GEOLOGY BASICS
Earth's Internal Structure
The Earth's interior is characterized by a
gradual increase in temperature, pressure and
density with depth.
At only 100 km depth, the temp is ~1300°C.
At the Earth's center, the temperature is
>6700°C.
The pressure in the crust increases ~280 bars
for every kilometer depth
Interior of the Earth (overall density = 5.5 g/cm3)
Earth’s interior is made up of three layers:
1. Crust is the thin (<100 km) outermost layer of the Earth and has a density of 2.5–3 g/cm3.
2. Mantle is the thick (2,900 km), solid layer between the crust and the Earth’s core. Density of the mantle is 3–9 g/cm3. The crust “floats” on top of the mantle.
3. Core is the central part of the Earth, composed of a solid inner core and a liquid outer core. Density of the core ranges from 9–13 g/cm3 and is probably composed of iron and nickel.
Layering by Strength Most of the Earth is not molten and most of the lava
from volcanoes rises upward from a narrow region of the mantle which is partially molten.
The shape of a planet is determined by the strength and fluidity of the inside as well as the strength of gravity Large worlds (> 500 km diameter) are round
Small worlds are irregular in shape
The crust and the top part of the mantle is relatively cool region of rock called the lithosphere that floats on the rest of the mantle.
Lithosphere (0 to ~100 km)
It's very stiff, and fractures if you push too
hard
The outer 75 km (with big variations between
10 and 300km) of the earth is a region which
does not get heated up to near-melting because
it is losing heat rapidly to the surface - it is
stuck at a temperature close to 0°C. This
relatively cool shell is called the lithosphere.
Plate Tectonics
A relatively recent theory that the Earth's crust is composed of rigid plates that move relative to one another.
Plate movements are on the order of a few centimeters/year - about the same rate as your fingernails grow!
There are 3 types of plate boundaries:
• 1. divergent
• 2. convergent
• 3. transform
Convergent boundaries - plates move
together forming a subduction zone and
mountain chains.
Divergent boundaries - plates move apart
forming the mid-ocean ridge(edge,fold) and
seafloor spreading.
Transform boundaries - plates grind past
one another. These boundaries subdivide the
mid-ocean ridge and also form the San
Andreas fault system.
• A simplified model of tectonic plates and the
location and nature of earthquakes.
Plate Boundaries
Plate boundaries are the boundaries where the real action occurs. The plates are all moving relative to each other. At the boundary between
two plates, there must be some motion of one relative to the other. You get three possibilities:
Spreading center: Divergent boundary
At the top of a rising convection limb. Heat is being brought up. Volcanism. Usually under-ocean. Often associated with a rift valley.
Collision zone: Convergent boundary
Cold lithosphere bends downward and begins sinking into the mantle (subduction). Mountains are squeezed up here by the collision. Most earthquakes occur here.
Parallel plate motion: Transform / Transcurrent / Strike Slip faulting
The San Andreas Fault is the most famous transform fault system.
Geologic Time
Absolute (Radiometric) Dating: Using radioactive decay of elements to determine the absolute
age of rocks. This is done using igneous and metamorphic
rocks.
Types of Rocks
There are three types of rocks
1. Igneous rocks
2. Sedimentary rocks
3. Metamorphic rocks
1. IGNEOUS ROCKS
• Two Types of Igneous Rocks
– Extrusive (Exterior)
– Intrusive (Interior)
Igneous rocks that solidify into rock beneath Earth’s
surface
Coarse Grained
Cool Slowly
Granite
Extrusive Igneous Rocks
Extrusive (Exterior)
Igneous rocks that solidify into rock on Earth’s
surface
Fine Grained
Cool Quickly
INTRUSIVE IGNEOUS ROCKS Intrusive (Interior)
Igneous rocks that solidify into rock beneath
Earth’s surface
Coarse Grained
Cool Slowly
Granite
IGNEOUS ROCK FORMATION
ORIGIN OF MAGMA
Where does the heat come from that melts rocks?
– Formation of Earth
– Heat from the decay of radioactive elements
Factors that Affect Magma Formation
• Temperature
– Increases with depth
• Pressure
– Increases with depth
• Water Content
– Decreases melting point
• Mineral Composition
– Different minerals, different melting points
2. METAMORPHIC ROCKS
Metamorphism
Transition of one rock into another by temperatures and/or pressures unlike those in which it formed
Metamorphic rocks are produced from
Igneous rocks
Sedimentary rocks
Other metamorphic rocks
Metamorphism progresses incrementally from low-grade to high-grade
During metamorphism the rock must remain solid
AGENTS OF METAMORPHISM
Heat
Most important agent
Recrystallization results in new, stable minerals
Two sources of heat
– Contact metamorphism – heat from magma
– An increase in temperature with depth -
geothermal gradient (20 to 30 degrees Celsius per
kilometer of depth)
Pressure and differential stress Increases with depth
Confining pressure applies forces equally in all directions
Rocks may also be subjected to differential stress which is unequal in different directions
Chemically active fluids Mainly water Enhances migration of ions Aids in recrystallization of existing minerals Sources of fluids
Pore spaces of sedimentary rocks Fractures in igneous rocks Hydrated minerals such as clays and micas
COMMON METAMORPHIC ROCKS
• Foliated rocks
Slate
– Very fine-grained
– Excellent rock cleavage
– Most often generated from low-grade metamorphism of shale, mudstone, or siltstone
Phyllite
– Gradational between slate and schist
– Platy minerals not large enough to be identified with the unaided eye
– Glossy sheen and wavy surfaces
– Exhibits rock cleavage
– Composed mainly of fine crystals of muscovite and/or chlorite
Slate (left) and Phyllite (right)
Schist
– Medium- to coarse-grained
– Platy minerals (mainly micas) predominate
– The term schist describes the texture
– To indicate composition, mineral names are used
(such as mica schist)
Gneiss
– Medium- to coarse-grained
– Banded appearance
– High-grade metamorphism
– Often composed of light-colored feldspar-rich layers
with bands of dark ferromagnesian minerals
Garnet-mica schist
• Nonfoliated rocks
Marble
– Coarse, crystalline
– Parent rock was limestone or dolostone
– Composed essentially of calcite or dolomite crystals
– Used as a decorative and monument stone
– Exhibits a variety of colors
Quartzite
– Formed from a parent rock of quartz-rich sandstone
– Quartz grains are fused together
Quartzite (Left) Marble (Right)
3. SEDIMENTARY ROCKS
Sediment becomes sedimentary rock through
lithification, which involves:
Compaction
Cementation
Recrystallization (of carbonate sediment)
Process of Sedimentary Rock Formation
Types of sedimentary rocks
Overview Terrigenous (detrital or clastic)
– Conglomerate or Breccia
– Sandstone
– Siltstone
– Shale
Chemical/biochemical – Evaporites
– Carbonate sedimentary rocks (limestones and dolostone)
– Siliceous sedimentary rocks
Organic (coals) – Other - ironstones
Sedimentary Structures
Sedimentary Environments
Fossils
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