Earth and Environment Interlude A

Rocks

geologic definition of rock

coherent, naturally occurring solid that consists of an aggregate of minerals or less commonly a body of glass

what does it mean that rocks are coherent?

rocks hold together and must be broken in order to be separated into smaller pieces; rocks can therefore form cliffs or be carved into construction blocks or sculptures (e.g. a pile of unattached mineral grains does not constitute a rock)

what does it mean that rocks are naturally occurring?

only materials formed by natural processes are considered rocks (manufactured products like concrete etc. are not rocks)

what does it mean that rocks have aggregates of minerals/bodies of glass?

most rocks consist of an aggregate of many mineral grains and/or crystals

grain

general term that can refer to a piece of mineral, fragment broken off a once bigger piece of a mineral or preexisting rock, or to a fragment of glass

crystal

continuous piece of a single mineral that grew to its present shape and may display crystal faces

what holds rocks together?

cement, composed of minerals that precipitated from water in the space between grains /or because they interlock with one another and fit together like pieces in a jigsaw puzzle

clastic rocks

rocks whose grains are held in place by cement

crystalline rocks

rocks whose crystals interlock with one another

what are the most common elements in rocks of the Earth’s crust and mantle?

oxygen (44.8% in mantle, 46,6% in crust), and silicon (21,5% in mantle, 27,7% in crust)

bedrock

remains attached to the Earth’s crust (exposure of bedrock - outcrop)

three basic rock classes

1) igneous rocks (form by the freezing of molten rocks)

2) sedimentary rocks (form by the cementing together of grains broken off preexisting rocks or by the precipitation of mineral crystals out of water solutions at or near the Earth’s surface)

3) metamorphic rocks (form when preexisting rocks change character in response to a change in pressure and temperature conditions and/or as a result of squashing, stretching or shearing under conditions such that the rocks do not crack and break, occurs in the solid state)

how does grain size and shape differ between different rock types?

some grains are too small to be seen with a microscope, others are as big as a car or larger, also differ in terms of the range of grain sizes that the rocks contain and in terms of grain shape, some rocks contain grains that are all the same size, other rocks contain grains of many different sizes; in some rocks all grains are equant, i.e. they have the same dimensions in all directions, whereas in others the grains are inequant, i.e. the grains do not have the same dimensions in all directions)

how does rock composition differ between different rock types?

this refers to the proportions of different chemicals that make up the rock, the proportions of chemicals in turn affect the proportions of different minerals constituting the rock; but two rocks with the same chemical composition can have different assemblages of minerals if each rock formed under different pressure and temperature conditions

how does texture differ between different rocks?

this refers to the configuration of grains in a rock, i.e. the way grains connect to one another and whether or not inequant grains align parallel to eachother

how does layering differ between different rocks?

some rocks contain distinct layers (defined by bands of different compositions, grain sizes or textures or by the alignment of inequant grains so that they parallel eachother)

layering in sedimentary rocks

bedding

layering in metamorphic rocks

metamorphic foliation

studying rock - outcrop observations

the study of rocks begins by observing rocks in an outcrop, if the outcrop is big enough an examination will reveal relationships between the rock we’re interested in and the rocks around it and will allow us to detect layering; geologists record observations of an outcrop, then use a hammer to break off a hand specimen (fist-sized piece of a rock) that they can examine more closely with a hand lens (type of high-quality magnifying glass), which can enable geologists to identify sand-sized or larger mineral grains and allow them to characterize the rock’s texture

thin-section study

done to examine rock composition and texture in detail to identify a rock and develop a hypothesis for how it formed; make a very thin slice (0.03 mm) and mount it on a glass slide; can be studied with a petrographic microscope (illuminates the thin section with transmitted polarized light) - the illuminating light beam first passes through a filter which makes all the light waves in the beam vibrate in the same plane, and then up through the thin section, before entering the polarized eyepiece; when illuminated with transmitted polarized light each type of mineral grain displays a unique suite of colors - the specific color the observer sees depends on both the identity of the grain and its orientation with respect to the waves of polarized light, for a crystal interferes with polarized light and allows only certain wavelengths to pass through - this method allows geologists to identify most of the minerals constituting a rock and can describe the way in which the grains connect to one another; a photomicrograph can be taken

how is the thin section for thin-section study made?

using a special rock saw with a very thin, rapidly spinning, water-cooled diamond studded blade, which slowly grinds a very thin groove into the rock (the diamonds embedded in the saw blade scratch and pulverize minerals as the saw blade rubs against the rock); four cuts result in a chip of rock. this chip is glued with an epoxy adhesive to a glass slide and the excess rock is cut off with the rock saw, by pressing the slide facedown against a lap the chip can be ground down until only a thin slice, still cemented to the glass slide, remains, ready for examination with a petrographic microscope

SEMs

scanning electron microscopes, can image the surface of a rock chip at extremely high magnification and map the distribution of elements in the chip

electron microbe

can focus a beam of electrons on a small part of a grain to create a signal that defines the chemical composition of the mineral

mass spectrometer

analyze the proportions of different isotopes of elements contained in a rock

X-ray diffractometers

identify minerals by looking at the way X-ray beams pass through crystals in a rock