There is in effect a stretch of approximately 300 million years for which no geologic record for rocks exists, and the evolution of this pregeologic period of time is, not surprisingly, the subject of much speculation.
To understand this little-known period, the following factors have to be considered: the age of formation at 4.6 billion years ago, the processes in operation until 4.3 billion years ago, the bombardment of Earth by meteorites, and the earliest zircon crystals.
This lesson is based on an online booklet that provides an introduction to the study of earth's history, published by the USGS.
Using careful analogies and written historical records, the authors help students understand the development of the geologic time scale, including how this depended on gathering evidence and making comparisons.
From the point at which the planet first began to form, the history of Earth spans approximately 4.6 billion years.
The oldest known rocks—the faux amphibolites of the Nuvvuagittuq greenstone belt in Quebec, Canada—however, have an isotopic age of 4.28 billion years.
It is taken to be the time when these bodies formed and, by inference, the time at which a significant part of the solar system developed.
When the evolution of the isotopes of lead deposits of different age on Earth, including oceanic sediments that represent a homogenized sample of Earth’s lead, the growth curve of terrestrial lead can be calculated, and, when this is extrapolated back in time, it is found to coincide with the age of about 4.6 billion years measured on lead isotopes in meteorites.
Melting at depth produced liquids that were gravitationally light and thus rose toward the surface and crystallized to form the earliest crust.
For example, you could present the content like this: Part I: Have students read the Most of the vocabulary in the booklet is clearly explained and used to enhance understanding of basic concepts.
If you'd like to emphasize some or all of the vocabulary, you could make a list of the terms found in the text ahead of time and ask students to record definitions as they read, building their own glossary.
These models place Earth’s age at approximately 4.5 billion years old.
Particles in the solar nebula condensed to form solid grains, and with increasing electrostatic and gravitational influences they eventually clumped together into fragments or chunks of rock. The constituent metallic elements sank toward the centre of the mass, while lighter elements rose toward the top.