Transcript Geologic Materials, Processes and Principles

Geologic Materials,
Processes and Principles
Dr. James D. Miller
Department of Geological Sciences
University of Minnesota Duluth
Geologic
History of
North
America
(incompletely)
recorded in its
rocks
The Rock Cycle
Showing the relationships between Earth Materials and Processes
IGNEOUS ROCKS
An aggregate of minerals crystallized from molten rock (magma)
Generation of Magma
Mafic – partial melting of the
mantle
Felsic – partial melting of the
crust
Igneous Environments
Fine-grained
= Volcanic
Coarse-grained
= Plutonic
SEDIMENTARY ROCK
Compacted and cemented accumulations of sediment,
which can be of two general types - clastic and chemical
Sedimentary Processes
Weathering
Mechanical
Chemical
Sedimentary Processes
Erosion, Transport, and Deposition
Sedimentary Processes
Compaction and Cementation
Criteria for Classifying Clastic
Sedimentary Rocks
Energy of Depositional System
Types of Clastic Sedimentary Rocks &
Environments of Formation

MUDSTONE/SHALE - Well-sorted, mature,
clay-sized particles ; generally implies deposition
into quiet water

SANDSTONE - Well-sorted, mature, commonly
bedded, sand-sized particles typically
transported by wind or moderate water
movement (e.g. rivers, beaches)

GREYWACKE - Moderately sorted, immature,
clay- to sand-sized particles commonly showing
graded bedding. Commonly deposited in deep
waters off mountainous coasts

CONGLOMERATE - Poorly sorted, immature,
clay to boulder-sized particles transported only a
short distance from their source and typically
deposited by fast moving water.

TILL - Very poorly sorted, clay to boulder-sized
particles; non-bedded; deposited from glaciers.
Environments of Clastic Sedimentation
Chemical Sedimentary Rocks
composed of minerals precipitated from water (usually ocean
water) due to evaporation or to the activity of organisms
IRON-FORMATION - Iron oxide minerals, usually magnetite
(taconite ore) or hematite (natural ore), interlayered with chert
(microcrystalline quartz) and clay minerals. Common chemical
sedimentary rock biogenically formed in shallow marine
environments older than about 1.8 billion years.
 LIMESTONE - Calcium carbonate (calcite) commonly containing
abundant marine fossils. Most common type of chemical
sediment forming today by biogenic processing of seawater.
Dolomite (or dolostone) is created by replacement of calcium by
magnesium after shallow burial of limestone. Forms in tropical
shallow marine environments.
 EVAPORITE DEPOSITS (Gypsum, Halite, Anhydrite) – mineral
precipitated from saline water in arid environments with high
evaporation rates (e.g., playa lakes)
 PEAT/COAL – Carbonaceous material created by the
accumulation, compaction and heating of organic matter.
Forms in temperate to tropical, low energy, terrestrial
environments (lagoons, floodplains).

Environments
of Chemical
Sedimentation
Evaporites
Coal
Limestone
Metamorphic Rocks
Metamorphism- changes in the mineral assemblage and
textures of igneous, sedimentary or other
metamorphic rocks due to prolonged exposure to
elevated temperatures and pressures (or increased
grades of metamorphism)
Controls on Rates of Metamorphic Change

Fluid content (particularly water)
of the rock. Water helps to
catalyze the mineral
transformations. Water gets
“baked” out of rocks during
prograde metamorphism.

Temperature - chemical
reactions occur faster at higher
temperatures.

Time - For a rock to develop a
new metamorphic mineral
assemblage corresponding to a
particular P & T, it must exist
under those condition for a
sufficiently long period of time
(generally tens of thousands to
millions of years).
Mineralogic Response
to Metamorphism
Minerals that form depend on:
 T and P conditions
 Bulk composition of the
source rock
Progressive
metamorphism
of a graywacke
(dirty sandstone)
Textural Response to Metamorphism
Reflects the intensity and directionality of pressure (or stress).
 Increased grain size - During prograde metamorphism or at a particular grade that is
maintained for a long period of time, minerals will tend to increase in size.
 Foliation - As new platy minerals grow, they will align themselves perpendicular to the
maximum stress direction. For clay mineral and fine-grained micas, the planar fabric that
results is referred to as a slaty cleavage. In higher grade rocks, coarser grained mica
minerals are said to impart a schistosity to the rock.
 Gneissic Banding - In very high grade rocks, the dark minerals tend to segregate from the
lighter colored minerals (feldspar and quartz) resulting in banded rock.
What Drives the Rock Cycle?
Plate Tectonic Theory
The Earth’s outer shell (lithosphere) is composed of rigid
plates that are moving relative to one another
OROGENESIS
Culmination of
Plate
Convergence
Continental
Rifting
The creation
of new ocean basins
Relative Time
Superposition
Faunal Succession
Geologic Time
Absolute Time
Radiometric Dating
Cross-cutting
Relationships
Unconformities
Gaps in the Geological Record
Siccar Point, Scotland
The Rock Cycle Recorded in the
Lake Superior Region