1.2a The universe is vast and estimated to be over ten billion years old. The current the-
ory is that the universe was created from an explosion called the Big Bang. Evidence for
this theory includes:
¥ cosmic background radiation
¥ a red-shift (the Doppler effect) in the light from very distant galaxies.
1.2b Stars form when gravity causes clouds of molecules to contract until nuclear fusion
of light elements into heavier ones occurs. Fusion releases great amounts of energy over
millions of years.
¥ The stars differ from each other in size, temperature, and age.
¥ Our Sun is a medium-sized star within a spiral galaxy of stars known as the Milky
Way. Our galaxy contains billions of stars, and the universe contains billions of such
1.2c Our solar system formed about five billion years ago from a giant cloud of gas and
debris. Gravity caused Earth and the other planets to become layered according to
density differences in their materials.
¥ The characteristics of the planets of the solar system are affected by each planetÕs
location in relationship to the Sun.
¥ The terrestrial planets are small, rocky, and dense. The Jovian planets are large,
gaseous, and of low density.
1.2d Asteroids, comets, and meteors are components of our solar system.
¥ Impact events have been correlated with mass extinction and global climatic change.
¥ Impact craters can be identified in Earths crust.
1.2e Earths early atmosphere formed as a result of the outgassing of water vapor,
carbon dioxide, nitrogen, and lesser amounts of other gases from its interior.
1.2f Earths oceans formed as a result of precipitation over millions of years. The pres-
ence of an early ocean is indicated by sedimentary rocks of marine origin, dating back
about four billion years.
1.2g Earth has continuously been recycling water since the outgassing of water early in
its history. This constant recirculation of water at and near Earths surface is described
by the hydrologic (water) cycle.
¥ Water is returned from the atmosphere to Earths surface by precipitation. Water
returns to the atmosphere by evaporation or transpiration from plants. A portion of
the precipitation becomes runoff over the land or infiltrates into the ground to
become stored in the soil or groundwater below the water table. Soil capillarity
influences these processes.
¥ The amount of precipitation that seeps into the ground or runs off is influenced by
climate, slope of the land, soil, rock type, vegetation, land use, and degree of
¥ Porosity, permeability, and water retention affect runoff and infiltration.
1.2h The evolution of life caused dramatic changes in the composition of Earths
atmosphere. Free oxygen did not form in the atmosphere until oxygen-producing
1.2i The pattern of evolution of life-forms on Earth is at least partially preserved in the
¥ Fossil evidence indicates that a wide variety of life-forms has existed in the past and
that most of these forms have become extinct.
¥ Human existence has been very brief compared to the expanse of geologic time.
1.2j Geologic history can be reconstructed by observing sequences of rock types and
fossils to correlate bedrock at various locations.
¥ The characteristics of rocks indicate the processes by which they formed and the environments in which these processes took place.
¥ Fossils preserved in rocks provide information about past environmental
¥ Geologists have divided Earth history into time units based upon the fossil
¥ Age relationships among bodies of rocks can be determined using principles of original horizontality, superposition, inclusions, cross-cutting relationships, contact metamorphism, and unconformities. The presence of volcanic ash layers, index fossils, and
meteoritic debris can provide additional information.
¥ The regular rate of nuclear decay (half-life time period) of radioactive isotopes allows
geologists to determine the absolute age of materials found in some rocks.