THE HYPOTHESIS OF DIFFERENTIAL EROSION
PLAYING WITH THE DATA - QUANTIFYING RESISTANCE TO EROSION

  • Once the data is assembled, it may be played with to develop a numerical measure (an "index") of the resistance of each of the geologic materials to erosion.
  • The numerical index may then be used together with elevation data to evaluate the prediction.

If the hypothesis is true, how would you expect elevation to change as resistance changes?

RESISTANCE TO EROSION

  • Major factors that control the resistance to erosion of geologic materials include:
    • how hard they are
    • their chemical reactivity
    • how well their components stick together (cohesion).
    The role of each of these three factors is considered for the eight materials that underlie the surface of the NYC area. The importance of each factor is expressed numerically as a 'Hardness Value', an 'Inertness Value', and a 'Cohesion Value'. These three values are then combined into an 'Erosion-Resistance Index' that indicates the relative resistance to erosion of each of the geologic materials.
  • Print out a copy of the 'Erosion-Resistance Index Calculation Table' (printout # Q-6).
  • As you determine the Hardness, Inertness and Cohesion Values, record them in the appropriate columns of this Table.
  • Then click on the red button at the bottom of the page to create an "Erosion-Resistance Index".

HARDNESS: Hardness will affect the resistance of individual mineral grains in geologic materials to being worn away by rubbing and scratching.
If the hypothesis is true, how would you expect elevation to change as hardness changes?

Press the blue button to determine the hardness of NYC minerals. Make sure you use the answer check table.
Press the blue button to calculate a 'Hardness Value' for NYC rocks and sediments. Make sure you use the answer check table.

CHEMICAL REACTIVITY: Inertness describes the resistance of individual mineral grains in geologic materials to being chemically altered or dissolved.
If the hypothesis is true, how would you expect elevation to change as chemical reactivity changes?

Press the blue button to determine the inertness of NYC minerals.
Press the blue button to calculate an 'Inertness Value' for NYC rocks and sediments.

COHESION: Cohesion measures how strongly the particles that compose geologic materials are joined together.
If the hypothesis is true, how would you expect elevation to change as cohesion changes?

Press the blue button to determine the effect of grain relationships on the cohesion strength of rocks and sediments.
Press blue the button to determine the cohesion strength of NYC materials
Press the blue button to calculate a 'Cohesion Value' for NYC rocks and sediments.
Now use the Hardness, Inertness, and Cohesion values to create an Erosion-Resistance Index. Click on the red button!

©2002
David J. Leveson