CRACKING CRUSTAL QUESTIONS
The crust of the Earth is divided into plates that move relative to one another. The interaction of these plates creates many of the earth's landforms. Provided information about crustal characteristics and plate boundaries, students will experiment with representative materials to understand crustal interactions.
Instructional Method: Experiment
Goal: Expose students to different crustal types and their interactions.
Objectives: Students will be able to:
- List the three types of plate interactions.
- Draw the three interactions.
- Name one landform that results from each plate interaction.
Setup: 20 mins.
Experiment: 20 mins.
Discussion: 20 mins.
- old cake pan
- paraffin wax
- heat source
- clear plastic container
- foam and cardboard strips 4 inches wide, 8 inches long
We are most familiar with the layer of the earth called the crust. It is the layer we live on. Surprisingly, it is the thinnest of Earth's layers. The crust ranges in thickness from 5 to 100 km, yet it represents less than 0.1% of the total volume of Earth! The crust is ridged and brittle, and when it cracks or moves, earthquakes occur. The two crust types are continental crust and oceanic crust.
Continental crust is about 35 to 100km thick. It is thickest beneath mountain ranges and plateaus. Continental crust is much lighter, or less dense, than oceanic crust. It contains a large portion of quartz material (SiO2), also known as felsic material.
Oceanic crust is the rock found below the world's oceans. It is the thinner, about 5-10 km thick, and more dense. It is composed of mafic (iron- and magnesium-rich) rock such as basalt. Because oceanic crust is more dense it tends to sink below the lighter continental crust when the two crust types collide.
On Earth the crust is fragmented into a dozen or more large and small regions known as plates. A single plate can be made of both continental and oceanic crust. These plates move relative to one another as they ride atop hotter, mobile material.
Moving plates create and destroy landforms such as mountain ranges, basins, trenches, and deep-sea ridges. Places where different plates touch, and sometimes collide, are called plate boundaries. There are three types of plate boundaries:
- continental crust to continental crust
- oceanic crust to oceanic crust
- continental crust to oceanic crust
Each boundary can be classified in one of three ways:
Divergent boundaries form when plates break and spread apart from one another. This occurs when a convection current develops below the crust. Upwelling hot magma forces plates to spread as the magma oozes through cracks at the surface, creating new crust. The place where new oceanic crust is formed is called a spreading ridge. An example of a spreading ridge is the Mid-Atlantic Ridge at the bottom of the Atlantic Ocean.
Divergent boundaries are not found only in the ocean. Sometimes a continental plate will tear apart due to the same upwelling forces that tear apart oceanic plates. For example, Lake Superior is believed to be the remnant of a failed continental rift. That rift did not succeed in tearing the continent into two separate pieces, but it did create a valley that is now filled by water.
Convergent boundaries are found where two plates collide. Large scale examples of this are seen when continents collide. When plates collide they buckle like the hood of a car in a head-on collision, forming huge mountain ranges. The mountains of Denali National Park are the result of continent - continent collision.
When continental and oceanic crusts collide, heavier oceanic plates slide under lighter continental plates. This process is known as subduction. As it slides below the continental crust, sinking deeper into the hot mantle, the oceanic crust begins to melt. This melted oceanic crust becomes less dense (because it is now in a liquid form) and therefore rises towards the surface causing continental crust to rise up above it, forming mountains and volcanoes.
Mountain ranges formed near subduction zones include the Andes, the Cascades and the Aleutian Islands in Alaska. The Andes formed as the Nazca Plate was subducted below the South American Plate. The Cascades and the Aleutian Islands are forming as the Pacific Plate slides under the North American Plate.
Deep underwater trenches form where continental and oceanic crusts collide. A trench is a depression in the sea floor that can be over a mile deep and hundreds of miles long. When oceanic crust collides with oceanic crust one plate subducts and begins to melt. The melting plate rises towards the surface and can push through the above plate forming underwater mountains and volcanoes. Volcanoes produced from oceanic-oceanic collisions can result in island chains. The Marianas Islands were produced from an oceanic-oceanic collision.
Transform boundaries form where two plates collide and slide next to each other along a fault instead of uplifting or subducting. A famous example of such a collision is the San Andreas fault in California. The thin strip of California to the west of the fault rides with the Pacific Plate as it slides along the side of the North American Plate. The San Andreas is very active, and this constant movement results in frequent earthquakes. Point Reyes National Park in California preserves the beauty associated with fault movement.
The following activities can be used to explain different crustal types, interactions between them, and fault boundaries.