A. Introducing Earth
Earth from Space Photo
1. Visiting alien idea: ask students what observations they would make as they approached this world, and what kinds of questions they might ask. visible: atmosphere, hydrosphere, geosphere or lithosphere, biosphere.
2. How big is Earth? [I like to throw quantitative questions at them to get them thinking how to make meaningful numerical estimates as it seems most of them don’t do this “automatically”.]
Make this a driving problem: the distance from Earth’s surface to its center is 6370 km. How long would it take to drive to the center of the Earth? (Triplehorn, 1994)
a. guestimate based on experience: has anyone driven across the country? How does the length of the U.S. compare to the radius of Earth?
b. calculate: Time = distance/rate constrain with assumptions:
- drive at legal speed, e.g. 100 km per hour (about 60 miles per hour)
- time = 6370 km / 100 km/hr = 63.7 hours
- at 8 hours per day: 63.7 h / 8 h per day = 8 days
3. How much water is there on Earth?
Water Demo: (Liukkonen, 1993, How Much Water is There?) one gallon represents all water on Earth; half cup is all fresh water; one drop = available fresh water.
4. Solid Earth: Let’s strip away the water and the air => reveals the surface of the Geosphere
Satellite image of Earth without atmosphere or hydrosphere; i.e. surface of Geosphere www.ngdc.noaa.gov/mgg/image/images.html/.
bi-modal surface: two average elevations, one above and one below sea-level. Why? A reflection of differences in composition.
crust = outer compositional layer of Earth; 2 basic varieties: oceanic and continental
rock samples: oceanic: basalt, gabbro / continental: granite
B. Earth Materials: how they form; properties; behavior
1. Minerals are the building blocks of rocks. Minerals can be used to study a number of the Ohio competencies, both in Earth and Space Sciences, as well as Physical Science. Those of you teaching chemistry, in particular, atomic bonds, may find that your students relate well to mineral examples. I like to start with mineral properties and then go to the atomic level to answer the question of why minerals exhibit the properties they do.
2. mineral properties which are controlled by bonding (Ohio competency 5) demos/student activity:
- hardness: the resistance of a material to being scratched
- glass plate, penknife or nail
- selection of minerals: talc or gypsum (scratch with finger nail); halite, calcite or fluorite (scratch with glass); feldspar (scratch with knife/nail); quartz (harder than knife/nail)
- lesson: ionically bonded minerals tend to be soft; covalently bonded materials are hard; hybrid (mixed character) are in between
- breakage: the characteristic shape that results when hit mineral with rock hammer
- demo: rock hammer and halite: hammer a chunk of halite and have the students observe the resulting shapes => 90 degree angles between planar cleavage surfaces.
- halite crystal model; quartz crystal model; mineral samples: mica; halite; galena; quartz
- lesson: variation in bond strength and arrangement controls breakage:
- all same strength as in quartz, or random variation as in glass => conchoidal breakage
- regular pattern of differences => cleavage directions
- mica - 1; halite - 3; galena - 3
3. What controls how atoms bond?
In chemistry the students learn about valence electrons and how to predict whether an element will likely bond ionically or covalently, or otherwise. But students also need to learn that the environmental conditions also play a controlling role. For example:
crystal models of diamond and graphite; samples of graphite and diamond;
minerals made of only carbon: in diamond, each carbon atom is bonded covalently to four other carbon atoms (and is the hardest natural substance!); O.K., so far, so good, but -graphite is also made of only carbon: soft, greasy, opaque; each carbon is covalently bonded to three other carbons, forming sheets; not perfectly charge balanced so Van der Waals forces hold sheets together, but not very strongly => cleavage; soft
diamond and graphite have the same composition but different structures; they form under different conditions (diamond at high pressure and temperature - mantle conditions; graphite at high temperature - metamorphic conditions in the crust); the mineral which grows is the one which is most stable under the conditions (T and P primarily).
other examples: calcite and aragonite; quartz polymorphs
4. We can continue the idea of stability by doing a simple demo:
HCl, calcite, dolomite, overhead projector, or students gather around
a. demo reaction between calcite and HCl. ask what is happening: fizz => reaction between weak acid and mineral calcite; composition of calcite is Ca CO3 => ask students composition of bubbles => CO2 Why does this happen? under these conditions, it is more stable for calcite to dissolve.
b. demo dolomite and HCl: no fizz Why not? dolomite is (Ca,Mg)CO3 and has lower solubility owing to the different composition.
c. environmental tangent: can discuss weathering of minerals at Earth’s surface due to naturally occurring acids (rainwater, soil water, groundwater in contact with calcite/limestone) => dissolution of exposed rock including limestone/marble used as building stone/sculpture and underground rock => sinkholes; caverns.
Let’s continue with the idea of stability and look at the formation of minerals to form the primary family of rocks, namely the igneous rocks.
5. Crystallization of molten rock (magma)
M&M Modeling of rocks: Crystallization of molten rock (magma)
K. Fryer June 2001
Students by now know the three rock families, and may know the major minerals in them, but may not know why certain minerals are only found with certain other minerals. The following simple activity furthers their understanding of process by illustrating the order of crystallization of minerals as the magma or lava cools from its initially fully molten state, by focusing on the conditions of formation, which for igneous rocks is primarily temperature.
Materials: M&M candies; smartie candies; fine sharpie makers; yarn or string
Rock samples: gabbro, diorite, granite, basalt, andesite, rhyolite
Approximately 95% of the minerals in igneous rocks are silicate minerals, with the following being the most common: olivine, pyroxene, amphibole, biotite, feldspar, muscovite , and quartz.
The first four of these are the ferromagnesian silicates -- they contain Fe and/or Mg in addition to Si , O, +/- other elements. These minerals each have a distinct freezing temperature, and so grow from the melt at different times, as the melt cools to their particular freezing temperature.
Crystallization order: olivine, pyroxene, amphibole, biotite.
The most abundant mineral group in Earth’s crust is the feldspars, which are also silicate minerals. The plagioclase feldspars contain Al, Si, and O, as well as a variable ratio between Ca and Na, ranging from all Ca (Ca-plagioclase, or anorthite) to all Na (Na-plagioclase, or albite). Ca-plag has a very high freezing temperature, while Na-plag has a relatively low freezing temperature, and there is a continuous change in ratio from high Ca to high Na as the temperature drops from the Ca-high temp to the Na-low temp.
As it happens the range of freezing temperatures for the ferromagnesian minerals and the plagioclase feldspars is the same, so that the first minerals to crystallize (highest temperature) include one ferromagnesian, olivine, along with the Ca-plagioclase. As the magma cools, olivine stops crystallizing and the plagioclase which continues to crystallize adds some Na to the mix. When the temperature reaches the freezing temperature of pyroxene, it crystallizes, and the plagioclase at this temperature contains both Ca and Na with Ca still more abundant. This pattern continues; at the freezing temperature of amphibole, the plagioclase is about equal parts Ca and Na; at the freezing temperature of biotite, the plagioclase is all Na.
Activity: M&M model
- Assign colors to minerals, e.g: olivine - green; pyroxene - blue; amphibole - red; biotite - brown; Ca-richer plag - orange; Ca=Na plag - using a sharpie, mark some orange with a line to indicate half & half; Na-richer plag - yellow
- Mark area of table with circle of yarn or string to represent magma chamber; place a pile of M&Ms (multiple of each color) within the chamber. Teacher, also dump in a handful of smarties, but don’t talk about them yet. Have students crystallize minerals by taking out the appropriate colors as temperature decreases. Put like-temperature crystals in piles together. When all have crystallized, have students note the different groupings and write down which minerals occur together.
Lesson: The three common igneous rocks found in Earth’s crust are:
- olivine + pyroxene + Ca-rich plagioclase = gabbro
- amphibole + Ca=Na plagioclase = diorite
- biotite + Na-rich plagioclase => we need to discuss this a bit more:
Once we get down to this relatively low temperature, some magmas are completely crystallized. However, silica (Si + O) -rich magmas still contain Si, O, Al, and K at this point, and so some other minerals crystallize as temperature continues to decrease. These include K-feldspar, muscovite, and quartz. These other minerals, because they contain no Fe or Mg, are very light in color, and are represented here by the smarties. These crystals go with the biotite + Na-plag, so tell the students to add them to that pile. Now they have formed granite.
Note: Now, those of you who know about fractional crystallization and Bowen’s Reaction Series know that magma processes are a bit more complicated, involving reactions between crystals and the melt as temperature decreases, but that is rather more advanced (if you do this with your class, talk to me as I know another activity to model this, using the students as ions (very low cost!).
Segue to magma extrusion: If magmas find their way out of the crust via a vent system, then they cool quickly at the surface, but still crystallize the same minerals in the same order. We call these rocks basalt, andesite, and rhyolite. Let’s look at an important aspect of magma/lava behavior that is controlled by composition.
6. Flow behavior [viscosity] of magmas/lavas:
- volcano pictures: show students pictures of shield and stratovolcanoes
- viscosity is controlled by composition, particularly silica content, and temperature
- There are a number of good exercises to demonstrate viscosity differences; two are referenced here. A non-messy one, from Gary Martindate, 1994, Hand-Held Magma Chamber, and a very messy one, Volcanoes!, 1997, from Project Primary. [References at end.]