Chapter 22, Problem 1GST

To determine

To explain: The reason regarding the Precambrian consisting of fewer divisions than the rest of the time scale (although it accounts for nearly 90 percent of the geologic time), by referring to Figure 22.3 in Chapter 22 in this textbook.

Answer to Problem 1GST

The reason regarding the Precambrian era consisting of fewer divisions than the rest of the time scale is because of having less knowledge about this time period. The primary reason is the absence of fossils in Precambrian rocks; many of these rocks are concealed within continental interiors or have been destroyed by tectonic processes.

Due to the lack in complete geologic record from this time, it is difficult to divide this span into smaller increments without more detailed data.

Explanation of Solution

The 4.5 billion years of the Earth’s history is marked by two major eons, namely the Phanerozoic Eon (in Greek, “visible life”), which started about 541 million years ago and the Precambrian Eon, which started approximately 4 billion years ago. The Precambrian Eon is further divided into Archaen, (‘archaios’ denotes ‘recent’) and Proterozoic Eon (‘proteros’ denotes ‘before’; ‘zoe’ denotes ‘life’).

Although, the Precambrian represents nearly 88 % of the Earth’s history, it is not divided into eras, periods and epochs. The reasons for the lack of detailed time scale are stated below:

• Not knowing enough about the detailed Precambrian history. The past 10 years of the Earth’s history is very well-documented because of its fresh, least distorted, and most observable record. As we go further back in time, the clues and records are more fragmented.
• The Precambrian was dominated by simple organisms like bacteria, algae, fungi, and worms. These organisms lacked hard parts which did not favor preservation. Owing to this reason, the Precambrian had very little fossil records.  Due to the lack of fossils, correlation with the exposed Precambrian rocks is usually difficult.
• The Precambrian rocks are extremely old and are subjected to a great deal of change. Most of them are greatly distorted metamorphic rocks, broadly eroded and often hidden beneath the younger geological beds and thus, it makes the interpretation of the past environment difficult because of many clues being destroyed in the original sedimentary rocks.