EARTH 222 Lecture 8: Clastic and Hydrothermal Sediments

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January 30, 2017
Clastic and Hydrothermal Sediments
Sediments
Types
Clastic sediments
Composed of fragments or grains derived from exiting rocks by weathering, erosion,
transport, and deposition
Includes clay, silt, sand, and gravel (decreasing fineness)
Deposited by water, wind, and glaciers
Clastic transport
Most clastic transported by water or air
Dominant source of sediments to the ocean is rivers, which carry 90% of clastic grains
to the ocean
Rivers carry 1.5e16g annually, enough to fill the Big House 4,000 times
The largest river in the world by discharge is the Amazon
The Ganges River drains the Himalaya Mountains and delivers the most
sediment to the ocean
Chemical sediments
Formed in place, typically by precipitation or crystallization from a fluid (seawater)
Biological sediments
Product of precipitation by organisms as they grow (clams, foraminifera, diatoms)
Thickness
Thick sediments are likely to be thicker on older crust
General bathymetry
Shelf
50-200m deep
10-100km wide
Composed of sand and mud
Slope
Where shelf drops off into the deep ocean
Mostly muds
Rise
Muds and turbidities
Abyssal plain
3,000-6,000m deep
Fine-grained muds and biogenic sediments
Decreasing energy from shelf to plain
Turbidity currents
Primary mechanism for moving sediments from shelf-slope to rise
Initiated by slumping of sediments on slope (often earthquake triggered), causing submarine
landslide down slope and rise to abyssal plains, where they come to rest
Creates dense turbid layer of mud and sand
Upon reaching gentler rise, flow slows and drops its coarse sediment load
Further slowing and sediment deposition occurs on abyssal plain
What do you think happens to water just above upwelling magma at mid-ocean ridges
It heats up and rises
Hydrothermal plumes
Metals (e.g., Fe, Mn, Ni, Cu) are leached from oceanic crust by hydrothermal circulation
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