The Rocktypes

The Rocktypes:

Notes for my students. I am reading the book “Petrology – Igneous, Sedimentary and Metamorphic” by Ehlers and Blatt after a long time. This is my own notes that I found most interesting from the first chapter named “Why Petrology?” There are many questions to be answered by studying various rock types.

What is the radius of Earth?
6,371 km {Note: crust is only outer .27% (.74% by volume)}

Did the early Earth have oxygen?
Yes. Hematitic Iron Ores were common and abundant even in earliest Precambrian rocks.

Past Vs present:

• Dolomite 3x more abundant in precambrian rocks than Limestone. Present day dolomite formation is extremely rare and restricted.
• Middle Precambrian rock contain 15% silica as Chert. Present day Chert formation is very rare.
• Evaporite formation is very common in present time, but very rare during the Precambrian.

Relative Surface Abundance of Rock Types:
about 66% of the surface is made of sedimentary rocks.

Crustal Abundance of Rocks:

Is Crust=Lithosphere? No. Crust = the outer shell of earth above the Mohorovicic discontinuity. Lithosphere = Crust + upper portion of mantle (about 100 km depth)
Asthenosphere – layer beyow lithosphere with lower seismic velocity than layer above.
What is the average thickness of crust? 17 km.
What is the average thickness of sediments on continental block? 1.8 km  (5905.51 feet) – 35% of the total crust
What is the average thickness of sediments on oceanic block? 0.3 km ( 984.25 feet) – about 65% of the total crust

What would be the average? Do math.
Continents – mainly granitic rocks
Oceanic basins – mainly basaltic rocks.