Programme For International Student Assessment Pisa / Sample Science Passage 3: The Grand Canyon


The Grand Canyon is located in a desert in the USA. It is a very large and deep canyon containing many  layers of rock. Sometime in the past, movements in the Earth's crust lifted these layers up. The Grand Canyon is now 1.6 km deep in parts. The Colorado River runs through the bottom of the canyon. 

See the picture below of the Grand Canyon taken from its south rim. Several different layers of rock can  be seen in the walls of the canyon. 




The temperature in the Grand Canyon ranges from below 0 oC to over 40 oC. Although it is a desert area, cracks in the rocks sometimes contain water. How do these temperature changes and the water in rock cracks help to speed up the breakdown of roc

Freezing water expands in the rock cracks.
Freezing water dissolves warm rocks.
Ice smoothes the surface of rocks.
Water cements rocks together.

There are many fossils of marine animals, such as clams, fish and corals, in the Limestone A layer of the Grand Canyon. What happened millions of years ago that explains why such fossils are found there?

In ancient times, people brought seafood to the area from the ocean.
An ocean covered this area at that time and then receded later.
Oceans were once much rougher and sea life washed inland on giant waves.
Some sea animals once lived on land before migrating to the sea.