Finding the Relative Age of Rocks Relative and
- Slides: 52
Finding the Relative Age of Rocks
Relative and Absolute Ages • Relative Age The age of a rock compared to the ages of rock layers. • Absolute Age The age of a rock given as the number of years since the rock formed.
Position of Rock Layers Its difficult to determine the absolute age so geologists use method to find a rock’s relative age. Use the: LAW OF SUPERPOSITION: in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.
Other Clues to Relative Age A. Clues from Igneous Rocks: 1. Lava that hardens on the surface is called an Extrusion (example – an eruption would put a layer of igneous rock on top of sedimentary rocks. Rock layers below an extrusion are always older than the extrusion.
The extrusion is in black
Now the extrusion is the youngest layer.
A. Clues from Igneous Rocks: 2. Magma that cools and pushes into bodies of rock and hardens is called an Intrusion An intrusion is always younger than the rock layers around and beneath it.
The intrusion (in red) is now younger than the surrounding rocks.
Draw this in your notes and label each of the parts listed below. 1. Intrusion 2. Extrusion 3. Oldest rock 4. Youngest rock
B. Clues from Faults: Fault: is a break in the Earth’s crust. • Forces inside the Earth cause movement of the rock on opposite sides of a fault. • Fault is always younger than the rock it cuts through.
GAPS IN THE GEOLOGIC RECORD - Record of sedimentary rock layers is not always complete - Deposition slowly builds layers upon layer of sedimentary rock, BUT some of these layers may erode away, exposing an older rock surface. Unconformity – is a gap in the geologic record. An unconformity shows where some rock layers have been lost because of erosion.
Unconformities *A break in the geologic record is called an unconformity. An unconformity shows that deposition stopped for a period of time, and rock may have been removed by erosion before deposition resumed.
*Unconformities There are three types of unconformities. 1. An unconformity in which stratified (layers) of rock rests upon unstratified rock is called a nonconformity. 2. The boundary between a set of tilted layers and a set of horizontal layers is called an angular unconformity. 3. The boundary between horizontal layers of old sedimentary rock and younger, overlying layers that are deposited on an eroded surface is called a disconformity. According to the Law of Superposition, all rocks beneath an unconformity are older than the rocks above the unconformity.
USING FOSSILS TO DATE ROCKS To date rock layers, geologists first give a relative age to a layer of rock at one location. THEN they can give the same age to matching layers of rock at other locations. Certain fossils, called Index Fossils help geologist match rock layers. INDEX FOSSILS – Fossils of widely distributed organisms that lived during only one short period.
*Index Fossils requirements: 1. It must be present in rocks scattered over a large region. 2. It must have features that clearly distinguish it from other fossils. 3. Organisms from which the fossil formed must have lived during a short span of geologic time. 4. The fossil must occur in fairly large numbers within the rock layers.
*Index Fossils To be an index fossil, a fossil must meet certain requirements: 1. It must be present in rocks scattered over a large region. 2. It must have features that clearly distinguish it from other fossils. 3. Organisms from which the fossil formed must have lived during a short span of geologic time. 4. The fossil must occur in fairly large numbers within the rock layers.
• Example of an Index Fossil: • Trilobites (hard shelled animals whose bodies had three distinct parts. • Trilobites evolved in shallow seas more than 500 million years ago. • Over time, many types have appeared. • They became extinct about 245 million years ago. • They have been found in many different places.
To become a Index Fossil … a trilobite must be different in some way from other trilobites. Example – type with large eyes These large-eyed. . bites survived for a time AFTER other bites became extinct. If a geologist finds large-eyed Trilobites in a rock layer, the geologist can infer that those rocks are younger than rocks containing other types of trilobites
A team of Canadian paleontologists working along Hudson Bay in northern Manitoba has discovered the world's largest recorded complete fossil of a trilobite, a many-legged, sea-dwelling animal that lived 445 million years ago. The giant creature is more than 70 cm long (about 28 inches), 70 percent larger than the previous record holder. "This is an important and amazing find, " says Bob Elias, a professor in the department of geological sciences at the University of Manitoba. "It looks like a huge bug!"
Fossils Essential Questions: What are fossils and how are they formed? What are the different kinds of fossils? What do fossils tell about how organisms have changed over time?
Fossils • Fossils- the preserved remains or evidence of living things
What do fossils tell about how organisms have changed over time? • Paleontologists use fossils to build up a picture of Earth’s environments in the past. • Fossils also provide evidence of Earth’s climate in the past. • Scientists can use fossils to learn about changes in Earth’s surface.
Fossilization § Organisms with hard parts are typically preserved. § Those without hard parts are fossilized by quick burial. How are fossils formed? § Mummification § Amber § Tar seeps § Freezing § Petrification
Types of Fossils • Original Remains- Unchanged remains of plants and animals – Examples • Hard parts: Bones, Shells • Animals trapped in ice: Wooly Mammoth • Animals trapped in tar or in amber
Mummification Drying of organisms; found in deserts.
Amber Hardened tree sap, insects get stuck in sap and sap hardens
La Brea Tar Pits • Thick petroleum • Animals get stuck in tar and it preserves them
Freezing Animals preserved because bacteria cannot survive in cold climate to decay the bodies
Types of Fossils • Replaced Remains: Hard parts of plants or animals are replaced by minerals over time. The minerals are in the shape of the organism. – This occurs when water containing minerals seeps through the plant/animal material. – Petrified wood forms in this way – Petrified means “turning to stone”
Petrification When organic material is replaced with minerals
Example: Petrified Wood Minerals have replaced all of the wood cells and turned this wood into rock!
Types of Fossils • Molds and Casts: Impressions and copies • Molds are created as mud or sediment becomes rock around dead organisms. • Casts are created as minerals get deposited into the molds and then harden. They make a copy of the fossil. Ice cube tray= Mold Ice= Cast
Types of Fossils continued… § Mold § Cast: when a mold fills in with sediment and becomes hardened.
Mold vs. Cast § Molds: animal remains dissolve away, but the shape of the animal remains. § Cast: when a mold fills in with sediment and becomes hardened.
Example: Cast of an Ammonite- marine mollusc
Molds and Casts
Types of fossils § Trace Fossils -Evidence that an animal existed. Ex. footprints
Trace Fossils
Carbonaceous Film HOW DOES A CARBON FILM FORM? When sediment buries an organism, some of the materials that make up the organism can become gases. These gases escape from the sediment, leaving carbon behind. Eventually, only a thin film of carbon remains.
Imprints- carbonized imprints Coprolites fossilized animal droppings (learn eating habits)
Trilobitemarine arthropods: Index fossil of Paleozoic • Graptolite: Index fossil of the Silurian
In Summary • Most fossils form when living things die and are quickly buried by sediment, which eventually hardens and preserves parts of the organisms. • The major kinds of fossils include petrified remains, molds, casts, carbon films, trace fossils, and preserved remains. • The fossil record shows that many different organisms have lived on Earth at different times and that groups of organisms have changed over time.
Fossils Preserved fossils amber tar Rock Fossils ice Trace fossils Casts and molds Petrified fossils Carbon films
Question: • Fossils are usually formed when the organism dies and gets buried in mud or sand. As a result, What type of rock are fossils usually found in?
Questions: • Why are fossils NOT usually found in igneous rock?
Questions: • In what settings (environments) could fossils be preserved?
Questions: • What are the 5 types of fossils?
- Sedimentary igneous and metamorphic
- Compaction and cementation
- What is reletive age
- Relative age of rocks diagram
- Iron age dates
- Iron age bronze age stone age timeline
- Extrusive rocks
- Granite and rhyolite
- Victorian age and modern age
- Neolithic period timeline
- Romantic age and victorian age
- Bone age greater than chronological age
- Paleolithic vs neolithic
- "age of trilobites" or "age of fish".
- During the later vedic period chamberlain was known as
- Stage 15 relative clauses and relative pronouns
- The person who phoned me last night is my teacher
- Relative adverbs and pronouns
- Relative age effect
- Hadean eon
- Uniformitarianism relative dating
- Dating
- Hình ảnh bộ gõ cơ thể búng tay
- Frameset trong html5
- Bổ thể
- Tỉ lệ cơ thể trẻ em
- Voi kéo gỗ như thế nào
- Tư thế worms-breton
- Alleluia hat len nguoi oi
- Các môn thể thao bắt đầu bằng tiếng đua
- Thế nào là hệ số cao nhất
- Các châu lục và đại dương trên thế giới
- Công thức tiính động năng
- Trời xanh đây là của chúng ta thể thơ
- Mật thư anh em như thể tay chân
- Phép trừ bù
- độ dài liên kết
- Các châu lục và đại dương trên thế giới
- Thơ thất ngôn tứ tuyệt đường luật
- Quá trình desamine hóa có thể tạo ra
- Một số thể thơ truyền thống
- Cái miệng xinh xinh thế chỉ nói điều hay thôi
- Vẽ hình chiếu vuông góc của vật thể sau
- Nguyên nhân của sự mỏi cơ sinh 8
- đặc điểm cơ thể của người tối cổ
- Ví dụ về giọng cùng tên
- Vẽ hình chiếu đứng bằng cạnh của vật thể
- Fecboak
- Thẻ vin
- đại từ thay thế
- điện thế nghỉ
- Tư thế ngồi viết
- Diễn thế sinh thái là