The Earth's Floating Plates-The Principle of Isostasy

Using various materials from around the house we can discover how and why things float. This discovery leads us to ask questions about how the Earth behaves.

Introduction

The rigid outer shell of the Earth, called the lithosphere, floats on a softer partially fluid layer of the Earth's mantle called the asthenosphere, similar to how wood floats in water. Like a "Fig Newton," the Earth's outer shell has layers that behave by breaking or stretching. The rigid lithosphere is cracked and broken like the shell of a hard boiled egg. These curved pieces of the outer shell can move around on the asthenosphere as the interior of the earth moves via convection. Convention is the result of heating from below by the Earth's hot core. We call these moving pieces of lithosphere, tectonic plates. The upper most part of the plates is called the crust. There are two types of crust; continental crust and oceanic crust. The type of crust that constitutes the upper lithosphere determines the nature of the plate at that place. The upper portion of continental lithosphere is granitic in composition and these rocks have a lower density than the rocks of basaltic composition that make up the oceanic lithosphere. In this investigation we will examine isostasy, the way things float, and see how plates can interact to form plate boundaries.

Draw a picture of the interior of the Earth showing the core, mantle and crust.






Draw a picture showing the lithosphere and asthenosphere.






Preliminary questions

What is the outer shell of the Earth called?

What is the process that describes how the pieces of outer shell move?

The interior of the Earth is moving because of this process?

What drives the movement of the interior of the Earth?

What is the partially molten layer on which the plates move?

What is the property of matter that determines how things float?


Predictions

Which of these 3 materials (foam, wood, sponge) do you think is the densest?

Which of these 3 materials do you think is the least dense?

Which of the materials do you think will float the highest?

Which of the materials do you think will float the lowest?


The Discovery

Using foam, wood, sponges, aluminum foil and a tray that can hold water we will explore the nature of how things float and become familiar with the movement of the Earth's tectonic plates

Part 1: Using the foam, wood and sponge, determine which of the materials floats the highest and the lowest

Where your predictions correct?

Which material would best represent continental lithosphere?

Which material would best represent oceanic lithosphere?

In our model of the Earth what represents the lithosphere? the asthenosphere?


Part 2: Using the 3 floating materials again, determine the various ways that pieces of lithosphere can interact

What happens when you push the sponge and the foam together? On Earth, when this happens we call it subduction A subduction zone is a type of plate boundary that is the result of a ____________plate diving below a ____________plate.



What happens when you push 2 pieces of foam or wood together? On Earth when this happens we call it a collision. A collision zone is a type of plate boundary that is the result of a ____________plate running into a ____________plate

Another way that plates can interact is by moving away from each other This process is called spreading or rifting. When plates move apart what is it that fills the space between them?


Part 3: Using 2 pieces of aluminum foil, roll one into a ball and make a boat out of the other First put the ball on the water and observe what happens, then place the boat on the water and see what happens Then load your boat with some stuff and see what happen.

Why does the aluminum foil behave differently in it's 2 different shapes?

Why can a ship that is made out of metal float in water?

When you load a boat what happens to it? What about when you unload it?

How could you put a load on a continent? How would it behave?


Summary questions

Why do continental rocks sit above sea level and the ocean floor rocks below sea level?





On the Earth, what type of material floats the highest and the lowest in the mantle?





Draw and label a picture showing what it looks like when a continental plate runs into an ocean plate.







Draw and label a picture showing what it would looks like if 2 continental plates run into each other.







Draw and label a picture showing what it looks like when continental or oceanic plates pull apart.





How do "Fig Newtons" represent the outer layers of the Earth?





Why and how does the sponge's float level change as it sits in water?