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Chapter 22

Across
Very stable and common mineral that scientists often use to age-date old rocks
Microscopic, photosynthetic prokaryotes that formed stromatolites and changed early Earth's atmosphere by generating oxygen
Unicellular organism that lacks a nucleus
Organisms composed of one or more cells each of which usually contains a nucleus; larger and more complex than a prokaryote
Alternating bands of iron oxide and chert; an iron-poor sedimentary rock
Process in which a planet becomes internally zoned, with the heavy materials sinking toward the center and the lighter materials accumulating near its surface
Fossils of various multicellular organisms from about 630 mya
A sedimentary rock deposit that contains oxidized iron; provides evidence that free oxygen existed in the atmosphere during the Proterozoic
Metallic or silica-rich object, 1 km to 950 km in diameter, that bombarded early Earth, generating heat energy; rocky remnant of the early solar system found mostly between the orbits of Mars and Jupiter in the asteroid belt
Down
Continental core formed from Archean or Proterozoic microcontinents; deepest and most stable part of a continent
A small fragment of granite-rich crust formed during the Archean
The top of a craton exposed at Earth's surface
Name given to the Precambrian shield in North America because much of it is exposed in Canada
Large mat or mound composed of billions of photosynthesizing cyanobacteria that dominated shallow oceans during the Proterozoic
A building block of proteins
A small fragment of an orbiting body that has fallen to Earth, generating heat; does not completely burn up in Earth's atmosphere and strikes Earth's surface, sometimes causing an impact crater
Ancient continent formed during the Proterozoic that is the core of modern-day North America