PETROLEUM GEOLOGY

 The Rock Cycle

There are four main layers that make up the earth:
1. Inner Core - A mass of iron with a temperature of about 7000 degrees F.
Although such temperatures would normally melt iron, immense pressure
on it keeps it in a solid form. The inner core is approximately 1,500 miles
in diameter.
2. Outer Core - A mass of molten iron about 1,425 miles deep that
surrounds the solid inner core. Electrical currents generated from this area
produce the earth's magnetic field.
3. Mantle - A rock layer about 1,750 miles thick that reaches about half the
distance to the center of the earth. parts of this layer become hot enough to
liquify and become slow moving molten rock or magma.
4. Crust - A layer from 4-25 miles thick consisting of sand and rock.
The core, mantle and crust of the earth can be envisioned as a giant rock recycling
machine. However, the elements that make up rocks are never created or destroyed
although they can be redistributed, transforming one rock type to another.
The recycling machine works something like this. Liquid (molten) rock material
solidifies either at or below the surface of the earth to form igneous rocks . Uplifting
occurs forming mountains made of rock. The exposure of rocks to weathering and
erosion at the earth's surface breaks them down into smaller grains producing soil. The
grains (soil) are transported by wind, water and gravity and eventually deposited as
sediments. This process is referred to as erosion.
The sediments are deposited in layers and become compacted and cemented (lithified)
forming sedimentary rocks. Variation in temperature, pressure, and/or the chemistry of
the rock can cause chemical and/or physical changes in igneous and sedimentary rocks to
form metamorphic rocks. When exposed to higher temperatures, metamorphic rocks (or
any other rock type for that matter) may be partially melted resulting in the creation once
again of igneous rocks starting the cycle all over again.

As you might expect - since most of the earth's surface is covered by water - molten
material from inside the earth often breaks through the floor of the ocean and flows from
fissures where it is cooled by the water resulting in the formation of igneous rocks. Some
low grade metamorphism often occurs during and after the formation of the rock due to
the intrusion of the material by the magma. As the molten material flows from the fissure,
it begins forming ridges adjacent to it.
If we examine the rock cycle in terms of plate tectonics, as depicted in figure 10 above,
we see that igneous rocks form on the sea floor as spreading ridges. As the rocks cool,
and more magma is introduced from below, the plate is forced away from the spreading
ridge, and acquires a sediment cover. As shown in the figure, in this case, the oceanic
plate eventually "dives" under the adjacent continental plate. As the oceanic plate travels
deeper, high temperature conditions cause partial melting of the crustal slab. When that
occurs, the surrounding "country rock" (existing adjacent rock) is metamorphosed at high
temperature conditions by the contact. The molten material is either driven to the surface
as volcanic eruptions, or crystallizes to form plutonic igneous rocks.