Geo 406 Lecture 1-Introduction
Goals for today
assess current knowledge
learn course goals
course organization
intro to petrology
review earth structure
Course Goals
Identify Igneous & Metamorphic Rocks
In hand specimen & thin section
Understand basics of their origin and history
Be able to read literature for more detailed info
Be able to infer tectonics from field occurrences
Get excited about rocks and their stories
Course Organization / Logistics - syllabus
Different type of course from Mineralogy
grades are primarily exam-based
labs are graded with checks, but I will read them more carefully. Keys will be
posted.
Must keep up with reading – there is lots to learn.
Some days will be lectures, others will be doing reading review problems in small
groups with mini lectures as needed.
Sometimes there will be no warning!
Exams
non-cumulative “final” – equal weight w/ midterm
Labs
graded with √, √-, √+
keys posted
Two lab exams, one on igneous, other on metamorphic rocks
MUST have hand lens at every lab; think about identifying minerals when they are
only a millimeter across!
Field Trip(s)
One for credit to BC: examining igneous & metamorphic rocks, camping
w/writeup
May 16-17
Learning objectives
Important! Online.
Course organization
Igneous rocks
Physical and chemical properties
Classification systems
Phase diagrams
Overview of plate tectonic environments, and the igneous rocks that form in each
Metamorphic
Similar: Chemistry and background, then real rocks
Lab:
Identifying textures, structures
Classifying rocks
Describing rocks
Assessment of knowledge – see Keynote
What is Petrology?
Study of rocks and their formation
We will first discuss how melts are created, then how they change and
crystallize (or not). In order to understand melts, we need to think about the
large-scale structure of the earth, then focus in.
Earth Structure
ASK & tell: Describe earth’s structure (without numbers, just with material
properties, compositions, equiv. rock types)
Key numbers: avg. base of crust = 30km (5-10 oceans, 100 Himalayas)
Asthenosphere: partially molten (3% melt), ductile
Lithosphere:
ocean: ~100 km
cont: ~200 km
Mantle mineralogy:
Moho - ~20km: Plag + ol (+ opx + cpx ± Fe-Cr oxide)
20 - ~60km: spinel + ol (+ opx + cpx ± Fe-Cr oxide)
> 60 km: garnet + ol (+ opx + cpx ± Fe-Cr oxide)
Core: don’t care, but core-mantle boundary:
“D’’ layer” in seismic data
Source of mantle plumes?
Large topography on core-mantle boundary: 3-13 km, depending on the study
How does pressure increase in the earth?
P = gh
Crust density = ~2.7 g/cc
Upper mantle density = ~3.3
ASK: Figure it out at the base of the continental crust (36 km, 2.8 g/cm^3):
P = (2.8 g cm-3)(9.8 m s-2)(33 km)
Note that 1 GPa = 1e9 Pa = 1e9 N m-2 = 1e9 (kg m s-2)(m-2) = 1e9 kg m-1 s-2
So P = 905 g m km cm-3 s-2
= (905 g m km cm-3 s-2)(1e6 cm3 m-3)(1e3 m km-1)(1e-3 kg g-1)
= 905 Pa = 0.9 GPa
P increases about 33 km per GPa (=10 kbar)
= 0.03 GPa / km
= 0.3 kbar / km
P at base of typical continental crust
Temperature: increases with depth
In all cases?
Where is this not true?
On a typical geotherm, where is the temperature relative to the melting temperature?
Somewhat below, except in Asthenosphere and outer core