| A | B |
|---|
| isotope | an atom of an element that has the same atomic number but different atomic mass |
| zircon | very stable mineral commonly found in granite through radiomatric dating; provides evidence of Earth's crust being at least 4.1 to 4.2 billion years old |
| asteroid | metallic or silica-rich object, 1 km to 950 km in diameter that bombarded early Earth generating heat and electricity |
| meteorite | small asteroid or asteroid fragment that fell on early Earth, generating heat |
| generate | to bring into existance or produce |
| lithosphere | term for the upper mantle and the crust |
| differentiation | process by which a planet becomes internally zoned with heavy materials in core and lighter materials near surface |
| Precambrian Shield | core of Archaen and Proterazoic rock found on the continents |
| Canadian Shield | Precambrian shield of North America |
| microcontinent | small pieces of continents |
| craton | the oldest, most stable part of the continent |
| Laurentia | ancient continent that became the core of North America |
| illustrate | to clarify, as by use examples or comparisons |
| atmosphere | thick blanket of gases surrounding Earth |
| cyanobacteria | microscopic, photosynthetic prokaryotes that formed stramatolites and changed early Earth's atmosphere by generating oxygen |
| stramatolites | large mats and mounds composed of billions of photosynthesizing cyanobacteria that dominated the Proterozoic's shallow oceans |
| banded iron formation | unique, shallow marine deposits made up of alternating bands of chert and iron oxides that formed due to locally high oxygen levels produced by stramatolites |
| red bed | sedimentary rock deposits that contain oxidized iron, providing evidence that free oxygen existed in the atmosphere during the Proteozoic |
| dominate | to occupy the preeminent position in or over |
| period | A division in the geologic time scale |
| amino acids | the building blocks of life |
| prokaryote | an organism that is made of a single cell and lacks a nucleus |
| eukaryote | an organism with cells that contain nuclei |
| Ediacara biota | immobile fossil invertebrates with no mouth, gut, ir anus that look very much like organisms from the Cambrian Era |
| constantly | continually occurring or recurring, persistant |
