Earth Science

Chapter 22.2

Riddle

Formation of the Crust Scientists believe that Earth consisted of hot molten

rock.

As the planet cooled minerals and elements became concentrated in specific density zones.

Differentiation Less dense materials float

on top of more dense materials

Iron- most dense- sank Silicon and oxygen- less

dense- floated to surface Differentiation: process by

which a planet becomes internally zoned when heavy materials sink toward the center and lighter materials accumulate near its surface.

Relative Densities

47%

28%

8%

5%

3%3% 3% 2% 1%

Earth’s Crust

Oxygen Silicon Aluminum

Iron Calcium Sodium

Potassium Magnesium All others

33%

31%

15%

2%

1%4% 14%

Entire Earth

Iron Oxygen Silicon Calcium

Aluminum Sulfur Magnesium

Earliest Crust Some type of early crust formed as soon as Earth’s

upper layer began to cool.

Similar to basaltic crust of Earth’s oceans

Early crust was recycled

Today it is recycled in subduction zones

Early recycling was vigorous

This vigorous recycling is not a process on Earth today

Continental Crust Early crustal pieces contained water.

This water was necessary for forming continental crust

Water reacted with mantle material to produce new less dense material.

This material crystallized to form small granite

Granite makes up most of the continental crust

These crustal fragments are called microcontinents

Microcontinents are not large enough to be considered continents.

Cratons Cratons- the oldest and most

stable part of a continent Most microcontinents formed

during the Archean and early Proterozoic still exist at the cores of today’s continents.

Cratons are made of granitic rocks Granite Gneiss Alternating bands of

metamorphosed basaltic rocks Represent 10% of Earth’s land

mass

Precambrian Shields Most cratons are buried beneath sedimentary rocks

Some places deep erosion has exposed the rocks of the craton Precambrian shield- the exposed rocks of the craton

Canadian Shield- the North American craton is called the Canadian shield Most of the exposed rock is in Canada

Also occurs in Greenland, northern Minnesota, Wisconsin and Michigan

Contain valuable minerals Nickel, silver and gold

Oldest rocks are about 3.8 billion years old.

Precambrian Shields

Canadian Shield

Growth of the Continents Earth’s continents were

once one big landmass

Pangaea

200 mya

Plate tectonics forces that formed Pangaea have been working since the end of the Archean

Mountain Building During the Proterozoic,

the microcontinents that formed during the Archean began to collide with each other

Formed massive mountains Orogenies-the mountain

building episodes The belts of rocks that are

deformed by the immense energy of the collisions are orogenies

Laurentia One of Earth’s largest

Proterozoic land masses

Laurentia- Ancient continent of

North America

Involved many different mountain-building events

Between 1.8 and 1.6 mya thousands of square kilometers were added

This collision was called Yavapai-Mazatzal Orogeny

First Supercontinent Collision of Laurentia with

Amazonia Occurred at the end of the

Proterozoic about 1.3 to 1 mya Coincided with formation of

Earth’s first supercontinent Rodinia

Positioned at the equator with Laurentia at its center.

By the time Rodinia formed, nearly 75% of Earth’s continental crust was in place.

The remaining 25% was added during the three eras of the Phanerozoic eon

The break up of this supercontinent began about 750 mya.