• Uniformitarianism (James Hutton, late 1700's) - (the present is the key to the past)

• physical, chemical, and biological laws that operate today have also operated in the past
• the forces and processes that we observe shaping the Earth today, have been operating for a very long time
• geologic processes occur over very long periods of time

• Geologic Time - is very long - Earth is 4.6 billion years old - based on "radiometric (absolute) dating" methods as well as "relative dating" techniques

• Geologic Time Scale - Eons, Eras, Periods, and Epochs

Geologic Time Scale

• Absolute Dating - absolute dating pinponts the time in history when some geologic event took place.  Our understanding of radioactivity allows us to accurately determine absolute dates for rock units that represent important events in Earth's past.

• Relative Dating - rocks and structures are placed in proper sequence or order - only the chronological order of events is determined - absolute ages are not determined - it is accomplished on the basis of the "law of superposition"

• Superposition - in an undeformed sequence of sedimentary rocks or lava flow, each bed is older than the one above, and younger than the one below

• Fossils - the remains or traces of prehistoric life

• Fossil Succession - fossil organisms  succeed one another in a definite and determinable order, and any time period can be recognized by its fossil content.

Fossil Succession

II. Earth's Structure

• the Earth's surface is continuously being altered

• these alterations are the result of two kinds of processes: "endogenic" & "exogenic" processes

• "endogenic" - processes inside the Earth that produce flows of heat and mass, resulting from radioactive decay - the Earth's surface moves, warps, & breaks (earthquakes & volcanoes) - tend to build surface features

• "exogenic" - processes at the Earth's surface & in the atmosphere (external processes); involve the movement of air, water, & ice; they shape & move the landscape - tend to reduce surface features

• the goal of the physical geographer is to have an overall understanding of the Earth's surface environment; & then understand how specific aspects of this surface environment affect society

• recall that the Earth is thought to have formed from a cloud of gas & dust (a nebula) ~ 4.6 billion years ago

• as the Earth became compacted, heat accumulated in its interior (heat was also added via radioactive decay)

• under the force of gravity, the heavier elements sank & the lighter elements rose toward the surface

• the Earth's interior is arranged in concentric layers - & each layer is differentiated by heat/chemistry

• heat moves outward via conduction & convection

• the Earth's internal structure has been determined on the basis of indirect evidence - "seismic evidence" (seismic waves (shock waves) pass through different materials in different ways and at differing speeds)

• earth's core - 1/3 of Earth's mass; 1/6 of it's volume

• "inner core" - composed of solid iron (Fe); density = 13.5g/cm3; its solid because of very high pressures

• "outer core" - is molton iron; density ~ 10.7 g/cm3

• outer (liquid) core is responsible for ~ 90% of Earth's magnetic field

• (hypothesis: circulation in the outer core produces currents which in turn induce the Earth's magnetic field)

• "mantle" - 80% of the Earth's total volume; density ~ 4.5 g/cm 3

• is separated from the outer core by the "Gutenberg Discontinuity" (a place with a change in physical properties)

• density decreases as you go up in the mantle

• the mantle's temperature increases gradually with depth

• upper mantle & lower mantle

• upper mantle:

(2) "asthenosphere" - from ~ 70-250 km - is a plastic layer; contains hot pockets due to radioactive decay; least rigid layer of the mantle; avg. density ~ 3.3 g/cm 3; movement here causes crustal movements; convection occurs

(3) remainder of upper mantle - 250-670 km - here rocks are solid because of increased pressures

• "crust" - .01% of Earth's mass

• "crust" + "high velocity zone" = "lithosphere"

• the boundary or discontinuity between the crust and the high velocity zone below, is referred to as the "Mohorovicic Discontinuity" (Moho)

• crust below mountains - 50-60 km depth

• crust below continental interiors - ~ 30 km depth - comprised of granite

• crust below oceans - ~ 5 km depth - comprised of basalt

• "isostasy" - with a greater load (e.g., when mountains lie above the crust) the crust rides lower in the asthenosphere; when some of the load is removed (e.g., due to erosion) the crust rides higher in the asthenosphere - "isosatic rebound"

• thus, the crust is always rising or sinking in response to its own changing weight

• the Earth's crust is in a constant state of flux

• "hydrologic cycle" - transports & deposits material
• "tectonic cycle" - moves heat energy and material to the surface, creating crustal movement & deformation
• "rock cycle" - generates the three basic rock types of the Earth's crust