Unconformities

An unconformity is a contact between two rock units in which the upper unit is usually much younger than the lower unit. Unconformities are typically buried erosional surfaces that can represent a break in the geologic record of hundreds of millions of years or more. For example, the contact between a 400-million-year-old sandstone that was deposited by a rising sea on a weathered bedrock surface that is 600 million years old is an unconformity that represents a time hiatus of 200 million years.

Types of unconformities

1)Disconformities. Disconformities are usually erosional contacts that are parallel to the bedding planes of the upper and lower rock units. Since disconformities are hard to recognize in a layered sedimentary rock sequence, they are often discovered when the fossils in the upper and lower rock units are studied.

2)Nonconformities. A nonconformity is the contact that separates a younger sedimentary rock unit from an igneous intrusive rock or metamorphic rock unit. A nonconformity suggests that a period of long-term uplift, weathering, and erosion occurred to expose the older, deeper rock at the surface before it was finally buried by the younger rocks above it. A nonconformity is the old erosional surface on the underlying rock

3)Angular unconformities. An angular unconformity (Figure 3) is the contact that separates a younger, gently dipping rock unit from older underlying rocks that are tilted or deformed layered rock. The contact is more obvious than a disconformity because the rock units are not parallel and at first appear cross-cutting.

CONTINENTAL CRUST VERSUS OCEANIC CRUST

CONTINENTAL CRUST

· Less Dense (lighter weight): average density = 2.7 gm per cubic centimeter

· Thicker: 0 to 40 km, average 35 km under high mountains, the crust can be thickened, up to 65 km. Under the craton, the average thickness is 30 km.

· Mineral Composition: granitic (felsic) rocks such as granodiorite and granite enriched in Potassium (K), Sodium (Na), Aluminum (Al), and Silica (Si)

· "Floats" on top of the Mantle

OCEANIC CRUST

· More Dense (heavier): average density = 3.0 gm per cubic centimeter

· Thinner: 0 to 10 km, average 5 km, thinnest at Mid-Ocean Ridges

· Mineral Composition: mafic rocks such as basalt and gabbro enriched in Magnesium (Mg) and Iron (Fe)

· Underlies Ocean Basins