Transcript Precambrian Time: Vast and Puzzling

Precambrian Time: Vast
and Puzzling
Chapter 13, Section 1
Precambrian History
• The Precambrian encompasses immense
geological time, from Earth’s distant beginnings
4.56 billion years ago until the start of the Cambrian
period, over 4 billion years later
• The Precambrian comprises about 88% of the
geologic time scale
• Most Precambrian rocks do not contain fossils,
making correlating rock layers difficult
• Many rocks are metamorphosed and deformed,
extremely eroded, and hidden by overlaying strata
Concept Check
• Why are specific events in Precambrian
history difficult to determine?
• Most Precambrian rocks do not contain
fossils, which makes correlating rock
layers difficult. Many Precambrian rocks
are metamorphosed and deformed,
extremely eroded, or hidden by overlying
strata.
Earth Forms
• Scientists hypothesize that Earth formed
as gravity pulled together dust, rock, and
ice in space; gravity increased as Earth
grew and began pulling in more materials
• The high velocity impacts caused the
planet to melt and divide into specific
layers based on density
• Over several hundred million years, the
crust and mantle cooled and hardened,
forming rock
Earth Forms
Earth’s Atmosphere Evolves
• Earth’s original atmosphere was made up of gases
similar to those released in volcanic eruptions today—
water vapor, carbon dioxide, nitrogen, and several trace
gases, but no oxygen
• Torrential rains continued and slowly filled low areas,
forming the oceans; this reduced the water vapor and
carbon dioxide in the atmosphere, leaving behind a
nitrogen rich atmosphere
• Primitive organisms evolved that used photosynthesis
and released oxygen
• Slowly, the oxygen content increased, seen in the banded
iron formations (which took the free oxygen out of the
atmosphere)
• Oxygen began to accumulate in the atmosphere about
2.5 billion years ago
Evidence of Early Oxygen
Concept Check
• Why did the amount of oxygen in Earth’s
atmosphere increase dramatically?
• The evolution of organisms that used
photosynthesis caused the release of
increasing amounts of oxygen into the
atmosphere.
Precambrian Rocks
• The lack of Precambrian rock illustrates the law
of superposition, it is covered by much younger
strata
• Precambrian rocks do show through the
surface where younger strata are extensively
eroded, such as in the Grand Canyon and in
some mountain ranges
• Shield – a large relatively flat expanse of
ancient metamorphic rock within the stable
continental interior
• Much of what we know about Precambrian
rocks comes from ores mined from shields
• The mining of iron, nickel, gold, and other
metals has provided Precambrian rock
samples for study
Remnants of Precambrian
Rocks
Precambrian Fossils
• The most common Precambrian fossils are
stromatolites
• Stromatolite – structure produced by algae
trapping sediment and forming layered mounds
of calcium carbonate
• Remains of cyanobacteria have been found
that extend the record of life back beyond 3.5
billion years
• Many of these ancient fossils are preserved in
chert—a hard, dense chemical sedimentary
rock
• Plant fossils date from the middle Precambrian,
but animal fossils date to the late Precambrian
(many are trace fossils)
Precambrian Fossils
Assignment
• Read Chapter 13, Section 1 (pg. 364-368)
• Do Chapter 13 Assessment #1-26 (pg. 389390)
• For Section 1: Do #’s 2, 11-13, 20-24