BIO 113 — Dinosaurs
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Lab 2
Types of Rocks

See also Fossils Page

See also Lagerstätte Page

Igneous Rocks

  • Formed by solidification of molten magma or lava
  • Most common type of rock
  • Scattered crystals of different minerals may or may not be visible to the naked eye
  • Do not contain fossils

granite outcrop granite 1 granite 2
Outcrops of granite rock. The dark color is due to desert varnish, not the rocks themselves. Piestewa Peak, AZ A red Granite. Granite. Note patches of different colored crystals. Usually light colored

basalt outcrop basalt 1 basalt 2
A basalt lava flow at Sunset Crater, AZ Basalt. Often has numerous small cavities Rhyolite. Rhyolites are lighter colored. Cavities may be few or absent in some

andeside & obsidian Peridotite 1 gabbro & diorite
Andesite (left) and Obsidian (right) Peridotite Gabbro (left) and Diorite (right)

Metamorphic Rocks

  • Rocks that have been distorted by high heat and pressure
  • Variable, but often contain layered patterns of minerals
  • Rarely contain fossils

gneiss Marble slate
Gneiss. Note the layers of different colored minerals Marble. An often whitish, soft rock (derived from limestone) Slate. A lightly transformed from shale; it can still contain fossils. Texture is fine-grained; hard and splits horizontally

Sedimentary Rocks

  • Form when sediments are fused together
  • Or form through the precipitation of dissolved minerals
  • Characteristics reflect the sediments on which it is based (such as grain size)
  • Contain the vast majority of fossils

Sandstone

Sandstones are composed of relatively coarse sand-sized particles that are visible and impart a grainy texture. Sandstones are usually a poor source of molds and casts (except trackways), but can contain permineralized bone or wood. Reddish colors are common.

sandstone outcrop sandstone sandstone
Layered sandstone at Glen Canyon NRA, AZ. Sandstone. The colored layers result from accumulations of different types of sand (not all sandstone is layered). Sandstone. Closeup showing grainy texture

Mudstones and Shales

Both mudstones (including both siltstones and claystones) and shales are composed of finer particles that cannot be distinguished by the naked eye, so they lack the grainy texture of sandstones. They differ in how the particles are cemented together. Mudstones are the least rock-like and easily crumble into irregular-shaped blocks, while shales tend to be harder and split into flat horizontal plates. Both are good sources of fossils.

sandstone outcrop mudstone shale
Mudstone cliffs at Petrified Forest National Monument, AZ. Note the eroded surface. Mudstone (specifically siltstone) rock. Shale splits into horizontal layers

Limestone

Limestone is characterized by a high concentration of calcite (calcium carbonate) that strongly cements the sediment particles together. Superficially it can resemble mudstones and shales, but it typically is harder and will fizz when exposed to acid. The amount of calcite can vary, so there is no definitive line between limestone and mudstone or shale. The calcium carbonate is often (but not always) derived from the shells of tiny marine organisms and algae, and thus is a good indicator of fossil-bearing aquatic sediments.

Limestone outcrop limestone coquina limestone
Limestone cliffs. The rounded edges and pitted surface are typical of limestone because, while hard and resistant to physical erosion, it can dissolve especially if the water is acidic. Natural Bridges S.P., KY. Limestone. The texture of limestone can resemble shale or mudstone, but it is usually harder and less likely to split or crumble. Fizzes when exposed to acid. Coquina Limestone. Limestone that is composed primarily of visible shelly material is called coquina.

Conglomerate & Breccia

In both conglomerate and breccia, large particles (gravel, stones, etc.) are cemented into a finer matrix. The particles in conglomerate rocks are rounded, indicating they have eroded, such as in a river. Particles in breccia have sharp angles, indicating they were broken off the parent rock (such as by landslides) without any opportunity for erosion. Neither are likely to contain fossils.

Conglomerate Conglomerate Breccia
Conglomerate alternating with sandstone. Conglomerate rock, cut and polished. Breccia. Differs from conglomerate in that the embedded rocks are angular rather than rounded

Other Chemical Sedimentary Rocks

As with limestones, these rocks are produced by chemical processes rather than simply cementing a substrate together.

Chert Tiger Iron Formation Coal
Chert. Chert is often associated with limestone but results from the precipitation of silica rather than calcite. Forms curved surfaces when split. Red, green, and brown coloration is common. It can contain small fossils. The red layers in this ancient "banded iron formation" are chert produced during anoxic periods that alternate with layers produceded by ancient bacteria 2.5 bya. Coal is a carbon-rich sedimentary rock (or metamorphic rock in the case of anthracite coal) derived from dense accumulations of plant material such as from peat swamps. It is considered a chemical fossil.

This page last updated 3 September 2014 by Udo M. Savalli ()
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