1b - I have to draw a cross section showing the relationship with gabbro cutting across sandstone and schist with schist being the oldest.
1c - I have to write a brief geological history of the region starting with the oldest events.
200 words.
Thanks
Desmond
I'm saving that question until last. I'm still stuck on radiometric dating! Any ideas to point me in the right direction? It's gonna be a late night tonight!
im stuck on both of these aswell so i will be watching for the replies aswell
Hol & Bex
I'm really stuck on q1 parts b&c as well, I think Q2b is something like this:
U238 decays to Pb at a known rate, if you know the ratio of Pb you can estimate the elapsed time since U was placed in the sample of granite - I think this is all that is required for this answer.
GOOD LUCK
It seems like you are all looking for some guidance on how to answer the questions related to radiometric dating and geologic history. Here are some steps and explanations that might help:
1. Radiometric Dating (Q2b):
Radiometric dating is a method used to determine the age of rocks and minerals based on the decay of radioactive isotopes. In this case, you need to explain how U238 decays to Pb at a known rate and how the ratio of Pb can be used to estimate the time since the U was placed in the sample of granite.
- Start by explaining that U238 is a radioactive isotope of uranium that naturally occurs in granite.
- Mention that over time, U238 decays into Pb, and this decay occurs at a well-established rate called the half-life.
- The half-life is the time it takes for half of the parent isotope (U238) to decay into the daughter isotope (Pb). For U238, the half-life is about 4.5 billion years.
- Explain that by measuring the ratio of Pb to U in the granite sample, scientists can determine how many half-lives have elapsed since the granite formed.
- The number of half-lives multiplied by the half-life duration gives the estimated age of the granite.
2. Cross Section (Q1b):
In order to draw a cross section showing the relationship between gabbro, sandstone, and schist, the first step is to understand the relative ages of the rocks.
- Based on the information given, schist is the oldest rock. So, start by drawing the schist layer at the bottom.
- Next, draw the sandstone layer on top of the schist. This indicates that the sandstone is younger than the schist.
- Finally, draw the gabbro cutting across both the sandstone and schist. This shows that the gabbro is the youngest rock, cutting across both the older sandstone and schist layers.
3. Geological History (Q1c):
When writing a brief geological history, you need to start with the oldest events and progress towards the most recent. Here's a general approach:
- Begin by stating the oldest event, which in this case is the formation of the schist. Explain how the schist formed, such as through intense heat and pressure metamorphosing pre-existing rocks.
- Move on to describe the deposition of the sandstone layer on top of the schist. Discuss the environmental conditions that led to the accumulation of sediment and the subsequent lithification into sandstone.
- Finally, mention the intrusion of the gabbro, cutting across both the schist and sandstone. Discuss how the gabbro formed, possibly due to the intrusion of molten material into the pre-existing rock layers.
Remember to keep your explanation within 200 words, using concise and clear language to describe the key geological events.
I hope this helps! If you have any more specific questions or need further assistance, feel free to ask.