Journal 24 Which rock is oldest? Which one is youngest?

The Rock Record

Today’s learning goal We will determine the relative age of rock layers. We will explain the law of superposition. We will state the principle of uniformitariansm.

Determining Relative Age James Hutton (18th Century Scottish physician) was a keen observer of geologic changes. He helped determine that the Earth is about 4.6 billion years old. He believed that studying the present is the key to understanding the past.

Uniformitarianism Uniformitarianism- a principle that geologic processes that occurred in the past can ben explained by geological processes. Examples: Vocanism, erosion This principle of one of the basic foundations of geology Geologists realize that although the processes of the past and present are the same, the rates of the processes vary over time

Earth’s Age Before Hutton, many people believed the Earth was 6,000 years old and all geologic features had formed at the same time. Hutton observed that forces that changed land operated very slowly. He reasoned that millions of years must be needed for those same forces to create the complicated rock structures observed in Earth’s crust. One way to learn about Earth’s past is to determine the order in which rock layers and other rock structures formed.

Let’s Review What We’ve Learned What evidence did Hutton propose to who that the Earth is very old? He observed that the forces that changed on the land operated very slowly. He reasoned that millions of years must be needed for those same forces to create the complicated rock structures in the Earth’s crust

Relative Age Layers of rock are called strata Strata show the sequence of events that took place in the past and who the order in which rock layers formed. Once scientists know the order in which rock layers formed, they can determine the relative age of each rock layer Relative Age- indicates that one layer is older or younger than another layer but does not indicate the rock’s age in years.

Law of Superposition

Law of Superposition Sedimentary rocks form when new sediments are deposited on top of old layers of sediment. As the sediments accumulate they are compressed and hardened into sedimentary rock layers which are called beds. The boundary between 2 beds is called the bedding plane.

Law of Superposition Scientists use a basic principle called the law of superposition to determined the relative age of a layer of sedimentary rock. This law states that an undeformed sedimentary rock layer is older than the layers above it and younger than the layers below it.

Principle of Original Horizontality Sedimentary rock generally forms in horizontal layers. The principle of original horizontality is that sedimentary rocks left undisturbed will remain in horizontal layers. Therefore, sedimentary rock layers that are not horizontal have been tilted or deformed by crustal movements that happened after the layers formed.

Principle of Original Horizontality \

Principle of Original Horizontality In some cases, tectonic forces push older layers on top of younger ones or overturn a group of rock layers. In such cases, the law of superposition cannot be easily applied. Grading beds Cross beds Ripple marks

Grading bedding One clue to the original position of rock layers in the size of the particles in the layers. In some environments, the largest particles of sediment are deposited in the bottom layers. The arrangement of layers in which coarse and heavy particles are located in the bottom layers is called graded bedding. If larger particles are located in the top layers, the layers may have been overturned by tectonic forces.

Cross Beds Another clue to the original position of rock layers is in the shape of the bedding planes. When sand is deposited, sandy sediment forms curved beds at an angle to the bedding plane. These beds are called cross-beds. The tops of these layers commonly erode before new layers are deposited. So, the sediment appears to be curved at the bottom of the layer and to be cut off at the top.

Cross Beds

Ripple Marks Ripple marks are small waves that form on the surface of sand because of the action of water or wind. When the sand becomes sandstone, the ripple marks may be preserved. In undisturbed sedimentary rock layers, the crests of the ripple marks point upward.

Ripple Marks

Crosscutting Relationships When layers have been disturbed by faults or intrusions, determining relative age may be difficult. A fault is a break or crack in the Earth’s crust along which rocks shift their position. An intrusion is a mass of igneous rock that forms when magma is injected into rock and then cools and solidifies.

Crosscutting Relationships The law of crosscutting relationships is that a fault or igneous intrusion is always younger than the rock layers it cuts through. If a fault or intrusion cuts through an unconformity, the fault or intrusion is younger than all the rocks it cuts through above and below the unconformity.

Crosscutting Relationships

Vocabulary Squares Law of superposition Graded bedding Cross-beds Ripple marks Law of crosscutting relationships

Today’s learning goal We will determine the relative age of rock layers. We will explain the law of superposition. We will state the principle of uniformitariansm.

Review Which rock is oldest? Which one is youngest?