to Earth Materials
- The Rock
and Geologic Time
Laws for Sedimentary Rocks
- Law of
- Law of
- Law of
Dating of Rocks
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.
rocks were recognized by how they formed:
rock: crystallize from molten rock material called magma or lava (when
cooled at the surface)
igneous rocks cool slowly beneath the surface
rocks cool rapidly as lava extrudes onto the surface
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.
rocks: form during lithification - sediment become compacted and
cemented during burial. Sandstone (see below), limestone, shale for
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.
schist, gneiss (see below), marble and quartzite for example.
a sequence of events using Relative
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.
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
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
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.
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.
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.
determine the relative age of the granite
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.
Relative Age Dating Rules
- Law of
- Law of
- Rock units
are laterally continuous
cutting relationships (younger intrudes into older).
fragment inclusions (older than surrounding rock)
principles can be applied as long as the rocks are in order and no
breaks or disruptions occur. (They are said to be conformable).
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.
unconformity is surface between layers that is formed by erosion or
an erosional surface between rock layers that represents missing
a surface between older igneous or metamorphic rocks and overlying
unconformity: a surface formed
when inclined rock units are topped by horizontal layers.
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.
scale uses information derived from the two types of age dating:
relative and radiometric age dating.
Geologic Time Scale
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