the schoolyard example of the tree and the leaf, real fossils sometimes come from environments very different than where you find them today. For example, this creature lived in a vast ocean, but do you see a vast ocean here today? What do you think happened? Not only does this place look pretty dry, but this photo was taken at an altitude of 1300 meters above sea-level (about 4000 feet). How did a sea creature get way up here? Geologists use fossils like this one to infer that two of the earth's plates crashed into one another and pushed up mountains.

Key Concepts:
  • Sedimentary rocks start out soft and squishy ("unconsolidated").
  • Fossils form when animals or plants die in the unconsolidated sediments and are covered by more layers.
  • Sediments can become hard over time if exposed to higher temperatures and pressures or certain minerals that cement the grains together.
  • Fossils are found in sedimentary rocks, but almost never in other rock types. (You need to start out with a soft material to make an imprint. Igneous rocks are very hot when they are soft/molten, so they burn up organic material. Metamorphic rocks are exposed to such intense heat and pressure that any fossils are destroyed).
Links for further Exploration:

Fossils, Rocks, and Time

Dinosaurs: Facts and Fiction

University of California Museum of Paleontology (Loads of superb teacher resources and student-friendly links)
Classroom Activities:

Mud Fossils (USGS Activity, Grades K-3)

University of California Museum of Paleontology (Loads of superb activities for K-12, with many activities emphasizing middle school)
Common Misconceptions: Misconception: Fossils are pieces of dead animals and plants.
Fact: Fossils are not actually pieces of dead animals and plants. They are only the impression or cast of the original living thing. The actual living parts decay away but their shape is permanently recorded in the rock as it hardens.

Misconception: Fossils of tropical plants cannot be found in deserts.
Fact: Fossils record ancient environments present at the time the rocks were deposited. The climate of a particular location can change because of a combination of 3 important factors: 1) Plate tectonics may cause land to move across much of the globe -- points that were once at the tropics may have moved to high latitude regions where the climate is dry. This motion can be tracked using magnetic signatures recorded in the rocks. Uplift from plate collisions can also raise areas from the bottom of the ocean up above beaches and to high mountains -- all different local climate zones; 2) The entire climate of the planet shifts. The planet has gone through wet and dry, hot and cold periods where the entire planet was different than it is now. Isotopic signatures in rocks record these changes; 3) Human accelerated climate change. Humans have impacted the local climatic conditions of small areas for several thousand years through agricultural practices. Deforestation and irrigation can cause dramatic local changes. Today, humans are causing changes through greenhouse gas emissions that may be big enough to change the entire global climate.

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