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Natural processes, including freezing and thawing

Natural processes, including freezing and thawing and the growth of roots, cause rocks to break down into smaller pieces.

The California Science Framework (page 64) distinguishes between chemical weathering and physical weathering. In both cases, the term weathering is used in its technical sense in geology meaning the breaking of rocks to smaller pieces. Rainwater and groundwater are generally somewhat acidic due to dissolved carbon dioxide from the air. The dissolved carbon dioxide forms carbonic acid, which is a weak acid. This acidity can chemically cause rocks to break apart or dissolve.

Plant roots that grow into cracks in rock can physically break rocks as they continue to grow. Freezing and thawing of water is another process that can physically break rocks. Students may not realize that water expands when it freezes. As recommended in the Framework, they can experience this effect by completely filling a soft plastic bottle with cold water, and then placing the tightly capped bottle in the freezer.

The properties of rocks and minerals reflect the processes that formed them

Waves, wind, water, and ice shape and reshape Earth’s land surface.

Rocks on the Earth’s surface are continually broken down by chemical and physical processes. Rainwater is slightly acidic because it contains dissolved carbon dioxide from the air. This weak acidity can chemically cause rocks to break apart and dissolve. Physical processes such as grinding by glaciers, pounding by waves, the freezing/thawing of water in ice cracks, and the growth of roots also break rocks into smaller pieces. Weathering is the technical term for both the chemical and physical processes that naturally break rocks.

While most people associate the term erosion with the breaking of rocks, technically erosion refers only to the movement of rock material from one location to another. Erosion eventually transports broken rock to the ocean as sediment. The net effect of this erosion is to lower the surface of the continents to sea level. From the point of view of geological time, mountains crumble rapidly. In the course of just 18 million years, the continents would be reduced to sea level and the oceans would cover the entire planet.

Why do we still have continents and mountains that reach miles into the air? Since the continents have existed for hundreds of millions of years, weathering and erosion must be balanced by a mountain building process. This process, known as plate tectonics, forms an important part of Grade 6 science standards.

For Grade 4, we set the stage for the later learning of mountain building. We introduce minerals and rocks (Standards 4a and 4b), and some of the processes that form igneous and metamorphic rock (Standard 4a). These processes and the rapid geologic events of earthquakes and volcanoes (Standard 5a) directly relate to plate tectonics.

The order in which the standards were written does not imply that they are supposed to be taught in that sequence. As in all the strands, these standards can be taught in many ways and in many sequences. The concept map below provides one way to organize these standards. The wording of some of the standards has been slightly changed for space reasons and to emphasize a particular conceptual flow.

A Concept Map for Grade 4 Earth Sciences

Students know how to differentiate among igneous, sedimentary, and metamorphic rocks by referring to their properties and methods of formation (the rock cycle).

Since rocks are made of minerals, the California Science Framework recommends teaching about minerals (Standard 4b) before rocks (Standard 4a). The order of content in the standards does not imply any sequence in teaching the standards.

The terms igneous, metamorphic, and sedimentary indicate the processes that formed the particular rocks. Since any given rock can be made from a wide variety of minerals, there are huge variations within each of these categories. An igneous rock composed mainly of minerals X, Y, and Z may have a much greater resemblance to a metamorphic rock made of minerals X, Y and D than to another igneous rock composed mostly of minerals J, K and L. Metamorphic rocks will differ based on how much temperature and pressure formed them. Igneous rocks will differ depending on whether they solidified above or below the ground.

The net result of these and other variations is that there are no simple rules that enable one to immediately identify a particular rock as being igneous, metamorphic, or sedimentary. The California Science Framework (page 63) provides very general guidance that is summarized in the Table below, but also clearly warns that there are many exceptions to these generalizations.

Very General Descriptions of Rock Types by Rock Properties
There Are Many Exceptions to These Generalizations
TYPE OF ROCK HARDNESS LAYERING OTHER
Igneous Hard No Interlocking crystalline textures
Sedimentary Soft Yes Fragmentary textures; look like broken grains of rock cemented together
Metamorphic Hard Minerals lined up or arranged in uneven layers