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