What Is a Blockchain? How Miners Add & Secure Blocks
A blockchain is a public ledger of transaction blocks, each linked to the last by its hash. How miners add blocks and why confirmed history is hard to rewrite.
Definition
A blockchain is a public record of transactions grouped into blocks and linked together in order. In cryptocurrency mining, miners compete to add the next valid block to this record, and each block points back to the block before it.
How It Works
When users send cryptocurrency transactions, those transactions wait to be confirmed by the network. Miners collect valid transactions from the mempool, which can be observed on public explorers such as mempool.space, and assemble them into a candidate block.
The miner then builds a block header. The header includes a reference to the previous block, a summary of the new transactions, the current difficulty target, and a nonce that miners can change. Bitcoin’s developer documentation describes these fields in its block headers reference.
In proof-of-work systems such as Bitcoin, mining means hashing the block header over and over, following the design introduced in the Bitcoin whitepaper. A valid block must produce a hash below the network’s target, which is controlled by mining difficulty. Because hash results are unpredictable, miners use hash rate to make many attempts quickly.
When a miner finds a valid block, it broadcasts it to the network. Other nodes check the transactions, verify the proof of work, and confirm that the block correctly links to the chain.
If accepted, the block becomes part of the blockchain. The successful miner earns the block reward, which can include new coins and transaction fees.
Why It Matters
The blockchain is the shared history that lets a cryptocurrency operate without a central bookkeeper. Everyone can verify the same record, but no single miner can simply edit it.
For miners, the blockchain defines what they are competing to extend. A miner is trying to produce the next block that the network will accept.
The chain structure also protects older transactions. Since each new block depends on the previous block’s hash, changing an old block would also change every block after it. An attacker would need to redo the proof of work and catch up with the honest network.
This is why confirmations matter. A transaction buried under several later blocks is much harder to reverse.
Related Terms
FAQ
What is a blockchain in cryptocurrency mining?
A blockchain is a public record of transactions grouped into blocks and linked in order, with each block pointing back to the one before it through its hash. In cryptocurrency mining, miners compete to add the next valid block, and that shared chain lets everyone verify the same history without a central bookkeeper.
How do miners add blocks to the blockchain?
Miners add blocks to the blockchain by collecting pending transactions from the mempool, building a block header, and hashing it repeatedly while changing the nonce until the result falls below the network target. The first miner to succeed broadcasts the block, and other nodes verify its proof of work before appending it to the chain.
Why is it hard to change a blockchain’s history?
A blockchain is hard to rewrite because each block depends on the previous block’s hash, so changing an old block would change every block after it. An attacker would have to redo that proof of work and catch up with the honest network, which is why transactions buried under several confirmations are much harder to reverse.