Isostatic Equilibrium Lab Why does the Earth have high places (the continents) and low places (the ocean basins)? What controls the elevation of mountains, lowlands, and various regions of the ocean floor? Adopted from a presentation by Ray Rector

Density Calculation The density of a substance is a measure of how much mass is present in a given unit of volume.  DENSITY = MASS / VOLUME                                                                                                                                                      Example: density of liquid water = 8.33 lb/gal = 64 lb/ft3 = 1.0 gm/cm3 2) We will measure all masses in grams and volumes in cubic centimeters (cm3), so all of our densities will be in grams per cubic centimeter (gm/cm3). 2

Density Calculation Measuring mass is easy; we use a gram scale. We’ll use two methods to measure volume: Linear dimensions (for objects that are rectilinear): Volume (cm3) = length (cm) x width (cm) x height (cm)                                                                                                                                                      2) Water displacement (for objects that have an irregular shape) 3

Density Calculation An object’s volume will displace an equal volume of water in a graduated cylinder. 4

Density Calculation An object’s density is a function of: Open spaces inside Chemical composition (atomic weight of the elements that make up the substance) Atomic weight = the number of protons plus neutrons in an element Example: wood is mostly carbon (C): atomic weight = 12 a fishing weight is mostly lead (Pb): atomic weight = 207 5

Isostatic Equilibrium Lab Why does the Earth have high places (the continents) and low places (the ocean basins)? What controls the elevation of mountains, lowlands, and various regions of the ocean floor? Answer: parts of the crust vary in thickness & density. Adopted from a presentation by Ray Rector

Earth’s Layered Structure Core Mantle Oceanic crust Continental crust Lithosphere: the crust (either oceanic or continental) plus the uppermost part of the mantle; forms a layer of cool, rigid rock that is broken up into moving plates. Crust

Earth’s Layered Structure The lithosphere is broken up into plates that move horizontally across the Earth.

Earth’s Layered Structure Today’s lab is not about horizontal movements of the Earth’s plates, but rather about vertical movements and vertical elevations of the Earth’s crust.

Elevation of Earth’s Crust

Two Main Types of Crust Continental Crust Oceanic Crust

The Concept of Isostasy Isostasy is the principle of buoyancy applied to the Earth’s crust. Isostasy: a state of gravitational equilibrium in which an area of crust “floats” in a balanced way on the denser rock of the mantle below. The elevation of any part of the Earth’s crust is a function of the THICKNESS and DENSITY of the crust.

The Concept of Isostasy These figures show how either thickness differences or density differences determine how high the wood blocks will float.

Isostasy and Isostatic Equilibrium The top of the wood will ride higher if: The wood block is thicker The wood block is less dense

Isostatic Adjustment – Ice Caps Isostatic Response to Changing Ice Weight Glacial Adjustment

Isostatic Adjustment - Orogeny