Question about the Zcash Blockexplorer, Block Times

#1

Today i was just playing around with the Zcash Block Explorer and i admit i’am not very familar with block explorers in generally other than checking if a given transaction went through and never gave it some attention. But i was in mood to dig a bit into it and have a few questions and maybe someone can answer them.

Question 1: When viewing a block on the explorer under summary for a given block there it writes “Time”. Which various. Shouldn’t this be allways 2.5 minutes are 150 seconds?

Question 2: Again about time. I found some blocks that have - time, for example -37 seconds? What’s the reason this happens?

Questions 3: I found a lot of blocks in 2018 (randomly choosen) that have a lot of huge transaction counts all with 0.0138 ZEC (if i remember right) or something like that and these seem to repeat very often. Some kind of spamming and if yes what’s the intention for someone doing it?

#2

Proof-of-Work mining is a statistical process: you solve a puzzle and then check whether your solution satisfies the required difficulty, which was set based on the target time of 150 seconds. It’s entirely possible that you stumble upon a valid and difficult-enough solution on your first try, allowing you to mine a block almost instantly. Conversely, it’s possible that you fail to find a solution for as long as 10 minutes. See this paper for an analysis of Bitcoin’s block times.

The timestamps in block headers are picked by mining pools, and thus cannot really be trusted individually, because they could be maliciously chosen. PoW systems instead rely on an average of timestamps when calculating the difficulty, to reduce the influence any one mining pool has. In the case you give, it’s more likely that two sequential blocks were mined by different mining pools with clocks that don’t agree.

My guess would be that they were generated by an exchange that was trying to merge small UTXOs in their wallet (usually the result of miners configuring mining pools to send payouts directly to exchange addresses).

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