University of Colorado GEOLOGY 1010 Class Note 1

Geological processes can be divided into those that are driven by an external heat engine (the sun), and those that are driven by an internal heat engine, the decay of naturally radioactive elements, principally uranium (U), thorium (Th), and potassium (K).

Earth's Surficial Processes and External Heat Engine

Surface processes are mostly driven by solar energy, the external heat engine. Water is evaporated from the oceans and rains down on the continents. Water chemically attacks (weathers) the silicate rocks and physically transports (erodes) the weathered products back to the ocean where they are laid down as horizontal sedimentary beds. Solidification of these sediments by burial to form sedimentary rocks is called lithification. This cycling of water is called the hydrologic cycle. Solar driven wind may also redistribute the sediments on the land surface.

Understanding sedimentary rocks and processes and the record of life on the surface contained in these rocks led to the first great revolution in the Earth sciences, the recognition of the vast amounts of time required for the formation of the planet and the evolution of the landforms and life forms of the surface. Geologists of the early part of the last century recognized that Earth history must be many millions of years, not thousands as advocated by theologians. By the end of the nineteenth century geologists and biologists (notably Darwin) wanted several billions of years of time but were opposed by the physicists (notably Kelvin) who, in ignorance of nuclear energy sources for the sun, wanted an upper limit of 40 million years. With the discovery of radioactivity and nuclear reactions, we obtained a heat source to allow the sun to be billions of years old, and a clock by which we can measure the age of the rocks.

Earth's Internal Structure and Heat Engine

The Earth is about 6370 km in radius. (40,000/2pi).

The outermost layer of the Earth is called the crust. It composes about 1/2% (0.005) of the Earth's total mass.

Oceanic crust is the layer below the deep ocean basins. It is basaltic (made up predominantly of the rock basalt), dense (density > = 3.0 to 3.2 g/cm3), thin (10-15 km) and young (<< 250My).

Continental crust is the layer that forms the continents. It is granitic (made up predominantly of the rock granite, plus overlying sediments), light (density = 2.7 g/cm3), thick (40-60km) and old (250 - 3700 My).

The lower boundary of the crust, both oceanic and continental, is a seismic discontinuity (reflector) called the Moho.

The crust (both oceanic and continental) together with the uppermost mantle behave as rigid, brittle, rocky plates and together form the lithosphere . The lithosphere is generally considered to be the upper 100 km of the Earth. Note that the lithosphere includes the crust, but also includes part of the upper mantle.

The region of the mantle below the lithosphere deforms plastically, although solid, and is termed the aesthenosphere. Note that the aesthenosphere includes most of the mantle, but excludes the uppermost part of the mantle and the crust.

The region between the Moho (base of the crust) and the top of the liquid metal core at 2900 km depth is the mantle. It composes 67% of the mass of the planet. The mantle is solid silicate (rock). Convection in the solid mantle is believed responsible for motion of the surface plates. This convection is driven by radioactive decay of the naturally radioactive elements, U (uranium), Th (thorium), and K (potassium). Within the solid silicate mantle, there is a prominent seismic discontinuity (reflector) at about 660 km depth and a less prominent discontinuity at 400 km depth. These are believed to be due to changes in the crystal structure of the silicate material in response to the increased pressure.

The core is the region below the mantle (2900-6370 km depth). It is made of metal, [Fe (iron), Ni (nickel), plus a small amount of a lighter element, probably S]. The outer core is liquid (molten) down to a depth of 5200km below the surface, and is solid metal below. Convection in the metal core is believed responsible for the magnetic field.

Do you remember what convection is? It is the rising of hot, light fluids and sinking of cool, dense fluids in a gravitational field.

Convection in the solid silicate mantle drives plate motion. How can a solid convect? Very slowly! Given high temperatures and lots of time, rocks flow like silly putty.

Upwelling occurs along mid-ocean ridges, and results in the generation of small amounts of magma (molten rock) that drives the rigid plates apart. This forms a divergent plate boundary. An example of a divergent boundary is the Mid-Atlantic Ridge.

Where plates collide, a convergent boundary is formed, and one plate is commonly subducted (drawn back into the mantle) beneath the other. Oceanic crust is subducted back into the mantle. Continental crust is too light to subduct and accumulates on the surface. Thus, continents have grown over geologic time. Examples of convergent plate boundaries are the west coast of South America, the Fiji-Tonga Trench, Phillipine Trench, and the Japan Trench.

If plates slide past one another, this forms a transform boundary. An example of a transform plate boundary is the west coast of North America at the San Andreas Fault.

The Earth is the third planet from the sun and has many features in common with the other planets.

The units we use in science are called Standard International (SI) Units. They are the meter (m), kilogram (kg), and second (s) and units based theron. Prefixes are used to denote every third power of ten of these units as in:
kilo-   103             milli-  10-3
mega-   106             micro-  10-6
giga-   109             nano-   10-9
Tera-   1012            pico-   10-12
                                   femto-  10-15
                                   ato-    10-18

GEOL 1010 Class Note 2

GEOL 1010 Syllabus

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