Geology and Fossils

Earth’s Surface – Rocks

Strata Strata are the horizontal layers of material usually arranged one on top of another. A bed or layer of sedimentary rock having approximately the same composition throughout.

Stratigraphy The study of rock strata, especially the distribution, deposition, and age of sedimentary rocks. Stratigraphy is a method of dating fossils by observing how deeply a fossil is buried. Sedimentary rock layers (strata) are formed episodically as earth is deposited horizontally over time. Newer layers are formed on top of older layers, pressurizing them into rocks. Paleontologists can estimate the amount of time that has passed since the stratum containing the fossil was formed. Generally, deeper rocks and fossils are older than those found above them.

Fossils Fossils are relics of organisms that lived and died long ago. (little or no organic matter left) The four types of fossils are: mold fossils (a fossilized impression made in the substrate - a negative image of the organism) cast fossils (formed when a mold is filled in) trace fossils = ichnofossils (fossilized nests, gastroliths, burrows, footprints, etc.) true form fossils (fossils of the actual animal or animal part). An animal dies and gets covered in sediment. Minerals dissolved in groundwater gradually replace organic material. Mineral-rich portions of the animal (bones, teeth, shells) may be left behind, becoming fossils. If mineral replacement is complete, the organism turns to stone (“stone trees” of the petrified forest). Mold Fossils = Sometimes, the organism decays, leaving and empty mold that fills with minerals dissolved in water. Then a shape is cast of the dead organism. Rarely, some organic material is preserved well enough for some study. Some fossil leaves are still green from millions of years ago. Whole organisms can be caught in tree resin (amber) or ice. Or, you can have a fossil of an ACTIVITY of an organism while is was alive. Coprolites are rocks that consist of petrified excrement. Fossilized dinosaur footprints. Few living organisms leave fossils behind.

Turning to Fossils There are six ways that organisms can turn into fossils, including: unaltered preservation (like insects or plant parts trapped in amber, a hardened form of tree sap) permineralization=petrification (in which rock-like minerals seep in slowly and replace the original organic tissues with silica, calcite or pyrite, forming a rock-like fossil - can preserve hard and soft parts - most bone and wood fossils are permineralized) replacement (An organism's hard parts dissolve and are replaced by other minerals, like calcite, silica, pyrite, or iron) carbonization=coalification (in which only the carbon remains in the specimen - other elements, like hydrogen, oxygen, and nitrogen are removed) recrystalization (hard parts either revert to more stable minerals or small crystals turn into larger crystals) authigenic preservation (molds and casts of organisms that have been destroyed or dissolved).

Fossil Animals Animal remains are most likely to be fossilized if their hard parts are covered by layers of sediments soon after death.   With time and pressure, these sediments, such as sand, plant debris, or ash, become compressed into rock.   Therefore, fossils are found in SEDIMENTARY ROCK, like sandstone, shale, limestone and coal. IGNEOUS ROCK, like granite and basalt, is formed by molten rock erupting from deep within the earth.   METAMORPHIC ROCK, such as marble, is formed by tremendous heat and pressure.   Fossils are not usually found in either igneous or metamorphic rocks. We generally look for fossils in sedimentary rock. The processes to produce igneous and metamorphic rock would destroy the fossil.

Let's look at a hypothetic fossil locality to see this process in action. 100 million years ago Scallops and other invertebrates inhabit the sea in this area. When they die, they are covered by sand that settles on them. 75 million years ago Pressure compacts the loose sandy sediments into hard sandstone. Dinosaurs now live, and die, in the locality. Like the scallops before them, their bodies are covered by sand.

50 million years ago There is a tremendous volcanic eruption in the area. The ash thickly covers the land, and for a while the land cannot support much life. 20 million years ago A Parahippus, a 3-toed horse common at the time, dies at the locality. Like the other creatures before it, its body is buried in sand and sediments by wave action.

3 million years ago The massive force of an earthquake causes layers of sediments to tilt and push upward. 2 years ago Forces of erosion, such as water moving against rock for millions of years, gradually wears away the land surface and a few of the rock layers beneath it. The fossilized bones of the horse, hidden for millions of years, are exposed above the ground.

Pseudoarctolepis sharpi Phyllocarid in unusual dorsal view showing stabilizing arms. Wheeler Formation, Cambrian, Wheeler Amphitheatre, House Range, Millard Co., Utah*

How Old? Relative ages from stratigraphy Absolute ages from radioactive age dating (although not completely accurate) Relative age from photogeology Photogeology is great for other planets/moons

Radioisotopes - Radioactivity Have an unstable nucleus that emits energy and particles a particles – 2 protons and 2 neutrons b particles – an electron g rays – a high energy photon Radioactive decay transforms radioisotope into a different element Decay occurs at a fixed rate Because decay happens at a fixed rate, you can use it for age dating! (C14 has a half-life of 5,700 years.)

Radioisotopes and Age Dating Radioisotopes have unstable nuclei that emit energy and particles Radioactive decay transforms radioisotope into a different element Decay occurs at a fixed rate Because decay happens at a fixed rate, you can use it for age dating!

The Age of the Solar System Ages of rocks can be measured through radioactive dating: Measure abundance of a radioactively decaying element to find the time since formation of the rock

The Age of the Solar System Dating of rocks on Earth, on the Moon, and meteorites all give ages of ~ 4.6 billion years.

Carbon-14 Dating