Presentation on theme: "Earth: The Biography Atmosphere. Sedimentary rocks are normally deposited as horizontal layers (law of original horizontality). Even when folded or tilted."— Presentation transcript:
Earth: The Biography Atmosphere
Sedimentary rocks are normally deposited as horizontal layers (law of original horizontality). Even when folded or tilted by faulting and tectonic forces, the originally horizontal layering is usually obvious. Upon close examination of “The Wave” however, one can’t help but notice that there are sets of layers or beds that are tilted at different angles. These layers of tilted beds are called cross-bedding, and are a clear indication that the environment of deposition was vast region of sand dunes!
Cross-bedding is a tell-tale indicator for ripples and dunes, with ripples being formed by the deposition of sand by water, and dunes being formed by the deposition of sand by wind.
Windward (Stoss) Slope Slipface (Leeward Slope)
Creep and saltation move sediments up the windward or stoss slope of the dunes. At the crest of the dune, the piles up. When the sediment pile reaches a height that is unstable—called the angle of repose—the grains will avalanche or tumble down the back side slipface (leeward side) of the pile. These tumbling grains form a thin layer along the slipface of the dune. Over time, multiple avalanching episodes will result in many thin parallel layers next to one another. These layers are called cross-bedding, because they are not the typical horizontal beds most often observed in deposition. Windward (Stoss) Slope Slipface (Leeward Slope) Windward (Stoss) Slope
Because Mount Shasta pokes up into the sky above, air is forced to rise above it. Mount Shasta is over 11,000 feet above the valley floor, and so the air is cooled significantly. The height where clouds begin to form indicates where the water vapor has cooled sufficiently to begin condensing out of the air. The lenticular or cap clouds seen above Mount Shasta are formed in this way. Sometimes there are several "saucers" in the air at one time. These clouds form in the "crests" of the wave of air caused by the peaks below or from undulating winds.
Interesting page on standing atmospheric waves and mountain clouds: Clouds form as air rises and cools. If the air cools enough, then the dew- point temperature is reached and the humidity is 100% (if not more), which causes clouds to form. The cloud dissipates as the air sinks down to the next trough but reforms again as it rises back at the top of the next wave downstream. Sometimes a large mountain can produce lenticular clouds that form hundreds of miles downstream as well
For more amazing photos of mountain and lenticular clouds: https://greatatmosphere.wordpress.com/tag/lenticular/ https://greatatmosphere.wordpress.com/tag/lenticular/
Kelvin-Helmholtz wave clouds are formed when there are two parallel layers of air that are usually moving at different speeds and in opposite directions. The upper layer of air usually moves faster than the lower layer because there is less friction. In order for us to see this shear layer, there must be enough water vapor in the air for a cloud to form. Even if clouds are not present to reveal the shear layer, pilots need to be aware of invisible atmospheric phenomenon.
Image from the Terra Satellite on Saturday showing clouds being produced by gravity waves. The waves were formed as winds flowed over the Appalachians. See the ripples in the atmosphere? This is the atmospheric equivalent of throwing a rock in a pond. Instead of a series of waves emanating from the spot the rock entered, we have cloud waves as the winds blow over the Appalachian Mountains. Sometimes the wavelength between the clouds makes it difficult to see from the ground, but from space, the ripples in the atmosphere are quite clear.
These are Morning Glory clouds, a very rare type of cloud formation seen in Australia. They look like long lines of rolling clouds moving across the landscape. They can be more than 1,000 km long and 1-2 km high. They move 60 km/h across the landscape.
Stromatolites: layered, organosedimentary structures created by cyanobacteria, which are the oldest oxygen-producing organisms on Earth. ntary/images/stromatolite.html
Banded Iron Formation, Australia Close-up of banded iron from Minnesota. Banded iron-formation composed of alternating layers of iron-rich material from Thabazimbi, South Africa.
BIFs are rocks of the Proterozoic Era ranging from 1.8 to 2.5 billion years in age and consist of alternating iron-rich and iron-poor layers, typically only millimeters to centimeters thick. Most BIFs are strikingly colorful with the dark layers being made up mainly of the iron oxide minerals, hematite (Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) and red layers of jasper, a variety of chalcedony, or very fine-grained quartz (SiO 2 ) (Mathez, 2006). Banded iron formations are found throughout the geological record, but the period from 2.5 to 2.0 billion years represents a unique time in Earth history, a time during which 92% of the Earth’s BIFs were laid down (Immenhauser, 2005). For this enormous accumulation of iron oxide to have occurred over such a vast time span meant that something about the chemistry of early earth was very different from today. The chemistry of rocks that date to the Proterozoic shows that oxygen was a rare gas in the atmosphere during this time. The key to understanding the chemical reactions occurring in the early oceans is in the relationship between the elements oxygen and iron. Iron forms two ionic states, namely, ferrous (Fe +2 ) and ferric (Fe +3 ). The +2 or +3 indicates the extent to which iron is oxidized. Iron will only dissolve in significant quantities in water that contains no oxygen (anoxic water). In anoxic water iron dissolves in the ferrous state as ions of hydrous Fe +2, or FeOH + (Mathez, 2006). Therefore, in order for iron-rich chemical precipitates to form, the early oceans must have been sufficiently anoxic to dissolve iron. Inference! Since the ocean and atmosphere exchange oxygen rapidly, the early atmosphere could not have contained much oxygen, either. Inference!
But oxygen was in the making! Primitive photosynthetic blue-green algae (cyanobacteria) were beginning to proliferate in shallow-water zones. Photosynthesis from cyanobacteria that dominated the early oceans would have created a net gain of oxygen, first in the ocean and later in the atmosphere (Attenborough, 2010). Oxygen (O 2 ) was a waste product for the photosynthesizing bacteria, and was released into the water. This free oxygen would combine with the iron dissolved in the ocean water to form iron oxides. The soluble ferrous iron that was dissolved in ocean water reacted with this oxygen and rained down onto the ocean floors as insoluble, rust colored deposits of hematite and magnetite. “As the cyanobacteria population expanded beyond the capacity for the available iron to combine with waste O 2, the oxygen content of the sea water rose to toxic levels and resulted in their large-scale die-off, which in turn gave rise to an iron poor layer of silica on the sea floor”. In other words, many early cyanobacteria colonies poisoned themselves over thousands of years! As time passed, cyanobacteria populations would re-established themselves, and a new iron- rich layer would begin to accumulate on ocean floors. This cycle was repeated and continued for hundreds of millions of years. “Each band in the iron formation is similar to an annual layer of sediment – or varve - to the extent that the banding is assumed to result from cyclic variations in available oxygen” (Kirschvink, 1992).
For over 2 billion years this went on, until the iron in earth’s oceans was depleted. Since there was no iron left in solution, the excess molecular oxygen bubbled up into the atmosphere and began accumulating from about 1700 million years ago (1.7 Ga), after two- thirds of Earth history. The vast layers of iron minerals stayed behind in the Banded-iron formations. The rise in the levels of oxygen after the massive depletion of iron meant that photosynthesizing bacteria would face near extinction as oxygen is a reactive and highly toxic gas (Southwood, 2003). Cells would eventually to adapt to this change in atmospheric conditions, and the excess oxygen would ultimately lead to the formation of an ozone layer. The proliferation of new life forms in an oxygenated world led to the so-called “Cambrian Explosion”.
2.1 billion years old rock containing black- banded ironstone, which has a weight of about 8.5 tons. The approximately two meter high, three meter wide, and one meter thick block of stone was found in North America