Ocean Structure - notes PDF

Preview

Summary

notes ...

Description

EVSC 1010 Lecture Outline Ocean Structure Hydrosphere  Seven seas, five oceans o Measured in depth, positive #’s above sea level negative #’s below sea level o Average depth of the oceans is -3800 m  Ocean basins o Mid-oceanic ridge: continuous submarine mountain chain that encircles the globe. It’s total length is 80,000 km, and it is more than 1,500 km wide in places.  Associated with a rift valley; As the plates separate, blocks of rock drop down along the fractures to form the rift valley. The moving blocks cause earthquakes.  Associated with divergent tectonic boundary o Abyssal plane  Vast majority of ocean made up of this  Layer of sediments covering up oceanic crust material  The three layers of oceanic crust.  Layer 1 consists of sediment. o Ocean floor sediments in the abyssal plain coming from direct precipitation of minerals from the water, or microscopic organisms’ remains (made up of calcium carbonate- bioclastic)  Layer 2 is pillow basalt. o Basalts that make up crust material = extrusive igneous rock; pillow lava rocks  Layer 3 consists of vertical dikes overlying gabbro.  Below layer 3 is the upper mantle. o Continental margin: zone of the ocean floor that separates the thin oceanic crust from the thick continental crust  Two broad categories: (see diagrams)  Active: associated with a tectonic boundary (mainly convergent). E.g. pacific coast. The continental shelf is narrow, the slope is steep, and the continental rise is nonexistent.  Passive: not associated with a tectonic boundary. E.g. Atlantic coast. Dominant process: deposition; continuous flow of fresh water/sediment eroded from the continent it’s carrying—builds up a broad continental shelf, slope, and rise  Ocean water o The hydrologic cycle shows that water circulates constantly among the sea, the atmosphere, and the land. Over history, oceans have become more salty b/c greater evaporation of ocean water than precipitation o Salinity: expressed by amount of salt found in 1,000 g of water. Avg ocean salinity: 35 parts per thousand (3.5 % salt).  Salinity as a function of 2 things

 Amount of fresh water coming in to ocean waters thru precipitation  Evaporation: amount of fresh water evaporating off the surface  Latitudinal patterns of ocean salinity  Salinity of the top layer of the ocean is closely linked with precipitation and evaporation. Evaporation leaves behind dissolved salts increasing salinity and precipitation "freshens" the top ocean layers.  Salinity is high in mid-latitudes where evaporation is high and precipitation is low (permanent low pressure systems with minimum amounts of precipitation coming in)  Salinity is low near the equator because precipitation is so high.  Very high latitudes can also see decreases in salinity where sea ice melts and "freshens" the water  Lower salinities along coastal environments b/c constantly being fed by incoming fresh water. o Availability of light  As light moves through, water & photoplankton absorb short-wave radiation  decrease in amount of sunlight as you move from surface to bottom of ocean  Light attenuation: the decline in light as you move down in depth

 o Vertical profile of water temperature

 

Turnover: The vertical mixing of water in a lake or ocean basin that occurs between the autumn and spring (in the temperate and polar regions)  Water temperature is a function of thermal energy content, so you might hypothesize that the general pattern of temperature with depth might parallel the observed pattern for light (shortwave radiation). However, this is not the case. Initially, the decline in water temperature “lags behind” the decline in light with depth. o Relationship between water temperature and density





Temperature and density share an inverse relationship. As temperature increases (after 4 degrees celcius), the space between water molecules increases—also known as density, which therefore decreases. If the temperature of water decreases its density increases, but only to a point. o Thermocline  Thin layer in a large body of fluid in which temperature changes more rapidly with depth than it does in the layers above or below.  (see vertical water temperature profile pic) o Pycnocline  A rapid change in water density with depth  In freshwater environments such as lakes this density change is primarily caused by water temperature  In seawater environments such as oceans the density change may be caused by changes in water temperature and/or salinity. o Seasonal dynamics of thermocline

 

Seasonal changes in water temperature, clearly evident in the mean temperature profile, penetrate the water column to a maximum depth of about 30 meters. The evolution of the thermocline with time and depth shows the lag between the solar solstice, which occurs around June 21, and the maximum depth of the thermocline, which occurs in mid September. Surface water is at its lowest temperature in February, while at 70 meters depth the water does not reach its lowest temperature until April o Surface waves – characteristics  Crest/base/height  Wavelength: horizontal distance b/w two crests

 

Depth of turbulence/wave action  When surface water molecules move by the force of the wind, they, in turn, drag deeper layers of water molecules below them.  Each layer of water molecules is moved by friction from the shallower layer, and each deeper layer moves more slowly than the layer above it, until the movement ceases at a depth of about 100 meters (330 feet).  Vertical motion extends to about ½ of the wavelength...