The crust is of two different types. One is the continental crust (under the land) and the other is the oceanic crust (under the ocean). The continental crust is thicker, 30 km (20 mi) to 50 km (30 mi) thick. It is mostly made of less dense, more felsic rocks, such as granite. The oceanic crust is thinner, 5 km (3 mi) to 10 km (6 mi) thick. It is made of denser, more mafic rocks, such as basalt.
The temperature of the crust increases with depth because of geothermal energy. Where the crust meets the mantle the temperatures can be between 200 °C (392 °F) to 400 °C (752 °F). The crust is the coldest layer because it is exposed to the atmosphere.
Igneous rocks make up over 90% of Earth's crust by volume. This is not noticeable because they are mostly covered by sedimentary and metamorphic rocks.
A single family of silicates, the feldspars, account for about half of the material in the crust (60% by weight), and quartz is a sizeable proportion of the rest. Other common minerals are mica and hornblende.
Only 8% of Earth's crust is non-silicate minerals, and this includes carbonates, sulfides, chlorides and oxides.
Earth's mantle and crust formed about 100 million years after the formation of the planet, about 4.6 billion years ago. At first the crust was very thin, and was probably changed often as the tectonic plates shifted around a lot more than they do now. The crust would have been destroyed many times by asteroids hitting Earth, which was much more common in the Late Heavy Bombardment.
The oldest oceanic basalt crust today is only about 200 million years. Most of the continental crust is much older. The oldest continental crustal rocks on Earth are cratons between 3.7 and 4.28 billion years old. These have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on the Fennoscandian Shield. A few zircons at least 4.3 billion years old have been found in the Narryer Gneiss Terrane in Western Australia.
The average age of Earth's continental crust is about 2.0 billion years. Most crustal rocks formed before 2.5 billion years ago are in cratons. Such old continental crust and the mantle below it are less dense than other places in the Earth. These are not easily destroyed when the plates shift. The making of new continental crust is linked to times of major orogeny or mountain building. This happens at the same time as the formation of the supercontinents such as Rodinia, Pangaea and Gondwana. The crust forms in part by the coming together of island arcs including granite and metamorphic fold belts. They are kept together partly by the subduction of the mantle below the crust, which makes a mantle on which the crust can float.
Food chain
A food chain shows the feeding relationship between different organisms in a particular environment and/or habitat. Plants are at the bottom of a food chain because they are producers that make their food from a process called photosynthesis . Organisms that can make their own food are called producers. Consumers are animals that eat the products of producers or other animals. The animal that eats the producer is called the primary consumer and it gains the most amount of energy. The animal that eats the primary consumer is called a secondary consumer which in the primary consumer's energy which is transferred becomes lower due to the primary consumer consuming some energy. The animal that eats the secondary consumer is called a tertiary consumer and so on to fourth, fifth, etc. The direction of arrows between the organisms shows who eats what and what gets eaten by what.
A food chain also represents a series of events and consumption in which food and energy are consumed from one organism in an ecosystem to another. Food chains show how energy is passed from the sun to producers, from producers to consumers, and from consumers to decomposes such as fungi. They also show how animals depend on other organisms for food.
In any ecosystem, many food chains overlap. Different food chains may include some of the same organisms. Several consumers may eat the same kind of plant or animal for food. When this happens, the food chain forms a food web. A food web shows how food chains are related within an ecosystem and how the cycles balanced between an equilibrium
Embryology
Embryology is the study of embryos and their development. The study of embryology starts with the fertilisation of an egg, and continues until the foetus stage. A broader term, developmental biology, covers the whole period of growth from the egg to adult life.
In mammals (and some other animals) embryos develop inside the mother, in the mother's womb. Otherwise it takes place after the egg is laid. It most animals, there are various stages between the egg and the adult animal. If the stages are well-defined the process is called metamorphosis.
Histology
Histology is the study of the microscopic anatomy of cells and tissues of plants and animals, particularly the tissues. It is a part of cytology, and an essential tool of biology and medicine.
Histology is usually done by looking at cells and tissues under a light microscope or electron microscope. The tissue has to be specially prepared beforehand.
The stages below are only described in outline. Laboratories which do histology work from schedules which are much more detailed.
Chemical fixatives are used to preserve tissue from decay. This preserves the structure of the cell and of sub-cellular components such as cell organelles (e.g., nucleus, endoplasmic reticulum, mitochondria). The most common fixative for light microscopy is formalin (4% formaldehyde in saline).
After fixing, the block of tissue is embedded in paraffin wax. This holds and preserves the tissue as a block.
The section is cut into a series of wafer-thin slices, each of which is put on a glass microscope slide. The machine which cuts the block is a mechanical guillotine which can be set to cut at a suitable depth for the tissue in question.
Stains are dyes, chemicals used to make cells and tissues easy to see under a microscope. There are many tissue stains, and each of them has advantages and disadvantages.
This is the most widely used stain in biology and medicine. Haematoxylin colours cell nuclei and eosin colours cell cytoplasm.
Camillo Golgi developed a silver nitrate stain for nerve cells. His idea was used by Santiago Ramón y Cajal in his famous work on the structure of brain tissue.
Electron microscopy is used often as well as light microscopy. This has its own procedures. Its advantage is that it resolves things which light cannot resolve. For example, viruses were first seen by electron microscopy..
Specialised selective stains using immunology or radioactive labelling are now routine. The advantage of using antibodies or radioactive labels is that they stick to "specific kinds of molecules".
Increasingly popular is tagging with a fluorescent stain, which shows up even if a tiny part of a cell is stained. Immunoinfluorescence is the name of this particular technique.
Ann Arbor, Michigan
Ann Arbor is a city in the US state of Michigan. It is the county seat of Washtenaw County. According to the 2010 United States Census, Ann Arbor was the 6th largest city in Michigan. In 2010, there were 113,934 people living there. The city is part of the Detroit Metropolitan Combined Statistial Area.
Ann Arbor was started in 1824. It is thought that the city is named after the wife, Ann, of one of the founders. The University of Michigan was moved to Ann Arbor in 1837. (Before that, the University had been in Detroit.) The city grew during the 1800s and 1900s. The only time the city did not grow was the Depression of 1873.
During the 1960s and 1970s, Ann Arbor was an important place for the Civil Rights Movement, the anti-Vietnam War movement, and for left-wing and liberal politics.