Earth History Lecture Notes: Fundamental Principles II

Fundamental Principles II

Introduction to Earth Materials

  • Minerals & Rocks
  • The Rock Cycle

Age dating and Geologic Time

Stenoís Laws for Sedimentary Rocks

  • Law of Superposition
  • Law of Original Horizontality
  • Law of Lateral continuity

Global Dating of Rocks

  • Intrusive Relationships
  • Principle of components
  • Fossil (Faunal) Succession

Minerals

A mineral is a naturally occurring, inorganic crystalline solid whose composition is known to exist within certain limits. Quartz, calcite, halite, feldspar, pyroxene for example.

Igneous Rock

Historically rocks were recognized by how they formed:

Igneous rock: crystallize from molten rock material called magma or lava (when cooled at the surface)

Intrusive igneous rocks cool slowly beneath the surface

Extrusive rocks cool rapidly as lava extrudes onto the surface

Sedimentary Rock

All rock exposed at the surface are subjected to physical and chemical weathering which break down rock materials and produce sediments of various size. Some sediments form by precipitation from seawater during evaporation. Sediments are transported by wind, water and glacial ice during erosion.

Sedimentary rocks: form during lithification - sediment become compacted and cemented during burial. Sandstone (see below), limestone, shale for example.

Metamorphic Rock

Rocks subject to intense pressure and increased temperature deep within the earth are transformed by changing the original rockís texture or by growth of new minerals. This is referred to as metamorphism. Note: rocks are not melted.

Slate, schist, gneiss (see below), marble and quartzite for example.

Reconstructing a sequence of events using Relative Age Dating

Rule 1: The law of superposition: Sediment is deposited in layers. Thus, new (younger) sediment is deposited on top of older sediments We say younger rocks are then formed on top of older sedimentary rock resulting in a sequence of rock called strata.

The Stratigraphic Record and Relative Age Dating

Rule 2: The principle of original horizontality: As sediment particles settle out of water under the influence of gravity, sediments usually form horizontal layers.

Therefore, if in the field an outcrop of rock is found to be lying at some angle, there must have been a force exerted on those layers after they were deposited.

Relative Age Dating

Rule 3.The principle of lateral continuity: states that sediment extends outward until it thins or by reaching the edge of a depositional basin. As deposition continues, a stratigraphic sequence is produced.

Relative Age Dating

4. The principle of cross-cutting relationships: An igneous intrusion or fault must be younger than the rocks they intrude.

Q. What is the correct order of formation (from oldest to youngest) for each of these rocks? Use cross-cutting relationships.

Relative Age Dating

Rule 5: The principle of inclusions states that rock fragments trapped within a layer of rock must be older than the rock they are in. For example, sandstone fragments that appear baked within a batholith.

Letís determine the relative age of the granite

Relative Age Dating

Rule 6: the principle of faunal succession: Fossils found in sedimentary layers at the bottom of a sequence are older than those at the top. Paleontologist look for short-live, widely distributed fossils to use as "index fossils". Presence of index fossils imply a restricted age of formation for the rock they occur in.

Summary of Relative Age Dating Rules

  • Law of superposition
  • Law of original horizontality
  • Rock units are laterally continuous
  • Cross cutting relationships (younger intrudes into older).
  • Rock fragment inclusions (older than surrounding rock)
  • Faunal successions

Conformable rock layers

 

These principles can be applied as long as the rocks are in order and no breaks or disruptions occur. (They are said to be conformable).

If a sequence of rock is missing (by erosion) or was not deposited, a break in geologic time occurs. This break can be represented by boundary surface in the rock layers and is said to be unconformable.

Unconformities

An unconformity is surface between layers that is formed by erosion or non-deposition (hiatus).

Disconformity: an erosional surface between rock layers that represents missing time.

Nonconformity: a surface between older igneous or metamorphic rocks and overlying sedimentary rock.

Angular unconformity: a surface formed when inclined rock units are topped by horizontal layers.

 

Geologic Time Scale

The geologic time scale was developed during the 19th and 20th century by a number scientists who studied various rock sequences or strata from around the globe. These scientist were able to recognized the relative order in which rocks sequences were formed.

This time scale uses information derived from the two types of age dating: relative and radiometric age dating.

North American Geologic Time Scale

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